Akritidis, Dimitris, Sara Bacer, Prodromos Zanis, Aristeidis K Georgoulias, Sourangsu Chowdhury, Larry W Horowitz, Vaishali Naik, Fiona M O'Connor, James Keeble, Philippe Le Sager, Twan van Noije, Putian Zhou, Steven T Turnock, J Jason West, Jos Lelieveld, and Andrea Pozzer, February 2024: Strong increase in mortality attributable to ozone pollution under a climate change and demographic scenario. Environmental Research Letters, 19(2), DOI:10.1088/1748-9326/ad2162. Abstract
Long-term exposure to ambient ozone (O3) is associated with excess respiratory mortality. Pollution emissions, demographic, and climate changes are expected to drive future ozone-related mortality. Here, we assess global mortality attributable to ozone according to an Intergovernmental Panel on Climate Change (IPCC) Shared Socioeconomic Pathway (SSP) scenario applied in Coupled Model Intercomparison Project Phase 6 (CMIP6) models, projecting a temperature increase of about 3.6 °C by the end of the century. We estimated ozone-related mortality on a global scale up to 2090 following the Global Burden of Disease (GBD) 2019 approach, using bias-corrected simulations from three CMIP6 Earth System Models (ESMs) under the SSP3-7.0 emissions scenario. Based on the three ESMs simulations, global ozone-related mortality by 2090 will amount to 2.79 M [95% CI 0.97 M–5.23 M] to 3.12 M [95% CI 1.11 M–5.75 M] per year, approximately ninefold that of the 327 K [95% CI 103 K–652 K] deaths per year in 2000. Climate change alone may lead to an increase of ozone-related mortality in 2090 between 42 K [95% CI −37 K–122 K] and 217 K [95% CI 68 K–367 K] per year. Population growth and ageing are associated with an increase in global ozone-related mortality by a factor of 5.34, while the increase by ozone trends alone ranges between factors of 1.48 and 1.7. Ambient ozone pollution under the high-emissions SSP3-7.0 scenario is projected to become a significant human health risk factor. Yet, optimizing living conditions and healthcare standards worldwide to the optimal ones today (application of minimum baseline mortality rates) will help mitigate the adverse consequences associated with population growth and ageing, and ozone increases caused by pollution emissions and climate change.
Allen, Robert J., James Gomez, Larry W Horowitz, and Elena Shevliakova, January 2024: Enhanced future vegetation growth with elevated carbon dioxide concentrations could increase fire activity. Communications Earth and Environment, 5, 54, DOI:10.1038/s43247-024-01228-7. Abstract
Many regions of the planet have experienced an increase in fire activity in recent decades. Although such increases are consistent with warming and drying under continued climate change, the driving mechanisms remain uncertain. Here, we investigate the effects of increasing atmospheric carbon dioxide concentrations on future fire activity using seven Earth system models. Centered on the time of carbon dioxide doubling, the multi-model mean percent change in fire carbon emissions is 66.4 ± 38.8% (versus 1850 carbon dioxide concentrations, under fixed 1850 land-use conditions). A substantial increase is associated with enhanced vegetation growth due to carbon dioxide biogeochemical impacts at 60.1 ± 46.9%. In contrast, carbon dioxide radiative impacts, including warming and drying, yield a negligible response of fire carbon emissions at 1.7 ± 9.4%. Although model representation of fire processes remains uncertain, our results show the importance of vegetation dynamics to future increases in fire activity under increasing carbon dioxide, with potentially important policy implications.
Kalisoras, Alkiviadis, Aristeidis K Georgoulias, Dimitris Akritidis, Robert J Allen, Vaishali Naik, Chaincy Kuo, Sophie Szopa, Pierre Nabat, Dirk Olivié, Twan van Noije, Philippe Le Sager, David Neubauer, Naga Oshima, Jane P Mulcahy, Larry W Horowitz, and Prodromos Zanis, July 2024: Decomposing the effective radiative forcing of anthropogenic aerosols based on CMIP6 Earth system models. Atmospheric Chemistry and Physics, 24(13), DOI:10.5194/acp-24-7837-20247837–7872. Abstract
Anthropogenic aerosols play a major role in the Earth–atmosphere system by influencing the Earth's radiative budget and precipitation and consequently the climate. The perturbation induced by changes in anthropogenic aerosols on the Earth's energy balance is quantified in terms of the effective radiative forcing (ERF). In this work, the present-day shortwave (SW), longwave (LW), and total (i.e., SW plus LW) ERF of anthropogenic aerosols is quantified using two different sets of experiments with prescribed sea surface temperatures (SSTs) from Earth system models (ESMs) participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6): (a) time-slice pre-industrial perturbation simulations with fixed SSTs (piClim) and (b) transient historical simulations with time-evolving SSTs (histSST) over the historical period (1850–2014). ERF is decomposed into three components for both piClim and histSST experiments: (a) ERFARI, representing aerosol–radiation interactions; (b) ERFACI, accounting for aerosol–cloud interactions (including the semi-direct effect); and (c) ERFALB, which is due to temperature, humidity, and surface albedo changes caused by anthropogenic aerosols. We present spatial patterns at the top-of-atmosphere (TOA) and global weighted field means along with inter-model variability (1 standard deviation) for all SW, LW, and total ERF components (ERFARI, ERFACI, and ERFALB) and for every experiment used in this study. Moreover, the inter-model agreement and the robustness of our results are assessed using a comprehensive method as utilized in the IPCC Sixth Assessment Report. Based on piClim experiments, the total present-day (2014) ERF from anthropogenic aerosol and precursor emissions is estimated to be −1.11 ± 0.26 W m−2, mostly due to the large contribution of ERFACI to the global mean and to the inter-model variability. Based on the histSST experiments for the present-day period (1995–2014), similar results are derived, with a global mean total aerosol ERF of −1.28 ± 0.37 W m−2 and dominating contributions from ERFACI. The spatial patterns for total ERF and its components are similar in both the piClim and histSST experiments. Furthermore, implementing a novel approach to determine geographically the driving factor of ERF, we show that ERFACI dominates over the largest part of the Earth and that ERFALB dominates mainly over the poles, while ERFARI dominates over certain reflective surfaces. Analysis of the inter-model variability in total aerosol ERF shows that SW ERFACI is the main source of uncertainty predominantly over land regions with significant changes in aerosol optical depth (AOD), with eastern Asia contributing mostly to the inter-model spread of both ERFARI and ERFACI. The global spatial patterns of total ERF and its components from individual aerosol species, such as sulfates, organic carbon (OC), and black carbon (BC), are also calculated based on piClim experiments. The total ERF caused by sulfates (piClim-SO2) is estimated at −1.11 ± 0.31 W m−2, and the OC ERF (piClim-OC) is −0.35 ± 0.21 W m−2, while the ERF due to BC (piClim-BC) is 0.19 ± 0.18 W m−2. For sulfates and OC perturbation experiments, ERFACI dominates over the globe, whereas for BC perturbation experiments ERFARI dominates over land in the Northern Hemisphere and especially in the Arctic. Generally, sulfates dominate ERF spatial patterns, exerting a strongly negative ERF especially over industrialized regions of the Northern Hemisphere (NH), such as North America, Europe, and eastern and southern Asia. Our analysis of the temporal evolution of ERF over the historical period (1850–2014) reveals that ERFACI clearly dominates over ERFARI and ERFALB for driving the total ERF temporal evolution. Moreover, since the mid-1980s, total ERF has become less negative over eastern North America and western and central Europe, while over eastern and southern Asia there is a steady increase in ERF magnitude towards more negative values until 2014.
We present a variable-resolution global chemistry-climate model (AM4VR) developed at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL) for research at the nexus of US climate and air quality extremes. AM4VR has a horizontal resolution of 13 km over the US, allowing it to resolve urban-to-rural chemical regimes, mesoscale convective systems, and land-surface heterogeneity. With the resolution gradually reducing to 100 km over the Indian Ocean, we achieve multi-decadal simulations driven by observed sea surface temperatures at 50% of the computational cost for a 25-km uniform-resolution grid. In contrast with GFDL's AM4.1 contributing to the sixth Coupled Model Intercomparison Project at 100 km resolution, AM4VR features much improved US climate mean patterns and variability. In particular, AM4VR shows improved representation of: precipitation seasonal-to-diurnal cycles and extremes, notably reducing the central US dry-and-warm bias; western US snowpack and summer drought, with implications for wildfires; and the North American monsoon, affecting dust storms. AM4VR exhibits excellent representation of winter precipitation, summer drought, and air pollution meteorology in California with complex terrain, enabling skillful prediction of both extreme summer ozone pollution and winter haze events in the Central Valley. AM4VR also provides vast improvements in the process-level representations of biogenic volatile organic compound emissions, interactive dust emissions from land, and removal of air pollutants by terrestrial ecosystems. We highlight the value of increased model resolution in representing climate–air quality interactions through land-biosphere feedbacks. AM4VR offers a novel opportunity to study global dimensions to US air quality, especially the role of Earth system feedbacks in a changing climate.
Lin, Meiyun, Larry W Horowitz, Lu Hu, and Wade Permar, August 2024: Reactive nitrogen partitioning enhances the contribution of Canadian wildfire plumes to US ozone air quality. Geophysical Research Letters, 51(15), DOI:10.1029/2024GL109369. Abstract
Quantifying the variable impacts of wildfire smoke on ozone air quality is challenging. Here we use airborne measurements from the 2018 Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption, and Nitrogen (WE-CAN) to parameterize emissions of reactive nitrogen (NOy) from wildfires into peroxyacetyl nitrate (PAN; 37%), NO3− (27%), and NO (36%) in a global chemistry-climate model with 13 km spatial resolution over the contiguous US. The NOy partitioning, compared with emitting all NOy as NO, reduces model ozone bias in near-fire smoke plumes sampled by the aircraft and enhances ozone downwind by 5–10 ppbv when Canadian smoke plumes travel to Washington, Utah, Colorado, and Texas. Using multi-platform observations, we identify the smoke-influenced days with daily maximum 8-hr average (MDA8) ozone of 70–88 ppbv in Kennewick, Salt Lake City, Denver and Dallas. On these days, wildfire smoke enhanced MDA8 ozone by 5–25 ppbv, through ozone produced remotely during plume transport and locally via interactions of smoke plume with urban emissions.
Ren, Fangxuan, Jintai Lin, Chenghao Xu, Jamiu A Adeniran, Jingxu Wang, Randall V Martin, Aaron Van Donkelaar, Melanie S Hammer, and Larry W Horowitz, et al., June 2024: Evaluation of CMIP6 model simulations of PM2.5 and its components over China. Geoscientific Model Development, 17(12), DOI:10.5194/gmd-17-4821-20244821-4836. Abstract
Earth system models (ESMs) participating in the latest Coupled Model Intercomparison Project Phase 6 (CMIP6) simulate various components of fine particulate matter (PM2.5) as major climate forcers. Yet the model performance for PM2.5 components remains little evaluated due in part to a lack of observational data. Here, we evaluate near-surface concentrations of PM2.5 and its five main components over China as simulated by 14 CMIP6 models, including organic carbon (OC; available in 14 models), black carbon (BC; 14 models), sulfate (14 models), nitrate (4 models), and ammonium (5 models). For this purpose, we collect observational data between 2000 and 2014 from a satellite-based dataset for total PM2.5 and from 2469 measurement records in the literature for PM2.5 components. Seven models output total PM2.5 concentrations, and they all underestimate the observed total PM2.5 over eastern China, with GFDL-ESM4 (−1.5 %) and MPI-ESM-1-2-HAM (−1.1 %) exhibiting the smallest biases averaged over the whole country. The other seven models, for which we recalculate total PM2.5 from the available component output, underestimate the total PM2.5 concentrations partly because of the missing model representations of nitrate and ammonium. Concentrations of the five individual components are underestimated in almost all models, except that sulfate is overestimated in MPI-ESM-1-2-HAM by 12.6 % and in MRI-ESM2-0 by 24.5 %. The underestimation is the largest for OC (by −71.2 % to −37.8 % across the 14 models) and the smallest for BC (−47.9 % to −12.1 %). The multi-model mean (MMM) reproduces the observed spatial pattern for OC (R = 0.51), sulfate (R = 0.57), nitrate (R = 0.70) and ammonium (R = 0.74) fairly well, yet the agreement is poorer for BC (R = 0.39). The varying performances of ESMs on total PM2.5 and its components have important implications for the modeled magnitude and spatial pattern of aerosol radiative forcing.
We describe the baseline model configuration and simulation characteristics of the Geophysical Fluid Dynamics Laboratory (GFDL)'s Land Model version 4.1 (LM4.1), which builds on component and coupled model developments over 2013–2019 for the coupled carbon-chemistry-climate Earth System Model Version 4.1 (ESM4.1) simulation as part of the sixth phase of the Coupled Model Intercomparison Project. Analysis of ESM4.1/LM4.1 is focused on biophysical and biogeochemical processes and interactions with climate. Key features include advanced vegetation dynamics and multi-layer canopy energy and moisture exchanges, daily fire, land use representation, and dynamic atmospheric dust coupling. We compare LM4.1 performance in the GFDL Earth System Model (ESM) configuration ESM4.1 to the previous generation component LM3.0 in the ESM2G configuration. ESM4.1/LM4.1 provides significant improvement in the treatment of ecological processes from GFDL's previous generation models. However, ESM4.1/LM4.1 likely overestimates the influence of land use and land cover change on vegetation characteristics, particularly on pasturelands, as it overestimates the competitiveness of grasses versus trees in the tropics, and as a result, underestimates present-day biomass and carbon uptake in comparison to observations.
Yang, Wenchang, Elizabeth Wallace, Gabriel A Vecchi, Jeffrey P Donnelly, Julien Emile-Geay, Gregory J Hakim, Larry W Horowitz, Richard M Sullivan, Robert Tardif, Peter J Van Hengstum, and Tyler S Winkler, January 2024: Last millennium hurricane activity linked to endogenous climate variability. Nature Communications, 15, 816, DOI:10.1038/s41467-024-45112-6. Abstract
Despite increased Atlantic hurricane risk, projected trends in hurricane frequency in the warming climate are still highly uncertain, mainly due to short instrumental record that limits our understanding of hurricane activity and its relationship to climate. Here we extend the record to the last millennium using two independent estimates: a reconstruction from sedimentary paleohurricane records and a statistical model of hurricane activity using sea surface temperatures (SSTs). We find statistically significant agreement between the two estimates and the late 20th century hurricane frequency is within the range seen over the past millennium. Numerical simulations using a hurricane-permitting climate model suggest that hurricane activity was likely driven by endogenous climate variability and linked to anomalous SSTs of warm Atlantic and cold Pacific. Volcanic eruptions can induce peaks in hurricane activity, but such peaks would likely be too weak to be detected in the proxy record due to large endogenous variability.
Ahsan, Hamza, Hailong Wang, Jingbo Wu, Mingxuan Wu, Steven J Smith, Susanne E Bauer, Harrison Suchyta, Dirk Olivié, Gunnar Myhre, Hitoshi Matsui, Huisheng Bian, Jean-Francois Lamarque, Kenneth S Carslaw, Larry W Horowitz, Leighton Regayre, Mian Chin, Michael Schulz, Ragnhild Bieltvedt-Skeie, Toshihiko Takemura, and Vaishali Naik, December 2023: The Emissions Model Intercomparison Project (Emissions-MIP): Quantifying model sensitivity to emission characteristics. Atmospheric Chemistry and Physics, 23(23), DOI:10.5194/acp-23-14779-202314779–14799. Abstract
Anthropogenic emissions of aerosols and precursor compounds are known to significantly affect the energy balance of the Earth–atmosphere system, alter the formation of clouds and precipitation, and have a substantial impact on human health and the environment. Global models are an essential tool for examining the impacts of these emissions. In this study, we examine the sensitivity of model results to the assumed height of SO2 injection, seasonality of SO2 and black carbon (BC) particulate emissions, and the assumed fraction of SO2 emissions that is injected into the atmosphere as particulate phase sulfate (SO4) in 11 climate and chemistry models, including both chemical transport models and the atmospheric component of Earth system models. We find large variation in atmospheric lifetime across models for SO2, SO4, and BC, with a particularly large relative variation for SO2, which indicates that fundamental aspects of atmospheric sulfur chemistry remain uncertain. Of the perturbations examined in this study, the assumed height of SO2 injection had the largest overall impacts, particularly on global mean net radiative flux (maximum difference of −0.35 W m−2), SO2 lifetime over Northern Hemisphere land (maximum difference of 0.8 d), surface SO2 concentration (up to 59 % decrease), and surface sulfate concentration (up to 23 % increase). Emitting SO2 at height consistently increased SO2 and SO4 column burdens and shortwave cooling, with varying magnitudes, but had inconsistent effects across models on the sign of the change in implied cloud forcing. The assumed SO4 emission fraction also had a significant impact on net radiative flux and surface sulfate concentration. Because these properties are not standardized across models this is a source of inter-model diversity typically neglected in model intercomparisons. These results imply a need to ensure that anthropogenic emission injection height and SO4 emission fraction are accurately and consistently represented in global models.
Chua, Glen, Vaishali Naik, and Larry W Horowitz, April 2023: Exploring the drivers of tropospheric hydroxyl radical trends in the Geophysical Fluid Dynamics Laboratory AM4.1 atmospheric chemistry–climate model. Atmospheric Chemistry and Physics, 23(8), DOI:10.5194/acp-23-4955-20234955-4975. Abstract
We explore the sensitivity of modeled tropospheric hydroxyl (OH) concentration trends to meteorology and near-term climate forcers (NTCFs), namely methane (CH4) nitrogen oxides (NOX = NO2 + NO) carbon monoxide (CO), non-methane volatile organic compounds (NMVOCs) and ozone-depleting substances (ODSs), using the Geophysical Fluid Dynamics Laboratory (GFDL)'s atmospheric chemistry–climate model, the Atmospheric Model version 4.1 (AM4.1), driven by emissions inventories developed for the Sixth Coupled Model Intercomparison Project (CMIP6) and forced by observed sea surface temperatures and sea ice prepared in support of the CMIP6 Atmospheric Model Intercomparison Project (AMIP) simulations. We find that the modeled tropospheric air-mass-weighted mean [OH] has increased by ∼5 % globally from 1980 to 2014. We find that NOx emissions and CH4 concentrations dominate the modeled global trend, while CO emissions and meteorology were also important in driving regional trends. Modeled tropospheric NO2 column trends are largely consistent with those retrieved from the Ozone Monitoring Instrument (OMI) satellite, but simulated CO column trends generally overestimate those retrieved from the Measurements of Pollution in The Troposphere (MOPITT) satellite, possibly reflecting biases in input anthropogenic emission inventories, especially over China and South Asia.
Stratospheric injections of sulfur dioxide from major volcanic eruptions perturb the Earth's global radiative balance and dominate variability in stratospheric sulfur loading. The atmospheric component of the GFDL Earth System Model (ESM4.1) uses a bulk aerosol scheme and previously prescribed the distribution of aerosol optical properties in the stratosphere. To quantify volcanic contributions to the stratospheric sulfur cycle and the resulting climate impact, we modified ESM4.1 to simulate stratospheric sulfate aerosols prognostically. Driven by explicit volcanic emissions of aerosol precursors and non-volcanic sources, we conduct ESM4.1 simulations from 1989 to 2014, with a focus on the Mt. Pinatubo eruption. We evaluate our interactive representation of the stratospheric sulfur cycle against data from Moderate Resolution Imaging Spectroradiometer, Multi-angle Imaging SpectroRadiometer, Advanced Very High Resolution Radiometer, High Resolution Infrared Radiation Sounder, and Stratospheric Aerosol and Gas Experiment II. To assess the key processes associated with volcanic aerosols, we performed a sensitivity analysis of sulfate burden from the Mt. Pinatubo eruption by varying injection heights, emission amount, and stratospheric sulfate's dry effective radius. We find that the simulated stratospheric sulfate mass burden and aerosol optical depth in the model are sensitive to these parameters, especially volcanic SO2 injection height, and the optimal combination of parameters depends on the metric we evaluate.
Gomez, James, Robert J Allen, Steven T Turnock, Larry W Horowitz, Kostas Tsigaridis, Susanne E Bauer, Dirk Olivié, Erik S Thomson, and Paul Ginoux, February 2023: The projected future degradation in air quality is caused by more abundant natural aerosols in a warmer world. Communications Earth and Environment, 4, 22, DOI:10.1038/s43247-023-00688-7. Abstract
Previous studies suggest that greenhouse gas-induced warming can lead to increased fine particulate matter concentrations and degraded air quality. However, significant uncertainties remain regarding the sign and magnitude of the response to warming and the underlying mechanisms. Here, we show that thirteen models from the Coupled Model Intercomparison Project Phase 6 all project an increase in global average concentrations of fine particulate matter in response to rising carbon dioxide concentrations, but the range of increase across models is wide. The two main contributors to this increase are increased abundance of dust and secondary organic aerosols via intensified West African monsoon and enhanced emissions of biogenic volatile organic compounds, respectively. Much of the inter-model spread is related to different treatment of biogenic volatile organic compounds. Our results highlight the importance of natural aerosols in degrading air quality under current warming, while also emphasizing that improved understanding of biogenic volatile organic compounds emissions due to climate change is essential for numerically assessing future air quality.
Schmidt, Gavin A., Timothy Andrews, Susanne E Bauer, Paul J Durack, Norman G Loeb, V Ramaswamy, Nathan P Arnold, Michael Bosilovich, Jason N S Cole, Larry W Horowitz, Gregory C Johnson, John M Lyman, Brian Medeiros, Takuro Michibata, Dirk Olonscheck, David J Paynter, Shiv Priyam Raghuraman, Michael Schulz, Daisuke Takasuka, Vijay Tallapragada, Patrick C Taylor, and Tilo Ziehn, July 2023: CERESMIP: A climate modeling protocol to investigate recent trends in the Earth's Energy Imbalance. Frontiers in Climate, 5, DOI:10.3389/fclim.2023.1202161. Abstract
The Clouds and the Earth's Radiant Energy System (CERES) project has now produced over two decades of observed data on the Earth's Energy Imbalance (EEI) and has revealed substantive trends in both the reflected shortwave and outgoing longwave top-of-atmosphere radiation components. Available climate model simulations suggest that these trends are incompatible with purely internal variability, but that the full magnitude and breakdown of the trends are outside of the model ranges. Unfortunately, the Coupled Model Intercomparison Project (Phase 6) (CMIP6) protocol only uses observed forcings to 2014 (and Shared Socioeconomic Pathways (SSP) projections thereafter), and furthermore, many of the ‘observed' drivers have been updated substantially since the CMIP6 inputs were defined. Most notably, the sea surface temperature (SST) estimates have been revised and now show up to 50% greater trends since 1979, particularly in the southern hemisphere. Additionally, estimates of short-lived aerosol and gas-phase emissions have been substantially updated. These revisions will likely have material impacts on the model-simulated EEI. We therefore propose a new, relatively low-cost, model intercomparison, CERESMIP, that would target the CERES period (2000-present), with updated forcings to at least the end of 2021. The focus will be on atmosphere-only simulations, using updated SST, forcings and emissions from 1990 to 2021. The key metrics of interest will be the EEI and atmospheric feedbacks, and so the analysis will benefit from output from satellite cloud observation simulators. The Tier 1 request would consist only of an ensemble of AMIP-style simulations, while the Tier 2 request would encompass uncertainties in the applied forcing, atmospheric composition, single and all-but-one forcing responses. We present some preliminary results and invite participation from a wide group of models.
Biogenic secondary organic aerosols (SOAs) contribute to a large fraction of fine aerosols globally, impacting air quality and climate. The formation of biogenic SOA depends on not only emissions of biogenic volatile organic compounds (BVOCs) but also anthropogenic pollutants including primary organic aerosol, sulfur dioxide (SO2), and nitrogen oxides (NOx). However, the anthropogenic impact on biogenic SOA production (AIBS) remains unclear. Here we use the decadal trend and variability in observed organic aerosol (OA) in the southeast US, combined with a global chemistry–climate model, to better constrain AIBS. We show that the reduction in SO2 emissions can only explain 40 % of the decreasing decadal trend of OA in this region, constrained by the low summertime month-to-month variability in surface OA. We hypothesize that the rest of the OA decreasing trend is largely due to a reduction in NOx emissions. By implementing a scheme for monoterpene SOA with enhanced sensitivity to NOx, our model can reproduce the decadal trend and variability in OA in this region. Extending to a centennial scale, our model shows that global SOA production increases by 36 % despite BVOC reductions from the preindustrial period to the present day, largely amplified by AIBS. Our work suggests a strong coupling between anthropogenic and biogenic emissions in biogenic SOA production that is missing from current climate models.
Bowman, Henry, Steven T Turnock, Susanne E Bauer, Kostas Tsigaridis, Makoto Deushi, Naga Oshima, Fiona M O'Connor, and Larry W Horowitz, et al., March 2022: Changes in anthropogenic precursor emissions drive shifts in the ozone seasonal cycle throughout the northern midlatitude troposphere. Atmospheric Chemistry and Physics, 22(5), DOI:10.5194/acp-22-3507-20223507-3524. Abstract
Simulations by six Coupled Model Intercomparison Project Phase 6 (CMIP6) Earth system models indicate that the seasonal cycle of baseline tropospheric ozone at northern midlatitudes has been shifting since the mid-20th century. Beginning in ∼ 1940, the magnitude of the seasonal cycle increased by ∼10 ppb (measured from seasonal minimum to maximum), and the seasonal maximum shifted to later in the year by about 3 weeks. This shift maximized in the mid-1980s, followed by a reversal – the seasonal cycle decreased in amplitude and the maximum shifted back to earlier in the year. Similar changes are seen in measurements collected from the 1970s to the present. The timing of the seasonal cycle changes is generally concurrent with the rise and fall of anthropogenic emissions that followed industrialization and the subsequent implementation of air quality emission controls. A quantitative comparison of the temporal changes in the ozone seasonal cycle at sites in both Europe and North America with the temporal changes in ozone precursor emissions across the northern midlatitudes found a high degree of similarity between these two temporal patterns. We hypothesize that changing precursor emissions are responsible for the shift in the ozone seasonal cycle; this is supported by the absence of such seasonal shifts in southern midlatitudes where anthropogenic emissions are much smaller. We also suggest a mechanism by which changing emissions drive the changing seasonal cycle: increasing emissions of NOx allow summertime photochemical production of ozone to become more important than ozone transported from the stratosphere, and increasing volatile organic compounds (VOCs) lead to progressively greater photochemical ozone production in the summer months, thereby increasing the amplitude of the seasonal ozone cycle. Decreasing emissions of both precursor classes then reverse these changes. The quantitative parameter values that characterize the seasonal shifts provide useful benchmarks for evaluating model simulations, both against observations and between models.
Morgenstern, Olaf, Douglas Kinnison, Michael J Mills, Martine Michou, Larry W Horowitz, and Pu Lin, et al., October 2022: Comparison of Arctic and Antarctic stratospheric climates in chemistry versus no-chemistry climate models. JGR Atmospheres, 127(20), DOI:10.1029/2022JD037123. Abstract
Using nine chemistry-climate and eight associated no-chemistry models, we investigate the persistence and timing of cold episodes occurring in the Arctic and Antarctic stratosphere during the period 1980–2014. We find systematic differences in behavior between members of these model pairs. In a first group of chemistry models whose dynamical configurations mirror their no-chemistry counterparts, we find an increased persistence of such cold polar vortices, such that these cold episodes often start earlier and last longer, relative to the times of occurrence of the lowest temperatures. Also the date of occurrence of the lowest temperatures, both in the Arctic and the Antarctic, is often delayed by 1–3 weeks in chemistry models, versus their no-chemistry counterparts. This behavior exacerbates a widespread problem occurring in most or all models, a delayed occurrence, in the median, of the most anomalously cold day during such cold winters. In a second group of model pairs there are differences beyond just ozone chemistry. In particular, here the chemistry models feature more levels in the stratosphere, a raised model top, and differences in non-orographic gravity wave drag versus their no-chemistry counterparts. Such additional dynamical differences can completely mask the above influence of ozone chemistry. The results point toward a need to retune chemistry-climate models versus their no-chemistry counterparts.
We analyze the relationship between fine particulate matter (PM2.5) and meteorology in winter in the Indo-Gangetic Plain (IGP). We find that the concentration of PM2.5 exhibits similar increase with decreasing surface wind speed in 15 out of 18 cities considered. Using this observed relationship, we estimate that the reduction of surface wind speed with increasing CO2 simulated by models participating in the Coupled Model Intercomparison Project Phase 6 will result in higher average wintertime PM2.5 concentrations (1% per degree K of global warming) and more frequent high-pollution events. This observation-based estimate is qualitatively consistent with the simulated response of black carbon to global warming inferred from the AerChemMIP ssp370SST and ssp370pdSST experiments. We hypothesize that a reduction in the frequency and intensity of western disturbances with increasing CO2 may contribute to the reduction in the surface wind in the IGP.
Xie, Yuanyu, Meiyun Lin, Bertrand Decharme, Christine Delire, Larry W Horowitz, David Lawrence, F Li, and Roland Séférian, March 2022: Tripling of western US particulate pollution from wildfires in a warming climate. Proceedings of the National Academy of Sciences, 119(14), DOI:10.1073/pnas.2111372119. Abstract
The air quality impact of increased wildfires in a warming climate has often been overlooked in current model projections, owing to the lack of interactive fire emissions of gases and particles responding to climate change in Earth System Model (ESM) projection simulations. Here, we combine multiensemble projections of wildfires in three ESMs from the Sixth Coupled Model Intercomparison Project (CMIP6) with an empirical statistical model to predict fine particulate (PM2.5) pollution in the late 21st century under a suite of Shared Socioeconomic Pathways (SSPs). Total CO2 emissions from fires over western North America during August through September are projected to increase from present-day values by 60 to 110% (model spread) under a strong-mitigation scenario (SSP1-2.6), 100 to 150% under a moderate-mitigation scenario (SSP2-4.5), and 130 to 260% under a low-mitigation scenario (SSP5-8.5) in 2080–2100. We find that enhanced wildfire activity under SSP2-4.5 and SSP5-8.5 could cause a twofold to threefold increase in PM2.5 pollution over the US Pacific Northwest during August through September. Even with strong mitigation under SSP1-2.6, PM2.5 in the western US would increase ∼50% by midcentury. By 2080–2100, under SSP5-8.5, the 95th percentile of late-summer daily PM2.5 may frequently reach unhealthy levels of 55 to 150 μg/m3. In contrast, chemistry-climate models using prescribed fire emissions of particles not responding to climate change simulate only a 7% increase in PM2.5. The consequential pollution events caused by large fires during 2017–2020 might become a new norm by the late 21st century, with a return period of every 3 to 5 y under SSP5-8.5 and SSP2-4.5.
Yang, Hui, Xinyuan Huang, Daniel M Westervelt, Larry W Horowitz, and Wei Peng, October 2022: Socio-demographic factors shaping the future global health burden from air pollution. Nature Sustainability, DOI:10.1038/s41893-022-00976-8. Abstract
Exposure to ambient particulate matter (PM2.5) currently contributes to millions of global premature deaths every year. Here, we assess the pollution and health futures in five 2015–2100 scenarios using an integrated modelling framework. On the basis of a global Earth System Model (GFDL-ESM4.1), we find lower ambient PM2.5 concentrations, both globally and regionally, in future scenarios that are less fossil fuel-dependent and with more stringent pollution controls. Across the five scenarios, the global cumulative PM2.5-related deaths vary by a factor of two. However, the projected deaths are not necessarily lower in scenarios with less warming or cleaner air. This is because while reducing PM2.5 pollution lowers the exposure level, increasing the size of vulnerable populations can significantly increase PM2.5-related deaths. For most countries, we find that changes in socio-demographic factors (for example, ageing and declining baseline mortality rates) play a more important role than the exposure level in shaping future health burden.
Zanis, Prodromos, Dimitris Akritidis, Steven T Turnock, Vaishali Naik, Sophie Szopa, Aristeidis K Georgoulias, Susanne E Bauer, Makoto Deushi, and Larry W Horowitz, et al., January 2022: Climate change penalty and benefit on surface ozone: A global perspective based on CMIP6 earth system models. Environmental Research Letters, 17(2), DOI:10.1088/1748-9326/ac4a34. Abstract
This work presents an analysis of the effect of climate change on surface ozone discussing the related penalties and benefits around the globe from the global modelling perspective based on simulations with five CMIP6 (Coupled Model Intercomparison Project Phase 6) Earth System Models. As part of AerChemMIP (Aerosol Chemistry Model Intercomparison Project) all models conducted simulation experiments considering future climate (ssp370SST) and present-day climate (ssp370pdSST) under the same future emissions trajectory (SSP3-7.0). A multi-model global average climate change benefit on surface ozone of −0.96 ± 0.07 ppbv °C−1 is calculated which is mainly linked to the dominating role of enhanced ozone destruction with higher water vapour abundances under a warmer climate. Over regions remote from pollution sources, there is a robust decline in mean surface ozone concentration on an annual basis as well as for boreal winter and summer varying spatially from −0.2 to −2 ppbv °C−1, with strongest decline over tropical oceanic regions. The implication is that over regions remote from pollution sources (except over the Arctic) there is a consistent climate change benefit for baseline ozone due to global warming. However, ozone increases over regions close to anthropogenic pollution sources or close to enhanced natural biogenic volatile organic compounds emission sources with a rate ranging regionally from 0.2 to 2 ppbv C−1, implying a regional surface ozone penalty due to global warming. Overall, the future climate change enhances the efficiency of precursor emissions to generate surface ozone in polluted regions and thus the magnitude of this effect depends on the regional emission changes considered in this study within the SSP3_7.0 scenario. The comparison of the climate change impact effect on surface ozone versus the combined effect of climate and emission changes indicates the dominant role of precursor emission changes in projecting surface ozone concentrations under future climate change scenarios.
Zeng, Guang, Olaf Morgenstern, Jonny Williams, Fiona M O'Connor, Paul T Griffiths, James Keeble, Makoto Deushi, Larry W Horowitz, Vaishali Naik, Louisa K Emmons, N Luke Abraham, Alexander T Archibald, Susanne E Bauer, Birgit Hassler, Martine Michou, Michael J Mills, Lee T Murray, Naga Oshima, Lori T Sentman, Simone Tilmes, Kostas Tsigaridis, and Paul J Young, August 2022: Attribution of stratospheric and tropospheric ozone changes between 1850 and 2014 in CMIP6 models. JGR Atmospheres, 127(16), DOI:10.1029/2022JD036452. Abstract
We quantify the impacts of halogenated ozone-depleting substances (ODSs), greenhouse gases (GHGs), and short-lived ozone precursors on ozone changes between 1850 and 2014 using single-forcing perturbation simulations from several Earth system models with interactive chemistry participating in the Coupled Model Intercomparison Project Aerosol and Chemistry Model Intercomparison Project. We present the responses of ozone to individual forcings and an attribution of changes in ozone columns and vertically resolved stratospheric and tropospheric ozone to these forcings. We find that whilst substantial ODS-induced ozone loss dominates the stratospheric ozone changes since the 1970s, in agreement with previous studies, increases in tropospheric ozone due to increases in short-lived ozone precursors and methane since the 1950s make increasingly important contributions to total column ozone (TCO) changes. Increases in methane also lead to substantial extra-tropical stratospheric ozone increases. Impacts of nitrous oxide and carbon dioxide on stratospheric ozone are significant but their impacts on TCO are small overall due to several opposing factors and are also associated with large dynamical variability. The multi-model mean (MMM) results show a clear change in the stratospheric ozone trends after 2000 due to now declining ODSs, but the trends are generally not significantly positive, except in the extra-tropical upper stratosphere, due to relatively small changes in forcing over this period combined with large model uncertainty. Although the MMM ozone compares well with the observations, the inter-model differences are large primarily due to the large differences in the models' representation of ODS-induced ozone depletion.
Allen, Robert J., Larry W Horowitz, Vaishali Naik, Naga Oshima, Fiona M O'Connor, Steven T Turnock, Sungbo Shim, Philippe Le Sager, Twan van Noije, Kostas Tsigaridis, Susanne E Bauer, Lori T Sentman, and Jasmin G John, et al., February 2021: Significant climate benefits from near-term climate forcer mitigation in spite of aerosol reductions. Environmental Research Letters, 16(3), DOI:10.1088/1748-9326/abe06b. Abstract
Near-term climate forcers (NTCFs), including aerosols and chemically reactive gases such as tropospheric ozone and methane, offer a potential way to mitigate climate change and improve air quality—so called 'win-win' mitigation policies. Prior studies support improved air quality under NTCF mitigation, but with conflicting climate impacts that range from a significant reduction in the rate of global warming to only a modest impact. Here, we use state-of-the-art chemistry-climate model simulations conducted as part of the Aerosol and Chemistry Model Intercomparison Project (AerChemMIP) to quantify the 21st-century impact of NTCF reductions, using a realistic future emission scenario with a consistent air quality policy. Non-methane NTCF (NMNTCF; aerosols and ozone precursors) mitigation improves air quality, but leads to significant increases in global mean precipitation of 1.3% by mid-century and 1.4% by end-of-the-century, and corresponding surface warming of 0.23 and 0.21 K. NTCF (all-NTCF; including methane) mitigation further improves air quality, with larger reductions of up to 45% for ozone pollution, while offsetting half of the wetting by mid-century (0.7% increase) and all the wetting by end-of-the-century (non-significant 0.1% increase) and leading to surface cooling of −0.15 K by mid-century and −0.50 K by end-of-the-century. This suggests that methane mitigation offsets warming induced from reductions in NMNTCFs, while also leading to net improvements in air quality.
Derwent, Richard G., David D Parrish, Alexander T Archibald, Makoto Deushi, Susanne E Bauer, Kostas Tsigaridis, Drew Shindell, Larry W Horowitz, M Anwar H Khan, and Dudley E Shallcross, March 2021: Intercomparison of the representations of the atmospheric chemistry of pre-industrial methane and ozone in earth system and other global chemistry-transport models. Atmospheric Environment, 248, 118248, DOI:10.1016/j.atmosenv.2021.118248. Abstract
An intercomparison has been set up to study the representation of the atmospheric chemistry of the pre-industrial troposphere in earth system and other global tropospheric chemistry-transport models. The intercomparison employed a constrained box model and utilised tropospheric trace gas composition data for the pre-industrial times at ninety mid-latitude surface locations. Incremental additions of four organic compounds: methane, ethane, acetone and propane, were used to perturb the constrained box model and generate responses in hydroxyl radicals and tropospheric ozone at each location and with each chemical mechanism. Although the responses agreed well across the chemical mechanisms from the selected earth system and other global tropospheric chemistry-transport models, there were differences in the detailed responses between the chemical mechanisms that could be tracked down by sensitivity analysis to differences in the representation of C1–C3 chemistry. Inter-mechanism ranges in NOx compensation points were about 0.17 ± 0.12 when expressed relative to the inter-mechanism average. Monte Carlo uncertainty analysis carried out with a single chemical mechanism put the intra-mechanism range a factor of three higher at 0.50 ± 0.12. Similar differences between inter-mechanism and intra-mechanism ranges were found for hydroxyl radical depletion but were up to a factor of six wider for ozone formation from incremental additions of organic compounds. The cause of the discrepancies between the inter- and intra-mechanism ranges was found to be the large uncertainties that are present in the laboratory determinations of the rate coefficients and product channel branching ratios of some key chemical reactions involving organic peroxy radicals and hydroperoxides. Whilst these large uncertainties are present in the laboratory determinations, there will be irreducible uncertainties in the predictions from the earth system and other chemistry-transport models of methane and tropospheric ozone trends since pre-industrial times and hence their contributions to the radiative forcing of climate change. Further definitive laboratory studies of the reaction rates and product yields of the reactions of the simple organic peroxy radicals and hydroperoxides are required to resolve and reduce current uncertainties in earth system and chemistry-transport model predictions.
Griffiths, Paul T., Lee T Murray, Guang Zeng, Youngsub Matthew Shin, N Luke Abraham, Alexander T Archibald, Makoto Deushi, Louisa K Emmons, Ian E Galbally, Birgit Hassler, Larry W Horowitz, James Keeble, Jane Liu, O Moeini, and Vaishali Naik, et al., March 2021: Tropospheric ozone in CMIP6 Simulations. Atmospheric Chemistry and Physics, 21(5), DOI:10.5194/acp-21-4187-20214187-4218. Abstract
The evolution of tropospheric ozone from 1850 to 2100 has been studied using data from Phase 6 of the Coupled Model Intercomparison Project (CMIP6). We evaluate long-term changes using coupled atmosphere–ocean chemistry–climate models, focusing on the CMIP Historical and ScenarioMIP ssp370 experiments, for which detailed tropospheric-ozone diagnostics were archived. The model ensemble has been evaluated against a suite of surface, sonde and satellite observations of the past several decades and found to reproduce well the salient spatial, seasonal and decadal variability and trends. The multi-model mean tropospheric-ozone burden increases from 247 ± 36 Tg in 1850 to a mean value of 356 ± 31 Tg for the period 2005–2014, an increase of 44 %. Modelled present-day values agree well with previous determinations (ACCENT: 336 ± 27 Tg; Atmospheric Chemistry and Climate Model Intercomparison Project, ACCMIP: 337 ± 23 Tg; Tropospheric Ozone Assessment Report, TOAR: 340 ± 34 Tg). In the ssp370 experiments, the ozone burden increases to 416 ± 35 Tg by 2100. The ozone budget has been examined over the same period using lumped ozone production (PO3) and loss (LO3) diagnostics. Both ozone production and chemical loss terms increase steadily over the period 1850 to 2100, with net chemical production (PO3-LO3) reaching a maximum around the year 2000. The residual term, which contains contributions from stratosphere–troposphere transport reaches a minimum around the same time before recovering in the 21st century, while dry deposition increases steadily over the period 1850–2100. Differences between the model residual terms are explained in terms of variation in tropopause height and stratospheric ozone burden.
The hydroxyl radical (OH) is extremely reactive in the atmosphere and able to destroy many other chemicals, such as methane, a strong greenhouse gas that contributes significantly to global warming. Therefore, OH is very important for methane concentrations and lifetime. Changes in the meteorological features (e.g., temperature, wind patterns, and relative humidity) would affect OH in the atmosphere. In this study, we use a three-dimensional numerical model to understand the meteorological impacts on OH and the resulting impacts on methane budget and lifetime over 1980–2017. With different meteorological datasets, we find there is a 2% difference in global mean tropospheric OH concentrations, with much larger differences over tropics. We calculate methane sources and loss due to OH and find an 11.2 Tg yr−1 difference in the global mean methane sources with 8 Tg yr−1 difference in the tropics, and 0.24 years difference in methane lifetime between the two meteorological datasets.
Keeble, James, Birgit Hassler, A Banerjee, Ramiro Checa-Garcia, G Chiodo, Sean Davis, Veronika Eyring, Paul T Griffiths, Olaf Morgenstern, P Nowack, Guang Zeng, Jiankai Zhang, G Bodeker, Susannah Burrows, Philip Cameron-Smith, D Cugnet, Christopher Danek, Makoto Deushi, and Larry W Horowitz, et al., March 2021: Evaluating stratospheric ozone and water vapour changes in CMIP6 models from 1850 to 2100. Atmospheric Chemistry and Physics, 21(6), DOI:10.5194/acp-21-5015-20215015-5061. Abstract
Stratospheric ozone and water vapour are key components of the Earth system, and past and future changes to both have important impacts on global and regional climate. Here, we evaluate long-term changes in these species from the pre-industrial period (1850) to the end of the 21st century in Coupled Model Intercomparison Project phase 6 (CMIP6) models under a range of future emissions scenarios. There is good agreement between the CMIP multi-model mean and observations for total column ozone (TCO), although there is substantial variation between the individual CMIP6 models. For the CMIP6 multi-model mean, global mean TCO has increased from ∼ 300 DU in 1850 to ∼ 305 DU in 1960, before rapidly declining in the 1970s and 1980s following the use and emission of halogenated ozone-depleting substances (ODSs). TCO is projected to return to 1960s values by the middle of the 21st century under the SSP2-4.5, SSP3-7.0, SSP4-3.4, SSP4-6.0, and SSP5-8.5 scenarios, and under the SSP3-7.0 and SSP5-8.5 scenarios TCO values are projected to be ∼ 10 DU higher than the 1960s values by 2100. However, under the SSP1-1.9 and SSP1-1.6 scenarios, TCO is not projected to return to the 1960s values despite reductions in halogenated ODSs due to decreases in tropospheric ozone mixing ratios. This global pattern is similar to regional patterns, except in the tropics where TCO under most scenarios is not projected to return to 1960s values, either through reductions in tropospheric ozone under SSP1-1.9 and SSP1-2.6, or through reductions in lower stratospheric ozone resulting from an acceleration of the Brewer–Dobson circulation under other Shared Socioeconomic Pathways (SSPs). In contrast to TCO, there is poorer agreement between the CMIP6 multi-model mean and observed lower stratospheric water vapour mixing ratios, with the CMIP6 multi-model mean underestimating observed water vapour mixing ratios by ∼ 0.5 ppmv at 70 hPa. CMIP6 multi-model mean stratospheric water vapour mixing ratios in the tropical lower stratosphere have increased by ∼ 0.5 ppmv from the pre-industrial to the present-day period and are projected to increase further by the end of the 21st century. The largest increases (∼ 2 ppmv) are simulated under the future scenarios with the highest assumed forcing pathway (e.g. SSP5-8.5). Tropical lower stratospheric water vapour, and to a lesser extent TCO, shows large variations following explosive volcanic eruptions.
The Coronavirus Disease 2019 (COVID‐19) pandemic led to a widespread reduction in aerosol emissions. Using satellite observations and climate model simulations, we study the underlying mechanisms of the large decreases in solar clear‐sky reflection (3.8 W m−2 or 7%) and aerosol optical depth (0.16 W m−2 or 32%) observed over the East Asian Marginal Seas in March 2020. By separating the impacts from meteorology and emissions in the model simulations, we find that about one‐third of the clear‐sky anomalies can be attributed to pandemic‐related emission reductions, and the rest to weather variability and long‐term emission trends. The model is skillful at reproducing the observed interannual variations in solar all‐sky reflection, but no COVID‐19 signal is discerned. The current observational and modeling capabilities will be critical for monitoring, understanding, and predicting the radiative forcing and climate impacts of the ongoing crisis.
The hydroxyl radical (OH) sets the oxidative capacity of the atmosphere and, thus, profoundly affects the removal rate of pollutants and reactive greenhouse gases. While observationally derived constraints exist for global annual mean present-day OH abundances and interannual variability, OH estimates for past and future periods rely primarily on global atmospheric chemistry models. These models disagree ± 30% in mean OH and in its changes from the preindustrial to late 21st century, even when forced with identical anthropogenic emissions. A simple steady-state relationship that accounts for ozone photolysis frequencies, water vapor, and the ratio of reactive nitrogen to carbon emissions explains temporal variability within most models, but not intermodel differences. Here, we show that departure from the expected relationship reflects the treatment of reactive oxidized nitrogen species (NOy) and the fraction of emitted carbon that reacts within each chemical mechanism, which remain poorly known due to a lack of observational data. Our findings imply a need for additional observational constraints on NOy partitioning and lifetime, especially in the remote free troposphere, as well as the fate of carbon-containing reaction intermediates to test models, thereby reducing uncertainties in projections of OH and, hence, lifetimes of pollutants and greenhouse gases.
Hydrogen (H2) has been proposed as an alternative energy carrier to reduce the carbon footprint and associated radiative forcing of the current energy system. Here, we describe the representation of H2 in the GFDL-AM4.1 model including updated emission inventories and improved representation of H2 soil removal, the dominant sink of H2. The model best captures the overall distribution of surface H2, including regional contrasts between climate zones, when vd(H2) is modulated by soil moisture, temperature, and soil carbon content. We estimate that the soil removal of H2 increases with warming (2–4% per K), with large uncertainties stemming from different regional response of soil moisture and soil carbon. We estimate that H2 causes an indirect radiative forcing of 0.84 mW m−2/(Tg(H2)yr−1) or 0.13 mW m−2 ppbv−1, primarily due to increasing CH4 lifetime and stratospheric water vapor production.
Thornhill, Gillian D., William J Collins, Ryan J Kramer, Dirk Olivié, Ragnhild B Skeie, Fiona M O'Connor, N Luke Abraham, Ramiro Checa-Garcia, Susanne E Bauer, Makoto Deushi, Louisa K Emmons, Piers M Forster, Larry W Horowitz, Ben Johnson, James Keeble, Jean-Francois Lamarque, Martine Michou, Michael J Mills, Jane P Mulcahy, Gunnar Myhre, Pierre Nabat, and Vaishali Naik, et al., January 2021: Effective radiative forcing from emissions of reactive gases and aerosols – a multi-model comparison. Atmospheric Chemistry and Physics, 21(2), DOI:10.5194/acp-21-853-2021853-874. Abstract
This paper quantifies the pre-industrial (1850) to present-day (2014) effective radiative forcing (ERF) of anthropogenic emissions of NOX, volatile organic compounds (VOCs; including CO), SO2, NH3, black carbon, organic carbon, and concentrations of methane, N2O and ozone-depleting halocarbons, using CMIP6 models. Concentration and emission changes of reactive species can cause multiple changes in the composition of radiatively active species: tropospheric ozone, stratospheric ozone, stratospheric water vapour, secondary inorganic and organic aerosol, and methane. Where possible we break down the ERFs from each emitted species into the contributions from the composition changes. The ERFs are calculated for each of the models that participated in the AerChemMIP experiments as part of the CMIP6 project, where the relevant model output was available.
The 1850 to 2014 multi-model mean ERFs (± standard deviations) are −1.03 ± 0.37 W m−2 for SO2 emissions, −0.25 ± 0.09 W m−2 for organic carbon (OC), 0.15 ± 0.17 W m−2 for black carbon (BC) and −0.07 ± 0.01 W m−2 for NH3. For the combined aerosols (in the piClim-aer experiment) it is −1.01 ± 0.25 W m−2. The multi-model means for the reactive well-mixed greenhouse gases (including any effects on ozone and aerosol chemistry) are 0.67 ± 0.17 W m−2 for methane (CH4), 0.26 ± 0.07 W m−2 for nitrous oxide (N2O) and 0.12 ± 0.2 W m−2 for ozone-depleting halocarbons (HC). Emissions of the ozone precursors nitrogen oxides (NOx), volatile organic compounds and both together (O3) lead to ERFs of 0.14 ± 0.13, 0.09 ± 0.14 and 0.20 ± 0.07 W m−2 respectively. The differences in ERFs calculated for the different models reflect differences in the complexity of their aerosol and chemistry schemes, especially in the case of methane where tropospheric chemistry captures increased forcing from ozone production.
Thornhill, Gillian D., William J Collins, Dirk Olivié, Ragnhild B Skeie, Alexander T Archibald, Susanne E Bauer, Ramiro Checa-Garcia, Stephanie Fiedler, Gerd Folberth, Ada Gjermundsen, Larry W Horowitz, Jean-Francois Lamarque, Martine Michou, Jane P Mulcahy, Pierre Nabat, Vaishali Naik, Fiona M O'Connor, and Fabien Paulot, et al., January 2021: Climate-driven chemistry and aerosol feedbacks in CMIP6 Earth system models. Atmospheric Chemistry and Physics, 21(2), DOI:10.5194/acp-21-1105-20211105-1126. Abstract
Feedbacks play a fundamental role in determining the magnitude of the response of the climate system to external forcing, such as from anthropogenic emissions. The latest generation of Earth system models includes aerosol and chemistry components that interact with each other and with the biosphere. These interactions introduce a complex web of feedbacks that is important to understand and quantify.
This paper addresses multiple pathways for aerosol and chemical feedbacks in Earth system models. These focus on changes in natural emissions (dust, sea salt, dimethyl sulfide, biogenic volatile organic compounds (BVOCs) and lightning) and changes in reaction rates for methane and ozone chemistry. The feedback terms are then given by the sensitivity of a pathway to climate change multiplied by the radiative effect of the change.
We find that the overall climate feedback through chemistry and aerosols is negative in the sixth Coupled Model Intercomparison Project (CMIP6) Earth system models due to increased negative forcing from aerosols in a climate with warmer surface temperatures following a quadrupling of CO2 concentrations. This is principally due to increased emissions of sea salt and BVOCs which are sensitive to climate change and cause strong negative radiative forcings. Increased chemical loss of ozone and methane also contributes to a negative feedback. However, overall methane lifetime is expected to increase in a warmer climate due to increased BVOCs. Increased emissions of methane from wetlands would also offset some of the negative feedbacks. The CMIP6 experimental design did not allow the methane lifetime or methane emission changes to affect climate, so we found a robust negative contribution from interactive aerosols and chemistry to climate sensitivity in CMIP6 Earth system models.
Allen, Robert J., Steven T Turnock, Pierre Nabat, David Neubauer, Ülrike Lohmann, Dirk Olivié, Naga Oshima, Martine Michou, Tongwen Wu, J Zhang, Toshihiko Takemura, M Schulz, Kostas Tsigaridis, Susanne E Bauer, Louisa K Emmons, Larry W Horowitz, Vaishali Naik, Twan van Noije, T Bergman, Jean-Francois Lamarque, Prodromos Zanis, I Tegen, Daniel M Westervelt, Philippe Le Sager, Peter Good, Sungbo Shim, Fiona M O'Connor, Dimitris Akritidis, Aristeidis K Georgoulias, Makoto Deushi, Lori T Sentman, Jasmin G John, S Fujimori, and William J Collins, August 2020: Climate and air quality impacts due to mitigation of non-methane near-term climate forcers. Atmospheric Chemistry and Physics, 20(16), DOI:10.5194/acp-20-9641-2020. Abstract
It is important to understand how future environmental policies will impact both climate change and air pollution. Although targeting near-term climate forcers (NTCFs), defined here as aerosols, tropospheric ozone, and precursor gases, should improve air quality, NTCF reductions will also impact climate. Prior assessments of the impact of NTCF mitigation on air quality and climate have been limited. This is related to the idealized nature of some prior studies, simplified treatment of aerosols and chemically reactive gases, as well as a lack of a sufficiently large number of models to quantify model diversity and robust responses. Here, we quantify the 2015–2055 climate and air quality effects of non-methane NTCFs using nine state-of-the-art chemistry–climate model simulations conducted for the Aerosol and Chemistry Model Intercomparison Project (AerChemMIP). Simulations are driven by two future scenarios featuring similar increases in greenhouse gases (GHGs) but with “weak” (SSP3-7.0) versus “strong” (SSP3-7.0-lowNTCF) levels of air quality control measures. As SSP3-7.0 lacks climate policy and has the highest levels of NTCFs, our results (e.g., surface warming) represent an upper bound. Unsurprisingly, we find significant improvements in air quality under NTCF mitigation (strong versus weak air quality controls). Surface fine particulate matter (PM2.5) and ozone (O3) decrease by −2.2±0.32 µg m−3 and −4.6±0.88 ppb, respectively (changes quoted here are for the entire 2015–2055 time period; uncertainty represents the 95 % confidence interval), over global land surfaces, with larger reductions in some regions including south and southeast Asia. Non-methane NTCF mitigation, however, leads to additional climate change due to the removal of aerosol which causes a net warming effect, including global mean surface temperature and precipitation increases of 0.25±0.12 K and 0.03±0.012 mm d−1, respectively. Similarly, increases in extreme weather indices, including the hottest and wettest days, also occur. Regionally, the largest warming and wetting occurs over Asia, including central and north Asia (0.66±0.20 K and 0.03±0.02 mm d−1), south Asia (0.47±0.16 K and 0.17±0.09 mm d−1), and east Asia (0.46±0.20 K and 0.15±0.06 mm d−1). Relatively large warming and wetting of the Arctic also occur at 0.59±0.36 K and 0.04±0.02 mm d−1, respectively. Similar surface warming occurs in model simulations with aerosol-only mitigation, implying weak cooling due to ozone reductions. Our findings suggest that future policies that aggressively target non-methane NTCF reductions will improve air quality but will lead to additional surface warming, particularly in Asia and the Arctic. Policies that address other NTCFs including methane, as well as carbon dioxide emissions, must also be adopted to meet climate mitigation goals.
Baublitz, C B., Arlene M Fiore, Olivia E Clifton, Jingqiu Mao, J Li, G Correa, Daniel M Westervelt, Larry W Horowitz, Fabien Paulot, and A Park Williams, April 2020: Sensitivity of tropospheric ozone over the Southeast USA to dry deposition. Geophysical Research Letters, 47(7), DOI:10.1029/2020GL087158. Abstract
Dry deposition (DD) is a major loss process for tropospheric ozone and some reactive nitrogen and carbon precursors. We investigate the response of summertime ozone and its production chemistry over the Southeast United States (USA) to variability in this sink. Turning off DD of oxidized nitrogen, ozone, or all species over the USA in the GFDL AM3 model increases regional mean surface ozone by 5, 18 or 25 ppb, respectively. Additional sensitivity simulations demonstrate that, assuming linearity, surface ozone has a similar sensitivity to ozone DD as to NOx emissions. Trends in ozone production efficiency derived from observed relationships between ozone and precursor oxidation products may not solely reflect precursor emission changes if ozone DD varies (e.g. with meteorology). We conclude that DD variability merits consideration when interpreting observed ozone trends. Quantifying the impact of changes in sinks versus sources will require long‐term DD measurements across the region of interest.
Identifying the contributions of chemistry and transport to observed ozone pollution using regional‐to‐global models relies on accurate representation of ozone dry deposition. We use a recently developed configuration of the NOAA GFDL chemistry‐climate model ‐‐ in which the atmosphere and land are coupled through dry deposition ‐‐ to investigate the influence of ozone dry deposition on ozone pollution over northern mid‐latitudes. In our model, deposition pathways are tied to dynamic terrestrial processes, such as photosynthesis and water cycling through the canopy and soil. Small increases in winter deposition due to more process‐based representation of snow and deposition to surfaces reduce hemispheric‐scale ozone through the lower troposphere by 5‐12 ppb, improving agreement with observations relative to a simulation with the standard configuration for ozone dry deposition. Declining snow cover by the end of the 21st century tempers the previously identified influence of rising methane on winter ozone. Dynamic dry deposition changes summer surface ozone by ‐4 to +7 ppb. While previous studies emphasize the importance of uptake by plant stomata, new diagnostic tracking of depositional pathways reveals a widespread impact of nonstomatal deposition on ozone pollution. Daily variability in both stomatal and nonstomatal deposition contribute to daily variability in ozone pollution. 21st‐century changes in summer deposition result from a balance among changes in individual pathways, reflecting differing responses to both high carbon dioxide (through plant physiology versus biomass accumulation) and water availability. Our findings highlight a need for constraints on the processes driving ozone dry deposition to test representation in regional‐to‐global models.
Clifton, Olivia E., Danica L Lombardozzi, Arlene M Fiore, Fabien Paulot, and Larry W Horowitz, November 2020: Stomatal conductance influences interannual variability and long-term changes in regional cumulative plant uptake of ozone. Environmental Research Letters, 15, DOI:10.1088/1748-9326/abc3f1. Abstract
Ambient ozone uptake by plant stomata degrades ecosystem and crop health and alters local-to-global carbon and water cycling. Metrics for ozone plant damage are often based solely on ambient ozone concentrations, overlooking the role of variations in stomatal activity. A better metric is the cumulative stomatal uptake of ozone (CUO), which indicates the amount of ozone entering the leaf over time available to cause physiological damage. Here we apply the NOAA GFDL global earth system model to assess the importance of capturing interannual variations and 21st century changes in surface ozone versus stomatal conductance for regional mean CUO using 20-year time-slice simulations at the 2010s and 2090s for a high-warming climate and emissions scenario. The GFDL model includes chemistry-climate interactions and couples atmospheric and land components through not only carbon, water, and energy exchanges, but also reactive trace gases—in particular, ozone dry deposition simulated by the land influences surface ozone concentrations. Our 20-year time slice simulations hold anthropogenic precursor emissions, well-mixed greenhouse gases, and land use distributions fixed at either 2010 or 2090 values. We find that CUO responds much more strongly to interannual and daily variability in stomatal conductance than in ozone. On the other hand, long-term changes in ozone explain 44%–90% of the annual CUO change in regions with decreases, largely driven by the impact of 21st century anthropogenic NOx emission trends on summer surface ozone. In some regions, increases in stomatal conductance from the 2010s to 2090s counteract the influence of lower ozone on CUO. We also find that summertime stomatal closure under high carbon dioxide levels can offset the impacts of higher springtime leaf area (e.g. earlier leaf out) and associated stomatal conductance on CUO. Our findings underscore the importance of considering plant physiology in assessing ozone vegetation damage, particularly in quantifying year-to-year changes.
We describe the baseline coupled model configuration and simulation characteristics of GFDL's Earth System Model Version 4.1 (ESM4.1), which builds on component and coupled model developments at GFDL over 2013–2018 for coupled carbon‐chemistry‐climate simulation contributing to the sixth phase of the Coupled Model Intercomparison Project. In contrast with GFDL's CM4.0 development effort that focuses on ocean resolution for physical climate, ESM4.1 focuses on comprehensiveness of Earth system interactions. ESM4.1 features doubled horizontal resolution of both atmosphere (2° to 1°) and ocean (1° to 0.5°) relative to GFDL's previous‐generation coupled ESM2‐carbon and CM3‐chemistry models. ESM4.1 brings together key representational advances in CM4.0 dynamics and physics along with those in aerosols and their precursor emissions, land ecosystem vegetation and canopy competition, and multiday fire; ocean ecological and biogeochemical interactions, comprehensive land‐atmosphere‐ocean cycling of CO2, dust and iron, and interactive ocean‐atmosphere nitrogen cycling are described in detail across this volume of JAMES and presented here in terms of the overall coupling and resulting fidelity. ESM4.1 provides much improved fidelity in CO2 and chemistry over ESM2 and CM3, captures most of CM4.0's baseline simulations characteristics, and notably improves on CM4.0 in (1) Southern Ocean mode and intermediate water ventilation, (2) Southern Ocean aerosols, and (3) reduced spurious ocean heat uptake. ESM4.1 has reduced transient and equilibrium climate sensitivity compared to CM4.0. Fidelity concerns include (1) moderate degradation in sea surface temperature biases, (2) degradation in aerosols in some regions, and (3) strong centennial scale climate modulation by Southern Ocean convection.
Changes in atmospheric methane abundance have implications for both chemistry and climate as methane is both a strong greenhouse gas and an important precursor for tropospheric ozone. A better understanding of the drivers of trends and variability in methane abundance over the recent past is therefore critical for building confidence in projections of future methane levels. In this work, the representation of methane in the atmospheric chemistry model AM4.1 is improved by optimizing total methane emissions (to an annual mean of 576 ± 32 Tg yr−1) to match surface observations over 1980–2017. The simulations with optimized global emissions are in general able to capture the observed global trend, variability, seasonal cycle, and latitudinal gradient of methane. Simulations with different emission adjustments suggest that increases in methane sources (mainly from energy and waste sectors) balanced by increases in methane sinks (mainly due to increases in OH levels) lead to methane stabilization (with an imbalance of 5 Tg yr−1) during 1999–2006, and that increases in methane sources combined with little change in sinks (despite small decreases in OH levels) during 2007–2012 lead to renewed methane growth (with an imbalance of 14 Tg yr−1 for 2007–2017). Compared to 1999–2006, both methane emissions and sinks are greater (by 31 Tg yr−1 and 22 Tg yr−1, respectively) during 2007–2017. Our results also indicate that the energy sector is more likely a major contributor to the methane renewed growth after 2006 than wetland, as increases in wetland emissions alone are not able to explain the renewed methane growth with constant anthropogenic emissions. In addition, a significant increase in wetland emissions would be required starting in 2006, if anthropogenic emissions declined, for wetland emissions to drive renewed growth in methane, which is a less likely scenario. Simulations with varying OH levels indicate that 1 % change in OH levels could lead to an annual mean of ~ 4 Tg yr−1 difference in the optimized emissions and 0.08 year difference in the estimated tropospheric methane lifetime. Continued increases in methane emissions along with decreases in tropospheric OH concentrations during 2008–2015 prolong methane lifetime and therefore amplify the response of methane concentrations to emission changes. Uncertainties still exist in the partitioning of emissions among individual sources and regions.
We describe the baseline model configuration and simulation characteristics of the Geophysical Fluid Dynamics Laboratory (GFDL)'s Atmosphere Model version 4.1 (AM4.1), which builds on developments at GFDL over 2013–2018 for coupled carbon‐chemistry‐climate simulation as part of the sixth phase of the Coupled Model Intercomparison Project. In contrast with GFDL's AM4.0 development effort, which focused on physical and aerosol interactions and which is used as the atmospheric component of CM4.0, AM4.1 focuses on comprehensiveness of Earth system interactions. Key features of this model include doubled horizontal resolution of the atmosphere (~200 to ~100 km) with revised dynamics and physics from GFDL's previous‐generation AM3 atmospheric chemistry‐climate model. AM4.1 features improved representation of atmospheric chemical composition, including aerosol and aerosol precursor emissions, key land‐atmosphere interactions, comprehensive land‐atmosphere‐ocean cycling of dust and iron, and interactive ocean‐atmosphere cycling of reactive nitrogen. AM4.1 provides vast improvements in fidelity over AM3, captures most of AM4.0's baseline simulations characteristics, and notably improves on AM4.0 in the representation of aerosols over the Southern Ocean, India, and China—even with its interactive chemistry representation—and in its manifestation of sudden stratospheric warmings in the coldest months. Distributions of reactive nitrogen and sulfur species, carbon monoxide, and ozone are all substantially improved over AM3. Fidelity concerns include degradation of upper atmosphere equatorial winds and of aerosols in some regions.
The semi-arid African Sahel region is highly sensitive to changes in monsoon precipitation, as much of the region’s workforce is employed in the agricultural industry (Hamro-Drotz and Programme 2011). Thus, studying the response of rainfall and aridity in this region to radiative perturbations is a matter of pressing humanitarian relevance. In addition, there is evidence to suggest that spatially asymmetric volcanic aerosols produce different hydroclimatic responses based on their hemispheric symmetry, both globally and in the Sahel. We use two different climate models, GFDL’s FLOR model (Vecchi et al. in J Clim 27(21):7994–8016, 2014) and NCAR’s CESM 1.1 model (Otto-Bliesner et al. in Bull Am Meteorol Soc 97(5):735–754, 2016), to characterize the response of rainfall in the Sahel to large volcanic eruptions based on the meridional symmetry of the volcanic eruptions. We find that in both the FLOR experiments simulating three large twentieth century eruptions and in the CESM Last Millennium Ensemble simulations of 46 historic volcanic eruptions, asymmetric Northern Hemisphere cooling causes a subsequent drying response in the Sahel, and Southern Hemisphere cooling causes a wetting, or “greening” response. Symmetric volcanic eruptions have a relatively small effect on rainfall in the Sahel. We also find that the FLOR results show a consistent response in the annual rainfall cycle in the Sahel for all three of the eruptions analyzed, with a reduction in rainfall in early summer followed by an increased rainfall in late summer. The annual cycle response of rainfall in the Sahel from the CESM experiments is different, in that the SH eruptions cause a rainfall maximum in August, NH eruptions cause a rainfall minimum in September, and symmetric eruptions show a slight increase in August and a decrease in October. Our results highlight the need for accurate meridional structures in historic volcanic forcing data used for climate models as well as the need for further study on regional effects of hemispherically asymmetric radiative forcing, especially as they might pertain to aerosol geoengineering.
Reducing surface ozone to meet the European Union’s target for human health has proven challenging despite stringent controls on ozone precursor emissions over recent decades. The most extreme ozone pollution episodes are linked to heatwaves and droughts, which are increasing in frequency and intensity over Europe, with severe impacts on natural and human systems. Here, we use observations and Earth system model simulations for the period 1960–2018 to show that ecosystem–atmosphere interactions, especially reduced ozone removal by water-stressed vegetation, exacerbate ozone air pollution over Europe. These vegetation feedbacks worsen peak ozone episodes during European mega-droughts, such as the 2003 event, offsetting much of the air quality improvements gained from regional emissions controls. As the frequency of hot and dry summers is expected to increase over the coming decades, this climate penalty could be severe and therefore needs to be considered when designing clean air policy in the European Union.
Morgenstern, Olaf, Fiona M O'Connor, Ben Johnson, Guang Zeng, Jane P Mulcahy, Jonny Williams, João Teixeira, Martine Michou, Pierre Nabat, Larry W Horowitz, Vaishali Naik, and Lori T Sentman, et al., October 2020: Reappraisal of the Climate Impacts of Ozone‐Depleting Substances. Geophysical Research Letters, 47(20), DOI:10.1029/2020GL088295. Abstract
We assess the effective radiative forcing due to ozone‐depleting substances using models participating in the Aerosols and Chemistry and Radiative Forcing Model Intercomparison Projects (AerChemMIP, RFMIP). A large intermodel spread in this globally averaged quantity necessitates an “emergent constraint” approach whereby we link the radiative forcing to ozone declines measured and simulated during 1979–2000, excluding two volcanically perturbed periods. During this period, ozone‐depleting substances were increasing, and several merged satellite‐based climatologies document the ensuing decline of total‐column ozone. Using these analyses, we find an effective radiative forcing of −0.05 to 0.13 W m−2. Our best estimate (0.04 W m−2) is on the edge of the “likely” range given by the Fifth Assessment Report of IPCC of 0.03 to 0.33 W m−2 but is in better agreement with two other literature results.
Recent laboratory and field studies point to an increase of sea salt aerosol (SSA) emissions with temperature, suggesting that SSA may lower climate sensitivity. We assess the impact of a strong (4.2 % K‐1) and weak (0.7% K‐1) temperature response of SSA emissions on the climate sensitivity of the coupled climate model CM4. We find that the stronger temperature dependence improves the simulation of marine aerosol optical depth sensitivity to temperature and lowers CM4 Transient Climate Response (‐0.12K) and Equilibrium Climate Sensitivity (‐0.5K). At CO2 doubling, the higher SSA emission sensitivity causes a negative radiative feedback (‐0.125 W m‐2 K‐1), which can only be partly explained by changes in the radiative effect of SSA (‐0.08 W m‐2 K‐1). Stronger radiative feedbacks are dominated by more negative low‐level clouds feedbacks in the Northern Hemisphere, which are partly offset by more positive feedbacks in the Southern Hemisphere associated with a weaker Atlantic Meridional Overturning Circulation.
The imprint of anthropogenic activities on the marine nitrogen (N) cycle remains challenging to represent in global models, in part because of uncertainties regarding the source of marine N to the atmosphere. While N inputs of terrestrial origin present a truly external perturbation, a significant fraction of N deposition over the ocean arises from oceanic ammonia (NH3) outgassing that is subsequently deposited in other ocean regions. Here, we describe advances in the Geophysical Fluid Dynamics Laboratory's (GFDL) Earth System Model 4 (ESM4.1) aimed at improving the representation of the exchange of N between atmosphere and ocean and its response to changes in ocean acidity and N deposition. We find that the simulated present‐day NH3 outgassing (3.1 TgN yr−1) is 7% lower than under preindustrial conditions, which reflects the compensating effects of increasing CO2 (−16%) and N enrichment of ocean waters (+9%). This change is spatially heterogeneous, with decreases in the open ocean (−13%) and increases in coastal regions (+15%) dominated by coastal N enrichment. The ocean outgassing of ammonia is shown to increase the transport of N from N‐rich to N‐poor ocean regions, where carbon export at 100 m increases by 0.5%. The implications of the strong response of NH3 ocean outgassing to CO2 for the budget of NH3 in the remote marine atmosphere and its imprint in ice cores are discussed.
Skeie, Ragnhild B., Gunnar Myhre, Øivind Hodnebrog, Philip Cameron-Smith, Makoto Deushi, Michaela I Hegglin, Larry W Horowitz, Ryan J Kramer, Martine Michou, Michael J Mills, Dirk Olivié, Fiona M O'Connor, and David J Paynter, et al., August 2020: Historical total ozone radiative forcing derived from CMIP6 simulations. npj Climate and Atmospheric Science, 3, 32, DOI:https://doi.org/10.1038/s41612-020-00131-0. Abstract
Radiative forcing (RF) time series for total ozone from 1850 up to the present day are calculated based on historical simulations of ozone from 10 climate models contributing to the Coupled Model Intercomparison Project Phase 6 (CMIP6). In addition, RF is calculated for ozone fields prepared as an input for CMIP6 models without chemistry schemes and from a chemical transport model simulation. A radiative kernel for ozone is constructed and used to derive the RF. The ozone RF in 2010 (2005–2014) relative to 1850 is 0.35 W m−2 [0.08–0.61] (5–95% uncertainty range) based on models with both tropospheric and stratospheric chemistry. One of these models has a negative present-day total ozone RF. Excluding this model, the present-day ozone RF increases to 0.39 W m−2 [0.27–0.51] (5–95% uncertainty range). The rest of the models have RF close to or stronger than the RF time series assessed by the Intergovernmental Panel on Climate Change in the fifth assessment report with the primary driver likely being the new precursor emissions used in CMIP6. The rapid adjustments beyond stratospheric temperature are estimated to be weak and thus the RF is a good measure of effective radiative forcing.
Stevenson, David S., Alcide Zhao, Vaishali Naik, Fiona M O'Connor, Simone Tilmes, Guang Zeng, Lee T Murray, William J Collins, Paul T Griffiths, Sungbo Shim, Larry W Horowitz, Lori T Sentman, and Louisa K Emmons, November 2020: Trends in global tropospheric hydroxyl radical and methane lifetime since 1850 from AerChemMIP. Atmospheric Chemistry and Physics, 20(21), DOI:10.5194/acp-20-12905-2020. Abstract
We analyse historical (1850–2014) atmospheric hydroxyl (OH) and methane lifetime data from Coupled Model Intercomparison Project Phase 6 (CMIP6)/Aerosols and Chemistry Model Intercomparison Project (AerChemMIP) simulations. Tropospheric OH changed little from 1850 up to around 1980, then increased by around 9 % up to 2014, with an associated reduction in methane lifetime. The model-derived OH trends from 1980 to 2005 are broadly consistent with trends estimated by several studies that infer OH from inversions of methyl chloroform and associated measurements; most inversion studies indicate decreases in OH since 2005. However, the model results fall within observational uncertainty ranges. The upward trend in modelled OH since 1980 was mainly driven by changes in anthropogenic near-term climate forcer emissions (increases in anthropogenic nitrogen oxides and decreases in CO). Increases in halocarbon emissions since 1950 have made a small contribution to the increase in OH, whilst increases in aerosol-related emissions have slightly reduced OH. Halocarbon emissions have dramatically reduced the stratospheric methane lifetime by about 15 %–40 %; most previous studies assumed a fixed stratospheric lifetime. Whilst the main driver of atmospheric methane increases since 1850 is emissions of methane itself, increased ozone precursor emissions have significantly modulated (in general reduced) methane trends. Halocarbon and aerosol emissions are found to have relatively small contributions to methane trends. These experiments do not isolate the effects of climate change on OH and methane evolution; however, we calculate residual terms that are due to the combined effects of climate change and non-linear interactions between drivers. These residual terms indicate that non-linear interactions are important and differ between the two methodologies we use for quantifying OH and methane drivers. All these factors need to be considered in order to fully explain OH and methane trends since 1850; these factors will also be important for future trends.
Turnock, Steven T., Robert J Allen, Martin Andrews, Susanne E Bauer, Louisa K Emmons, Peter Good, Larry W Horowitz, Jasmin G John, Martine Michou, Pierre Nabat, and Vaishali Naik, et al., November 2020: Historical and future changes in air pollutants from CMIP6 models. Atmospheric Chemistry and Physics, 20(23), DOI:10.5194/acp-20-14547-2020. Abstract
Poor air quality is currently responsible for large impacts on human health across the world. In addition, the air pollutants ozone (O3) and particulate matter less than 2.5 µm in diameter (PM2.5) are also radiatively active in the atmosphere and can influence Earth's climate. It is important to understand the effect of air quality and climate mitigation measures over the historical period and in different future scenarios to ascertain any impacts from air pollutants on both climate and human health. The Coupled Model Intercomparison Project Phase 6 (CMIP6) presents an opportunity to analyse the change in air pollutants simulated by the current generation of climate and Earth system models that include a representation of chemistry and aerosols (particulate matter). The shared socio-economic pathways (SSPs) used within CMIP6 encompass a wide range of trajectories in precursor emissions and climate change, allowing for an improved analysis of future changes to air pollutants. Firstly, we conduct an evaluation of the available CMIP6 models against surface observations of O3 and PM2.5. CMIP6 models consistently overestimate observed surface O3 concentrations across most regions and in most seasons by up to 16 ppb, with a large diversity in simulated values over Northern Hemisphere continental regions. Conversely, observed surface PM2.5 concentrations are consistently underestimated in CMIP6 models by up to 10 µg m−3, particularly for the Northern Hemisphere winter months, with the largest model diversity near natural emission source regions. The biases in CMIP6 models when compared to observations of O3 and PM2.5 are similar to those found in previous studies. Over the historical period (1850–2014) large increases in both surface O3 and PM2.5 are simulated by the CMIP6 models across all regions, particularly over the mid to late 20th century, when anthropogenic emissions increase markedly. Large regional historical changes are simulated for both pollutants across East and South Asia with an annual mean increase of up to 40 ppb for O3 and 12 µg m−3 for PM2.5. In future scenarios containing strong air quality and climate mitigation measures (ssp126), annual mean concentrations of air pollutants are substantially reduced across all regions by up to 15 ppb for O3 and 12 µg m−3 for PM2.5. However, for scenarios that encompass weak action on mitigating climate and reducing air pollutant emissions (ssp370), annual mean increases in both surface O3 (up 10 ppb) and PM2.5 (up to 8 µg m−3) are simulated across most regions, although, for regions like North America and Europe small reductions in PM2.5 are simulated due to the regional reduction in precursor emissions in this scenario. A comparison of simulated regional changes in both surface O3 and PM2.5 from individual CMIP6 models highlights important regional differences due to the simulated interaction of aerosols, chemistry, climate and natural emission sources within models. The projection of regional air pollutant concentrations from the latest climate and Earth system models used within CMIP6 shows that the particular future trajectory of climate and air quality mitigation measures could have important consequences for regional air quality, human health and near-term climate. Differences between individual models emphasise the importance of understanding how future Earth system feedbacks influence natural emission sources, e.g. response of biogenic emissions under climate change.
Westervelt, Daniel M., N R Mascioli, Arlene M Fiore, A J Conley, Jean-Francois Lamarque, Drew Shindell, G Faluvegi, M Previdi, G Correa, and Larry W Horowitz, March 2020: Local and remote mean and extreme temperature response to regional aerosol emissions reductions. Atmospheric Chemistry and Physics, 20(5), DOI:10.5194/acp-20-3009-2020. Abstract
The climatic implications of regional aerosol and precursor emissions reductions implemented to protect human health are poorly understood. We investigate the mean and extreme temperature response to regional changes in aerosol emissions using three coupled chemistry-climate models: NOAA GFDL-CM3, NCAR-CESM1, and NASA GISS-E2. Our approach contrasts a long present-day control simulation from each model (up to 400 years with perpetual year 2000 or 2005 emissions) with fourteen individual aerosol emissions perturbation simulations (160–240 years each). We perturb emissions of sulfur dioxide (SO2) and/or carbonaceous aerosol within six world regions and assess the statistical significance of mean and extreme temperature responses relative to internal variability determined by the control simulation and across the models. In all models, the global mean surface temperature response (perturbation minus control) to SO2 and/or carbonaceous aerosol is mostly positive (warming), statistically significant, and ranges from +0.17 K (Europe SO2) to −0.06 K (US BC). The warming response to SO2 reductions is strongest in the US and Europe perturbation simulations, both globally and regionally, with Arctic warming up to 1 K due to a removal of European anthropogenic SO2 emissions alone; however, even emissions from regions remote to the Arctic, such as SO2 from India, significantly warm the Arctic by up to 0.5 K. Arctic warming is the most robust response across each model and several aerosol emissions perturbations. The temperature response in the northern hemisphere mid-latitudes is most sensitive to emissions perturbations within that region. In the tropics, however, the temperature response to emissions perturbations is roughly the same in magnitude from emissions perturbations either within or outside of the tropics. We find that climate sensitivity to regional aerosol perturbations ranges from 0.5 to 1.0 K per W m−2 depending on the region and aerosol composition, and is larger than the climate sensitivity to a doubling of CO2 in two of three models. We update previous estimates of Regional Temperature Potential (RTP), a metric for estimating the regional temperature responses to a regional emissions perturbation that can facilitate assessment of climate impacts with integrated assessment models without requiring computationally demanding coupled climate model simulations. These calculations indicate a robust regional response to aerosol forcing within the northern hemisphere mid-latitudes, regardless of where the aerosol forcing is located longitudinally. We show that regional aerosol perturbations can significantly increase extreme temperatures on the regional scale. Except in the Arctic in the summer, extreme temperature responses largely mirror mean temperature responses to regional aerosol perturbations through a shift of the temperature distributions and are mostly dominated by local rather than remote aerosol forcing.
Xie, Yuanyu, Meiyun Lin, and Larry W Horowitz, July 2020: Summer PM2.5 Pollution Extremes Caused by Wildfires Over the Western United States During 2017–2018. Geophysical Research Letters, 47(16), DOI:10.1029/2020GL089429. Abstract
Using observations and model simulations (ESM4.1) during 1988–2018, we show large year‐to‐year variability in western U.S. PM2.5 pollution caused by regional and distant fires. Widespread wildfires, combined with stagnation, caused summer PM2.5 pollution in 2017 and 2018 to exceed 2 standard deviations over long‐term averages. ESM4.1 with a fire emission inventory constrained by satellite‐derived fire radiative energy and aerosol optical depth captures the observed surface PM2.5 means and extremes above the 35 μg/m3 U.S. air quality standard. However, aerosol emissions from the widely used Global Fire Emissions Database (GFED) must be increased by 5 times for ESM4.1 to match observations. On days when observed PM2.5 reached 35–175 μg/m3, wildfire emissions can explain 90% of total PM2.5, with smoke transported from Canada contributing 25–50% in northern states, according to model sensitivity simulations. Fire emission uncertainties pose challenges to accurately assessing the impacts of fire smoke on air quality, radiation, and climate.
Zhang, Li, Meiyun Lin, Andrew O Langford, and Larry W Horowitz, et al., September 2020: Characterizing sources of high surface ozone events in the southwestern U.S. with intensive field measurements and two global models. Atmospheric Chemistry and Physics, 20(17), DOI:10.5194/acp-20-10379-2020. Abstract
The detection and attribution of high background ozone (O3) events in the southwestern US is challenging but relevant to the effective implementation of the lowered National Ambient Air Quality Standard (NAAQS; 70 ppbv). Here we leverage intensive field measurements from the Fires, Asian, and Stratospheric Transport−Las Vegas Ozone Study (FAST-LVOS) in May–June 2017, alongside high-resolution simulations with two global models (GFDL-AM4 and GEOS-Chem), to study the sources of O3 during high-O3 events. We show possible stratospheric influence on 4 out of the 10 events with daily maximum 8 h average (MDA8) surface O3 above 65 ppbv in the greater Las Vegas region. While O3 produced from regional anthropogenic emissions dominates pollution events in the Las Vegas Valley, stratospheric intrusions can mix with regional pollution to push surface O3 above 70 ppbv. GFDL-AM4 captures the key characteristics of deep stratospheric intrusions consistent with ozonesondes, lidar profiles, and co-located measurements of O3, CO, and water vapor at Angel Peak, whereas GEOS-Chem has difficulty simulating the observed features and underestimates observed O3 by ∼20 ppbv at the surface. On days when observed MDA8 O3 exceeds 65 ppbv and the AM4 stratospheric ozone tracer shows 20–40 ppbv enhancements, GEOS-Chem simulates ∼15 ppbv lower US background O3 than GFDL-AM4. The two models also differ substantially during a wildfire event, with GEOS-Chem estimating ∼15 ppbv greater O3, in better agreement with lidar observations. At the surface, the two models bracket the observed MDA8 O3 values during the wildfire event. Both models capture the large-scale transport of Asian pollution, but neither resolves some fine-scale pollution plumes, as evidenced by aerosol backscatter, aircraft, and satellite measurements. US background O3 estimates from the two models differ by 5 ppbv on average (greater in GFDL-AM4) and up to 15 ppbv episodically. Uncertainties remain in the quantitative attribution of each event. Nevertheless, our multi-model approach tied closely to observational analysis yields some process insights, suggesting that elevated background O3 may pose challenges to achieving a potentially lower NAAQS level (e.g., 65 ppbv) in the southwestern US.
We describe GFDL's CM4.0 physical climate model, with emphasis on those aspects that may be of particular importance to users of this model and its simulations. The model is built with the AM4.0/LM4.0 atmosphere/land model and OM4.0 ocean model. Topics include the rationale for key choices made in the model formulation, the stability as well as drift of the pre‐industrial control simulation, and comparison of key aspects of the historical simulations with observations from recent decades. Notable achievements include the relatively small biases in seasonal spatial patterns of top‐of‐atmosphere fluxes, surface temperature, and precipitation; reduced double Intertropical Convergence Zone bias; dramatically improved representation of ocean boundary currents; a high quality simulation of climatological Arctic sea ice extent and its recent decline; and excellent simulation of the El Niño‐Southern Oscillation spectrum and structure. Areas of concern include inadequate deep convection in the Nordic Seas; an inaccurate Antarctic sea ice simulation; precipitation and wind composites still affected by the equatorial cold tongue bias; muted variability in the Atlantic Meridional Overturning Circulation; strong 100 year quasi‐periodicity in Southern Ocean ventilation; and a lack of historical warming before 1990 and too rapid warming thereafter due to high climate sensitivity and strong aerosol forcing, in contrast to the observational record. Overall, CM4.0 scores very well in its fidelity against observations compared to the Coupled Model Intercomparison Project Phase 5 generation in terms of both mean state and modes of variability and should prove a valuable new addition for analysis across a broad array of applications.
Solar geoengineering (SG) has the potential to restore average surface temperatures by increasing planetary albedo1–4, but this could reduce precipitation5–7. Thus, although SG might reduce globally aggregated risks, it may increase climate risks for some regions8–10. Here, using the high-resolution forecast-oriented low ocean resolution (HiFLOR) model—which resolves tropical cyclones and has an improved representation of present-day precipitation extremes11,12—alongside 12 models from the Geoengineering Model Intercomparison Project (GeoMIP), we analyse the fraction of locations that see their local climate change exacerbated or moderated by SG. Rather than restoring temperatures, we assume that SG is applied to halve the warming produced by doubling CO2 (half-SG). In HiFLOR, half-SG offsets most of the CO2-induced increase of simulated tropical cyclone intensity. Moreover, neither temperature, water availability, extreme temperature nor extreme precipitation are exacerbated under half-SG when averaged over any Intergovernmental Panel on Climate Change (IPCC) Special Report on Extremes (SREX) region. Indeed, for both extreme precipitation and water availability, less than 0.4% of the ice-free land surface sees exacerbation. Thus, while concerns about the inequality of solar geoengineering impacts are appropriate, the quantitative extent of inequality may be overstated13.
The response of ozone (O3) dry deposition to ecosystem‐atmosphere interactions is poorly understood but is central to determining the potential for extreme pollution events under current and future climate conditions. Using observations and an interactive dry deposition scheme within two dynamic vegetation land models (GFDL LM3.0/LM4.0) driven by observation‐based meteorological forcings over 1948‐2014, we investigate the factors controlling seasonal and interannual variability (IAV) in O3 deposition velocities (Vd,O3). Stomatal activity in this scheme is determined mechanistically, depending on phenology, soil moisture, vapor pressure deficit, and CO2 concentration. Soil moisture plays a key role in modulating the observed and simulated Vd,O3 seasonal changes over evergreen forests in Mediterranean Europe, South Asia, and the Amazon. Analysis of multi‐year observations at forest sites in Europe and North America reveals drought stress to reduce Vd,O3 by ~50%. Both LM3.0 and LM4.0 capture the observed Vd,O3 decreases due to drought; however, IAV is weaker by a factor of two in LM3.0 coupled to atmospheric models, particularly in regions with large precipitation biases. IAV in summertime Vd,O3 to forests, driven primarily by the stomatal pathway, is largest (15‐35%) in semi‐arid regions of western Europe, eastern North America, and northeastern China. Monthly mean Vd,O3 for the highest year is two to four times that of the lowest, with significant implications for surface O3 variability and extreme events. Using Vd,O3 from LM4.0 in an atmospheric chemistry model improves the simulation of surface O3 abundance and spatial variability (reduces mean biases by ~10 ppb) relative to the widely‐used Wesely scheme.
Black carbon (BC) mitigation can reduce adverse environmental impacts on climate, air quality, human health, and water resource availability. To facilitate the identification of mitigation priorities, we use a state-of-the-science global chemistry-climate coupled model (AM3), with additional tagged BC tracers representing regional (East Asia, South Asia, Europe and North America) and sectoral (land transport, residential, industry) anthropogenic BC emissions to identify sources with the largest impacts on air quality, human health and glacial deposition. We find that within each tagged region, domestic emissions dominate BC surface concentrations and associated premature mortality (generally over 90%), as well as BC deposition on glaciers (∼40–95% across glaciers). BC emissions occurring within each tagged source region contribute roughly 1–2 orders of magnitude more to their domestic BC concentrations, premature mortality, and BC deposition on regional glaciers than that caused by the same quantity of BC emitted from foreign regions. At the sectoral level, the South Asian residential sector contributes ∼60% of BC associated premature mortality in South Asia and ∼40–60% of total BC deposited on southern Tibetan glaciers. Our findings imply that BC mitigation within a source region, particularly from East and South Asian residential sectors, will bring the largest reductions in BC associated air pollution, premature mortality, and glacial deposition.
A central strategy in achieving greenhouse gas mitigation targets is the transition of vehicles from internal combustion engines to electric power. However, due to complex emission sources and nonlinear chemistry, it is unclear how such a shift might impact air quality. Here we apply a prototype version of the new-generation NOAA GFDL global Atmospheric Model, version 4 (GFDL AM4) to investigate the impact on U.S. air quality from an aggressive conversion of internal combustion vehicles to battery-powered electric vehicles (EVs). We examine a suite of scenarios designed to quantify the effect of both the magnitude of EV market penetration and the source of electricity generation used to power them. We find that summer surface ozone (O3) decreases in most locations due to widespread reductions of traffic NOx emissions. Summer fine particulate matter (PM2.5) increases on average and largest in areas with increased coal-fired power generation demands. Winter O3 increases due to reduced loss via traffic NOx while PM2.5 decreases since larger ammonium nitrate reductions offset increases in ammonium sulfate. The largest magnitude changes are simulated at the extremes of the probability distribution. Increasing the fraction of vehicles converted to EVs further decreases summer O3, while increasing the fraction of electricity generated by “emission-free” sources largely eliminates the increases in summer PM2.5 at high EV adoption fractions. Ultimately, the number of conventional vehicles replaced by EVs has a larger effect on O3 than PM2.5, while the source of the electricity for those EVs exhibit greater control on PM2.5.
Explosive volcanic eruptions have large climate impacts, and can serve as observable tests of the climatic response to radiative forcing. Using a high resolution climate model, we contrast the climate responses to Pinatubo, with symmetric forcing, and those to Santa Maria and Agung, which had meridionally asymmetric forcing. Although Pinatubo had larger global‐mean forcing, asymmetric forcing strongly shifts the latitude of tropical rainfall features, leading to larger local precipitation/TC changes. For example, North Atlantic TC activity over is enhanced/reduced by SH‐forcing (Agung)/NH‐forcing (Santa Maria), but changes little in response to the Pinatubo forcing. Moreover, the transient climate sensitivity estimated from the response to Santa Maria is 20% larger than that from Pinatubo or Agung. This spread in climatic impacts of volcanoes needs to be considered when evaluating the role of volcanoes in global and regional climate, and serves to contextualize the well‐observed response to Pinatubo.
Conley, A J., Daniel M Westervelt, Jean-Francois Lamarque, Arlene M Fiore, Drew Shindell, G Correa, G Faluvegi, and Larry W Horowitz, March 2018: Multi-model surface temperature responses to removal of U.S. sulfur dioxide emissions. Journal of Geophysical Research: Atmospheres, 123(5), DOI:10.1002/2017JD027411. Abstract
Three Earth System models are used to derive surface temperature responses to removal of U.S. anthropogenic SO2 emissions. Using multi-century perturbation runs with and without U.S. anthropogenic SO2 emissions, the local and remote surface temperature changes are estimated. In spite of a temperature drift in the control and large internal variability, 200-year simulations yield statistically significant regional surface temperature responses to the removal of U.S. SO2 emissions. Both local and remote surface temperature changes occur in all models, and the patterns of changes are similar between models for Northern Hemisphere land regions. We find a global average temperature sensitivity to U.S. SO2 emissions of 0.0055 K per Tg(SO2) per year with a range of [0.0036, 0.0078]. We examine global and regional responses in SO4 burdens, aerosol optical depths (AOD), and effective radiative forcing (ERF). While changes in AOD and ERF are concentrated near the source region (U.S.), the temperature response is spread over the northern hemisphere with amplification of the temperature increase towards the Arctic. In all models, we find a significant response of dust concentrations, which affects the AOD but has no obvious effect on surface temperature. Temperature sensitivity to the effective radiative forcing of U.S. SO2 emissions is found to differ from the models’ sensitivity to radiative forcing of doubled CO2.
Dhomse, S S., Douglas Kinnison, M P Chipperfield, I Cionni, Michaela I Hegglin, N Luke Abraham, Hideharu Akiyoshi, Alexander T Archibald, E M Bednarz, S Bekki, P Braesicke, Neal Butchart, M Dameris, Makoto Deushi, S Frith, Steven C Hardiman, Birgit Hassler, Larry W Horowitz, R-M Hu, Patrick Jöckel, Beatrice Josse, O Kirner, S Kremser, U Langematz, J Lewis, M Marchand, and Meiyun Lin, et al., June 2018: Estimates of Ozone Return Dates from Chemistry-Climate Model Initiative Simulations. Atmospheric Chemistry and Physics, 18(11), DOI:10.5194/acp-18-8409-2018. Abstract
We analyse simulations performed for the Chemistry-Climate Model Initiative (CCMI) to estimate the return dates of the stratospheric ozone layer from depletion caused by anthropogenic stratospheric chlorine and bromine. We consider a total of 155 simulations from 20 models, including a range of sensitivity studies which examine the impact of climate change on ozone recovery. For the control simulations (unconstrained by nudging towards analysed meteorology) there is a large spread (±20 DU in the global average) in the predictions of the absolute ozone column. Therefore, the model results need to be adjusted for biases against historical data. Also, the interannual variability in the model results need to be smoothed in order to provide a reasonably narrow estimate of the range of ozone return dates. Consistent with previous studies, but here for a Representative Concentration Pathway (RCP) of 6.0, these new CCMI simulations project that global total column ozone will return to 1980 values in 2047 (with a 1-σ uncertainty of 2042–2052). At Southern Hemisphere mid-latitudes column ozone is projected to return to 1980 values in 2046 (2042–2050), and at Northern Hemisphere mid-latitudes in 2034 (2024–2044). In the polar regions, the return dates are 2062 (2055–2066) in the Antarctic in October and 2035 (2025–2040) in the Arctic in March. The earlier return dates in the NH reflect the larger sensitivity to dynamical changes. Our estimates of return dates are later than those presented in the 2014 Ozone Assessment by approximately 5–15 years, depending on the region. In the tropics only around half the models predict a return to 1980 values, at around 2040, while the other half do not reach this value. All models show a negative trend in tropical total column ozone towards the end of the 21st century. The CCMI models generally agree in their simulation of the time evolution of stratospheric chlorine, which is the main driver of ozone loss and recovery. However, there are a few outliers which show that the multi-model mean results for ozone recovery are not as tightly constrained as possible. Throughout the stratosphere the spread of ozone return dates to 1980 values between models tends to correlate with the spread of the return of inorganic chlorine to 1980 values. In the upper stratosphere, greenhouse gas-induced cooling speeds up the return by about 10–20 years. In the lower stratosphere, and for the column, there is a more direct link in the timing of the return dates, especially for the large Antarctic depletion. Comparisons of total column ozone between the models is affected by different predictions of the evolution of tropospheric ozone within the same scenario, presumably due to differing treatment of tropospheric chemistry. Therefore, for many scenarios, clear conclusions can only be drawn for stratospheric ozone columns rather than the total column. As noted by previous studies, the timing of ozone recovery is affected by the evolution of N2O and CH4. However, the effect in the simulations analysed here is small and at the limit of detectability from the few realisations available for these experiments compared to internal model variability. The large increase in N2O given in RCP 6.0 extends the ozone return globally by ~ 15 years relative to N2O fixed at 1960 abundances, mainly because it allows tropical column ozone to be depleted. The effect in extratropical latitudes is much smaller. The large increase in CH4 given in the RCP 8.5 scenario compared to RCP 6.0 also changes ozone return by ~ 15 years, again mainly through its impact in the tropics. For future assessments of single forcing or combined effects of CO2, CH4, and N2O on the stratospheric column ozone return dates, this work suggests that is more important to have multi-member (at least 3) ensembles for each scenario from each established participating model, rather than a large number of individual models.
Fiore, Arlene M., E V Fischer, S P Deolal, O Wild, D A Jaffe, J Staehelin, Olivia E Clifton, G P Milly, D J Bergmann, William J Collins, Frank Dentener, R Doherty, B N Duncan, B Fischer, S Gilge, Peter G Hess, and Larry W Horowitz, et al., October 2018: Peroxy acetyl nitrate (PAN) measurements at northern midlatitude mountain sites in April: a constraint on continental source–receptor relationships. Atmospheric and Climate Sciences, 18(20), DOI:10.5194/acp-18-15345-2018. Abstract
Peroxy acetyl nitrate (PAN) is the most important reservoir species for nitrogen oxides (NOx) in the remote troposphere. Upon decomposition in remote regions, PAN promotes efficient ozone production. We evaluate monthly mean PAN abundances from global chemical transport model simulations (HTAP1) for 2001 with measurements from five northern mid-latitude mountain sites (four European and one North American). The multi-model mean generally captures the observed monthly mean PAN but individual models simulate a factor of ~ 4–8 range in monthly abundances. We quantify PAN source-receptor relationships at the measurement sites with sensitivity simulations that decrease regional anthropogenic emissions of PAN (and ozone) precursors by 20 % from North America (NA), Europe (EU), and East Asia (EA). The HTAP1 models attribute more of the observed PAN at Jungfraujoch (Switzerland) to emissions in NA and EA, and less to EU, than a prior trajectory-based estimate. The trajectory-based and modeling approaches agree that EU emissions play a role in the observed springtime PAN maximum at Jungfraujoch. The signal from anthropogenic emissions on PAN is strongest at Jungfraujoch and Mount Bachelor (Oregon, U.S.A.) during April. In this month, PAN source-receptor relationships correlate both with model differences in regional anthropogenic volatile organic compound (AVOC) emissions and with ozone source-receptor relationships. PAN observations at mountaintop sites can thus provide key information for evaluating models, including links between PAN and ozone production and source-receptor relationships. Establishing routine, long-term, mountaintop measurements is essential given the large observed interannual variability in PAN.
Li, J, Jingqiu Mao, Arlene M Fiore, R C Cohen, John D Crounse, A P Teng, P O Wennberg, B H Lee, Felipe D Lopez-Hilfiker, J A Thornton, Jeff Peischl, I B Pollack, Thomas B Ryerson, P R Veres, J M Roberts, J Andrew Neuman, John B Nowak, G M Wolfe, T F Hanisco, A Fried, H B Singh, Jack E Dibb, Fabien Paulot, and Larry W Horowitz, February 2018: Decadal changes in summertime reactive oxidized nitrogen and surface ozone over the Southeast United States. Atmospheric Chemistry and Physics, 18(3), DOI:10.5194/acp-18-2341-2018. Abstract
Widespread efforts to abate ozone (O3) smog have significantly reduced emissions of nitrogen oxides (NOx) over the past 2 decades in the Southeast US, a place heavily influenced by both anthropogenic and biogenic emissions. How reactive nitrogen speciation responds to the reduction in NOx emissions in this region remains to be elucidated. Here we exploit aircraft measurements from ICARTT (July–August 2004), SENEX (June–July 2013), and SEAC4RS (August–September 2013) and long-term ground measurement networks alongside a global chemistry–climate model to examine decadal changes in summertime reactive oxidized nitrogen (RON) and ozone over the Southeast US. We show that our model can reproduce the mean vertical profiles of major RON species and the total (NOy) in both 2004 and 2013. Among the major RON species, nitric acid (HNO3) is dominant (∼ 42–45 %), followed by NOx (31 %), total peroxy nitrates (ΣPNs; 14 %), and total alkyl nitrates (ΣANs; 9–12 %) on a regional scale. We find that most RON species, including NOx, ΣPNs, and HNO3, decline proportionally with decreasing NOx emissions in this region, leading to a similar decline in NOy. This linear response might be in part due to the nearly constant summertime supply of biogenic VOC emissions in this region. Our model captures the observed relative change in RON and surface ozone from 2004 to 2013. Model sensitivity tests indicate that further reductions of NOx emissions will lead to a continued decline in surface ozone and less frequent high-ozone events.
Mao, Jingqiu, A G Carlton, R C Cohen, W Brune, Steven S Brown, G M Wolfe, J L Jimenez, Havala O T Pye, N L Ng, Lu Xu, V Faye McNeill, Kostas Tsigaridis, B C McDonald, C Warneke, A Guenther, M J Alvarado, J A de Gouw, L J Mickley, E M Leibensperger, R. Mathur, Christopher G Nolte, R W Portmann, Nadine Unger, M Tosca, and Larry W Horowitz, February 2018: Southeast Atmosphere Studies: learning from model-observation syntheses. Atmospheric Chemistry and Physics, 18(4), DOI:10.5194/acp-18-2615-2018. Abstract
Concentrations of atmospheric trace species in the United States have changed dramatically over the past several decades in response to pollution control strategies, shifts in domestic energy policy and economics, and economic development (and resulting emission changes) elsewhere in the world. Reliable projections of the future atmosphere require models to not only accurately describe current atmospheric concentrations, but to do so by representing chemical, physical and biological processes with conceptual and quantitative fidelity. Only through incorporation of the processes controlling emissions and chemical mechanisms that represent the key transformations among reactive molecules can models reliably project the impacts of future policy, energy and climate scenarios. Efforts to properly identify and implement the fundamental and controlling mechanisms in atmospheric models benefit from intensive observation periods, during which collocated measurements of diverse, speciated chemicals in both the gas and condensed phases are obtained. The Southeast Atmosphere Studies (SAS, including SENEX, SOAS, NOMADSS and SEAC4RS) conducted during the summer of 2013 provided an unprecedented opportunity for the atmospheric modeling community to come together to evaluate, diagnose and improve the representation of fundamental climate and air quality processes in models of varying temporal and spatial scales.
This paper is aimed at discussing progress in evaluating, diagnosing and improving air quality and climate modeling using comparisons to SAS observations as a guide to thinking about improvements to mechanisms and parameterizations in models. The effort focused primarily on model representation of fundamental atmospheric processes that are essential to the formation of ozone, secondary organic aerosol (SOA) and other trace species in the troposphere, with the ultimate goal of understanding the radiative impacts of these species in the southeast and elsewhere. Here we address questions surrounding four key themes: gas-phase chemistry, aerosol chemistry, regional climate and chemistry interactions, and natural and anthropogenic emissions. We expect this review to serve as a guidance for future modeling efforts.
Nolte, Christopher G., Patrick D Dolwick, Neal Fann, Larry W Horowitz, and Vaishali Naik, et al., 2018: Air Quality In Impacts, Risks, and Adaptation in the United States: Fourth National Climate Assessment, Volume II [Reidmiller, D.R., C.W. Avery, D.R. Easterling, K.E. Kunkel, K.L.M. Lewis, T.K. Maycock, and B.C. Stewart (eds.)], Washington, DC, USA, U.S. Global Change Research Program, DOI:10.7930/NCA4.2018.CH13512-538.
We present observation and model-based estimates of the changes in the direct shortwave effect of aerosols under clear-sky (SDRECS) from 2001 to 2015. Observation-based estimates are obtained from changes in the outgoing shortwave clear-sky radiation (Rsutcs) measured by the Clouds and the Earth's Radiant Energy System (CERES), accounting for the effect of variability in surface albedo, water vapor, and ozone. We find increases in SDRECS (i.e., less radiation scattered to space by aerosols) over Western Europe (0.7–1 W m−2 dec−1) and the Eastern US (0.9–1.8 W m−2 dec−1), decreases over India (−0.5– −1.9 W m−2 dec−1) and no significant change over Eastern China. Comparisons with the GFDL chemistry climate model AM3, driven by CMIP6 historical emissions, show that changes in SDRECS over Western Europe and the Eastern US are well captured, which largely reflects the mature understanding of the sulfate budget in these regions. In contrast, the model overestimates the trends in SDRECS over India and Eastern China. Over China, this bias can be partly attributed to the decline of SO2 emissions after 2007, which is not captured by the CMIP6 emissions. In both India and Eastern China, we find much larger contributions of nitrate and black carbon to changes in SDRECS than in the US and Europe, which highlights the need to better constrain their precursors and chemistry. Globally, our model shows that changes in the all-sky aerosol direct forcing between 2001 and 2015 (+0.03 W m−2) are dominated by black carbon (+0.12 W m−2) with significant offsets from nitrate (−0.03 W m−2) and sulfate (−0.03 W m−2). Changes in the sulfate (+7 %) and nitrate (+60 %) all-sky direct forcing between 2001 and 2015 are only weakly related to changes in the emissions of their precursors (−12.5 % and 19 % for SO2 and NH3, respectively), due mostly to chemical non linearities.
Paulot, Fabien, Sergey Malyshev, T B Nguyen, John D Crounse, Elena Shevliakova, and Larry W Horowitz, December 2018: Representing sub-grid scale variations in nitrogen deposition associated with land use in a global Earth System Model: implications for present and future nitrogen deposition fluxes over North America. Atmospheric Chemistry and Physics, 18(24), DOI:10.5194/acp-18-17963-2018. Abstract
Reactive nitrogen (N) emissions have increased over the last 150 years as a result of greater fossil fuel combustion and food production. The resulting increase in N deposition can alter the function of ecosystems, but characterizing its ecological impacts remains challenging, in part because of uncertainties in model-based estimates of N dry deposition. Here, we leverage the tiled structure of the land component (LM3) of the Geophysical Fluid Dynamics Laboratory (GFDL) Earth System Model to represent the impact of physical, hydrological, and ecological heterogeneities on the surface removal of chemical tracers. We show that this framework can be used to estimate N deposition at more ecologically-relevant scales (e.g., natural vegetation, water bodies) than from the coarse-resolution global chemistry–climate model (GFDL-AM3). Focusing on North America, we show that the faster removal of N over forested ecosystems relative to cropland and pasture implies that coarse resolution estimates of N deposition from global models systematically underestimate N deposition to natural vegetation by 10 to 30% in the Central and Eastern US. Neglecting the subgrid scale heterogeneity of dry deposition velocities also results in an underestimate (overestimate) of the amount of reduced (oxidized) nitrogen deposited to water bodies. Overall, changes in land cover associated with human activities are found to slow down the removal of N from the atmosphere, causing a reduction in the dry oxidized, dry reduced, and total N deposition over the contiguous US of 8%, 26%, and 6%, respectively. We also find that the reduction in the overall rate of removal of N associated with land-use change tends to increase N deposition on the remaining natural vegetation and facilitate N export to Canada. We show that subgrid scale differences in the surface removal of oxidized and reduced nitrogen imply that near-term (2010–2050) changes in oxidized (−47%) and reduced (+40%) US N emissions will cause opposite changes in N deposition to water bodies (increase) and natural vegetation (decrease) in the Eastern US, with potential implications for acidification and ecosystems.
Paynter, David J., Thomas L Frölicher, Larry W Horowitz, and Levi G Silvers, February 2018: Equilibrium Climate Sensitivity Obtained from Multi-Millennial Runs of Two GFDL Climate Models. Journal of Geophysical Research: Atmospheres, 123(4), DOI:10.1002/2017JD027885. Abstract
Equilibrium climate sensitivity (ECS), defined as the long-term change in global mean surface air temperature in response to doubling atmospheric CO2, is usually computed from short atmospheric simulations over a mixed layer ocean, or inferred using a linear regression over a short-time period of adjustment. We report the actual ECS from multi-millenial simulations of two GFDL general circulation models (GCMs), ESM2M and CM3 of 3.3 K and 4.8 K, respectively. Both values are ~1 K higher than estimates for the same models reported in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change obtained by regressing the Earth's energy imbalance against temperature. This underestimate is mainly due to changes in the climate feedback parameter (−α) within the first century after atmospheric CO2 has stabilized. For both GCMs it is possible to estimate ECS with linear regression to within 0.3 K by increasing CO2 at 1% per year to doubling and using years 51-350 after CO2 is constant. We show that changes in −α differ between the two GCMs and are strongly tied to the changes in both vertical velocity at 500 hPa (ω500) and estimated inversion strength (EIS) that the GCMs experience during the progression towards the equilibrium. This suggests that while cloud physics parametrizations are important for determining the strength of −α, the substantially different atmospheric state resulting from a changed SST pattern may be of equal importance.
Rieder, H E., Arlene M Fiore, Olivia E Clifton, G Correa, Larry W Horowitz, and Vaishali Naik, November 2018: Combining model projections with site-level observations to estimate changes in distributions and seasonality of ozone in surface air over the USA. Atmospheric Environment, 193, DOI:10.1016/j.atmosenv.2018.07.042. Abstract
While compliance with air quality standards is evaluated at individual monitoring stations, projections of future ambient air quality for global climate and emission scenarios often rely on coarse resolution models. We describe a statistical transfer approach that bridges the spatial gap between air quality projections, averaged over four broad U.S. regions, from a global chemistry-climate model and the local level (at specific U.S. CASTNet sites). Our site-level projections are intended as a line of evidence in planning for possible futures rather than the sole basis for policy decisions. We use a set of transient sensitivity simulations (2006–2100) from the Geophysical Fluid Dynamics Laboratory (GFDL) chemistry-climate model CM3, designed to isolate the effects of changes in anthropogenic ozone (O3) precursor emissions, climate warming, and global background CH4 on surface O3. We find that surface maximum daily 8-h average (MDA8) O3 increases despite constant precursor emissions in a warmer climate during summer, particularly in the low tail of the MDA8 O3 distribution for the Northeastern U.S., while MDA8 O3 decreases slightly throughout the distribution over the West and Southeast during summer and fall. Under scenarios in which non-methane O3 precursors decline as climate warms (RCP4.5 and RCP8.5), summertime MDA8 O3 decreases with NOx emissions, most strongly in the upper tail of the MDA8 O3 distribution. In a scenario where global methane abundances roughly double over the 21st century (RCP8.5), winter and spring MDA8 O3 increases, particularly in the lower tail and over the Western U.S. In this RCP8.5 scenario, the number of days when MDA8 O3 exceeds 70 ppb declines in summer with NOx emissions, but increases in spring (and winter); by the end of the century, the majority of sites in the WE and NE show probabilistic return values of the annual 4th highest MDA8 O3 concentration above 70 ppb (the current O3 NAAQS level). Continued increases in global CH4 abundances can be thought of as a “methane penalty”, offsetting benefits otherwise attainable by controlling non-CH4 O3 precursors.
Northern India (23° N–31° N, 68° E–90° E) is one of the most densely populated and polluted regions in world. Accurately modeling pollution in the region is difficult due to the extreme conditions with respect to emissions, meteorology, and topography, but it is paramount in order to understand how future changes in emissions and climate may alter the region's pollution regime. We evaluate the ability of a developmental version of the new-generation NOAA GFDL Atmospheric Model, version 4 (AM4) to simulate observed wintertime fine particulate matter (PM2.5) and its relationship to meteorology over Northern India. We compare two simulations of GFDL-AM4 nudged to observed meteorology for the period 1980–2016 driven by pollutant emissions from two global inventories developed in support of the Coupled Model Intercomparison Project Phases 5 (CMIP5) and 6 (CMIP6), and compare results with ground-based observations from India's Central Pollution Control Board (CPCB) for the period 1 October 2015–31 March 2016. Overall, our results indicate that the simulation with CMIP6 emissions, produces improved concentrations of pollutants over the region relative to the CMIP5-driven simulation.
While the particulate concentrations simulated by AM4 are biased low overall, the model generally simulates the magnitude and daily variability of observed total PM2.5. Nitrate and organic matter are the primary components of PM2.5 over Northern India in the model. On the basis of correlations of the individual model components with total observed PM2.5 and correlations between the two simulations, meteorology is the primary driver of daily variability. The model correctly reproduces the shape and magnitude of the seasonal cycle of PM2.5, but the simulated diurnal cycle misses the early evening rise and secondary maximum found in the observations. Observed PM2.5 abundances are by far the highest within the densely populated Indo-Gangetic Plain, where they are closely related to boundary layer meteorology, specifically relative humidity, wind speed, boundary layer height, and inversion strength. The GFDL AM4 model reproduces the overall observed pollution gradient over Northern India as well as the strength of the meteorology-PM2.5 relationship in most locations.
Turner, A J., I Y Fung, Vaishali Naik, Larry W Horowitz, and R C Cohen, September 2018: Modulation of hydroxyl variability by ENSO in the absence of external forcing. Proceedings of the National Academy of Sciences, 115(36), DOI:10.1073/pnas.1807532115. Abstract
The hydroxyl radical (OH) is the primary oxidant in the troposphere, and the impact of its fluctuations on the methane budget has been disputed in recent years, however measurements of OH are insufficient to characterize global interannual fluctuations relevant for methane. Here, we use a 6,000-y control simulation of preindustrial conditions with a chemistry-climate model to quantify the natural variability in OH and internal feedbacks governing that variability. We find that, even in the absence of external forcing, maximum OH changes are 3.8 ± 0.8% over a decade, which is large in the context of the recent methane growth from 2007–2017. We show that the OH variability is not a white-noise process. A wavelet analysis indicates that OH variability exhibits significant feedbacks with the same periodicity as the El Niño–Southern Oscillation (ENSO). We find intrinsically generated modulation of the OH variability, suggesting that OH may show periods of rapid or no change in future decades that are solely due to the internal climate dynamics (as opposed to external forcings). An empirical orthogonal function analysis further indicates that ENSO is the dominant mode of OH variability, with the modulation of OH occurring primarily through lightning NOx. La Niña is associated with an increase in convection in the Tropical Pacific, which increases the simulated occurrence of lightning and allows for more OH production. Understanding this link between OH and ENSO may improve the predictability of the oxidative capacity of the troposphere and assist in elucidating the causes of current and historical trends in methane.
Westervelt, Daniel M., A J Conley, Arlene M Fiore, Jean-Francois Lamarque, Drew Shindell, M Previdi, N R Mascioli, G Faluvegi, G Correa, and Larry W Horowitz, August 2018: Connecting regional aerosol emissions reductions to local and remote precipitation responses. Atmospheric Chemistry and Physics, 18(16), DOI:10.5194/acp-18-12461-2018. Abstract
The unintended climatic implications of aerosol and precursor emission reductions implemented to protect public health are poorly understood. We investigate the precipitation response to regional changes in aerosol emissions using three coupled chemistry-climate models: NOAA Geophysical Fluid Dynamics Laboratory Coupled Model 3 (GFDL-CM3), NCAR Community Earth System Model (CESM1), and NASA Goddard Institute for Space Studies ModelE2 (GISS-E2). Our approach contrasts a long present-day control simulation from each model (up to 400 years with perpetual year 2000 or 2005 emissions) with fourteen individual aerosol emissions perturbation simulations (160–240 years each). We perturb emissions of sulfur dioxide and/or carbonaceous aerosol within six world regions and assess the significance of precipitation responses relative to internal variability determined by the control simulation and across the models. Global and regional precipitation mostly increases when we reduce regional aerosol emissions in the models, with the strongest responses occurring for sulfur dioxide emissions reductions from Europe and the United States. Precipitation responses to aerosol emissions reductions are largest in the tropics and project onto the El Niño-Southern Oscillation (ENSO). Regressing precipitation onto an Indo-Pacific zonal sea level pressure gradient index (a proxy for ENSO) indicates that the ENSO component of the precipitation response to regional aerosol removal can be as large as 20 % of the total simulated response. Precipitation increases in the Sahel in response to aerosol reductions in remote regions because an anomalous interhemispheric temperature gradient alters the position of the Intertropical Convergence Zone (ITCZ). This mechanism holds across multiple aerosol reduction simulations and models.
In this two-part paper, a description is provided of a version of the AM4.0/LM4.0 atmosphere/land model that will serve as a base for a new set of climate and Earth system models (CM4 and ESM4) under development at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL). This version, with roughly 100km horizontal resolution and 33 levels in the vertical, contains an aerosol model that generates aerosol fields from emissions and a “light” chemistry mechanism designed to support the aerosol model but with prescribed ozone. In Part I, the quality of the simulation in AMIP (Atmospheric Model Intercomparison Project) mode – with prescribed sea surface temperatures (SSTs) and sea ice distribution – is described and compared with previous GFDL models and with the CMIP5 archive of AMIP simulations. The model's Cess sensitivity (response in the top-of-atmosphere radiative flux to uniform warming of SSTs) and effective radiative forcing are also presented. In Part II, the model formulation is described more fully and key sensitivities to aspects of the model formulation are discussed, along with the approach to model tuning.
In Part II of this two-part paper, documentation is provided of key aspects of a version of the AM4.0/LM4.0 atmosphere/land model that will serve as a base for a new set of climate and Earth system models (CM4 and ESM4) under development at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL). The quality of the simulation in AMIP (Atmospheric Model Intercomparison Project) mode has been provided in Part I. Part II provides documentation of key components and some sensitivities to choices of model formulation and values of parameters, highlighting the convection parameterization and orographic gravity wave drag. The approach taken to tune the model's clouds to observations is a particular focal point. Care is taken to describe the extent to which aerosol effective forcing and Cess sensitivity have been tuned through the model development process, both of which are relevant to the ability of the model to simulate the evolution of temperatures over the last century when coupled to an ocean model.
Choi, Hyun-Deok, H Liu, A J Crawford, D B Considine, D J Allen, B N Duncan, and Larry W Horowitz, et al., July 2017: Global O3-CO Correlations in a Chemistry and Transport Model during July–August: Evaluation with TES Satellite Observations and Sensitivity to Input Meteorological Data and Emissions. Atmospheric Chemistry and Physics, 17(13), DOI:10.5194/acp-17-8429-2017. Abstract
We examine the capability of the Global Modeling Initiative (GMI) chemistry and transport model to reproduce global mid-tropospheric (618 hPa) O3-CO correlations determined by the measurements from Tropospheric Emission Spectrometer (TES) aboard NASA’s Aura satellite during boreal summer (July–August). The model is driven by three meteorological data sets (fvGCM with sea surface temperature for 1995, GEOS4-DAS for 2005, and MERRA for 2005), allowing us to examine the sensitivity of model O3-CO correlations to input meteorological data. Model simulations of radionuclide tracers (222Rn, 210Pb, and 7Be) are used to illustrate the differences in transport-related processes among the meteorological data sets. Simulated O3 values are evaluated with climatological ozone profiles from ozonesonde measurements and satellite tropospheric O3 columns. Despite the fact that the three simulations show significantly different global and regional distributions of O3 and CO concentrations, all simulations show similar patterns of O3-CO correlations on a global scale. These patterns are consistent with those derived from TES observations, except in the tropical easterly biomass burning outflow regions. Discrepancies in regional O3-CO correlation patterns in the three simulations may be attributed to differences in convective transport, stratospheric influence, and subsidence, among other processes. To understand how various emissions drive global O3-CO correlation patterns, we examine the sensitivity of GMI/MERRA model-calculated O3 and CO concentrations and their correlations to emission types (fossil fuel, biomass burning, biogenic, and lightning NOx emissions). Fossil fuel and biomass burning emissions are mainly responsible for the strong positive O3-CO correlations over continental outflow regions in both hemispheres. Biogenic emissions have a relatively smaller impact on O3-CO correlations than other emissions, but are largely responsible for the negative correlations over the tropical eastern Pacific, reflecting the fact that O3 is consumed and CO generated during the atmospheric oxidation process of isoprene under low NOx conditions. We find that lightning NOx emissions degrade both positive correlations at mid-/high- latitudes and negative correlations in the tropics because ozone production downwind of lightning NOx emissions is not directly related to the emission and transport of CO. Our study concludes that O3-CO correlations may be used effectively to constrain the sources of regional tropospheric O3 in global 3-D models, especially for those regions where convective transport of pollution plays an important role.
The ozone (O3) dry depositional sink and its contribution to observed variability in tropospheric O3 are both poorly understood. Distinguishing O3 uptake through plant stomata versus other pathways is relevant for quantifying the O3 influence on carbon and water cycles. We use a decade of O3, carbon, and energy eddy covariance (EC) fluxes at Harvard Forest to investigate interannual variability (IAV) in O3 deposition velocities ( math formula). In each month, monthly mean math formula for the highest year is twice that for the lowest. Two independent stomatal conductance estimates, based on either water vapor EC or gross primary productivity, vary little from year to year relative to canopy conductance. We conclude that nonstomatal deposition controls the substantial observed IAV in summertime math formula during the 1990s over this deciduous forest. The absence of obvious relationships between meteorology and math formula implies a need for additional long-term, high-quality measurements and further investigation of nonstomatal mechanisms.
Guo, Yixin, Junfeng Liu, D L Mauzerall, X Li, and Larry W Horowitz, et al., May 2017: Long-Lived Species Enhance Summertime Attribution of North American Ozone to Upwind Sources. Environmental Science & Technology, 51(9), DOI:10.1021/acs.est.6b05664. Abstract
Ground-level ozone (O3), harmful to most living things, is produced from both domestic and foreign emissions of anthropogenic precursors. Previous estimates of the linkage from distant sources rely on the sensitivity approach (i.e., modeling the change of ozone concentrations that result from modifying precursor emissions) as well as the tagging approach (i.e., tracking ozone produced from specific O3 precursors emitted from one region). Here, for the first time, we tag all O3 precursors (i.e., nitrogen oxides (NOx), carbon monoxide (CO), and volatile organic compounds (VOCs)) from East Asia and explicitly track their physicochemical evolution without perturbing the nonlinear O3 chemistry. We show that, even in summer, when intercontinental influence on ozone has typically been found to be weakest, nearly 3 parts per billion by volume (ppbv) seasonal average surface O3 over North America can be attributed to East Asian anthropogenic emissions, compared with 0.7 ppbv using the sensitivity approach and 0.5 ppbv by tagging reactive nitrogen oxides. Considering the acute effects of O3 exposure, approximately 670 cardiovascular and 300 respiratory premature mortalities within North America could be attributed to East Asia. CO and longer-lived VOCs, largely overlooked in previous studies, extend the influence of regional ozone precursors emissions and, thus, greatly enhance O3 attribution to source region.
Li, Z, Junfeng Liu, D L Mauzerall, X Li, Songmiao Fan, and Larry W Horowitz, et al., March 2017: A potential large and persistent black carbon forcing over Northern Pacific inferred from satellite observations. Scientific Reports, 7, 43429, DOI:10.1038/srep43429. Abstract
Black carbon (BC) aerosol strongly absorbs solar radiation, which warms climate. However, accurate estimation of BC’s climate effect is limited by the uncertainties of its spatiotemporal distribution, especially over remote oceanic areas. The HIAPER Pole-to-Pole Observation (HIPPO) program from 2009 to 2011 intercepted multiple snapshots of BC profiles over Pacific in various seasons, and revealed a 2 to 5 times overestimate of BC by current global models. In this study, we compared the measurements from aircraft campaigns and satellites, and found a robust association between BC concentrations and satellite-retrieved CO, tropospheric NO2, and aerosol optical depth (AOD) (R2 > 0.8). This establishes a basis to construct a satellite-based column BC approximation (sBC*) over remote oceans. The inferred sBC* shows that Asian outflows in spring bring much more BC aerosols to the mid-Pacific than those occurring in other seasons. In addition, inter-annual variability of sBC* is seen over the Northern Pacific, with abundances varying consistently with the springtime Pacific/North American (PNA) index. Our sBC* dataset infers a widespread overestimation of BC loadings and BC Direct Radiative Forcing by current models over North Pacific, which further suggests that large uncertainties exist on aerosol-climate interactions over other remote oceanic areas beyond Pacific.
Lin, Meiyun, Larry W Horowitz, R Payton, Arlene M Fiore, and G Tonnesen, March 2017: US surface ozone trends and extremes from 1980 to 2014: quantifying the roles of rising Asian emissions, domestic controls, wildfires, and climate. Atmospheric Chemistry and Physics, 17(4), DOI:10.5194/acp-17-2943-2017. Abstract
Surface ozone (O3) responds to varying global-to-regional precursor emissions, climate, and extreme weather, with implications for designing effective air quality control policies. We examine these conjoined processes with observations and global chemistry-climate model (GFDL-AM3) hindcasts over 1980–2014. The model captures the salient features of observed trends in daily maximum 8-hour average O3; (1) increases over East Asia (up to 2 ppb yr−1), (2) springtime increases at western US (WUS) rural sites (0.2–0.5 ppb yr−1) with a ‘baseline’ sampling approach, (3) summertime decreases, largest at the 95th percentile, and wintertime increases in the 50th to 5th percentiles over the eastern US (EUS). Asian NOx emissions tripled since 1990, contributing as much as 65 % to modeled springtime background O3 increases (0.3–0.5 ppb yr−1) over the WUS, outpacing O3 decreases attained via US domestic emission controls. Methane increases over this period raise WUS background O3 by 15 %. During summer, increasing Asian emissions approximately offset the effects of US emission reductions, leading to weak or insignificant observed O3 trends at WUS rural sites. While wildfire emissions can enhance summertime monthly mean O3 at individual sites by 2–8 ppb, high temperatures and the associated buildup of O3 produced from regional anthropogenic emissions contribute most to elevating observed summertime O3 throughout the USA. Rising Asian emissions and global methane under the RCP8.5 scenario increase mean springtime O3 above the WUS by ~ 10 ppb from 2010 to 2030. Historical EUS O3 decreases, driven by regional emission controls, were most pronounced in the Southeast with an earlier onset of biogenic isoprene emissions and NOx-sensitive O3 production. Regional NOx reductions also alleviated the O3 buildup during the recent heat waves of 2011 and 2012 relative to earlier heat waves (e.g., 1988; 1999). Without emission controls, the 95th percentile summertime O3 in the EUS would have increased by 0.2–0.4 ppb yr−1 over 1988–2014 due to more frequent hot extremes and rising biogenic isoprene emissions.
The soluble fraction of aerosol Fe, mainly Fe(II), represents a large source of nutrient iron to the open ocean. Fe(II) may also play an important role in the adverse health effects of ambient aerosols. Our current understanding of the reduction of Fe(III) to Fe(II) in aerosols suggests that the major pathway is the photoreduction of Fe(III)–oxalate complexes, but this pathway cannot explain the observed nighttime Fe(II) in ambient aerosols and is also limited by the supply of oxalate. Here we propose a new pathway initiated by gaseous HO2 uptake, followed by Cu–Fe redox coupling, which can sustain nighttime Fe(II) and also dominate Fe(III) reduction in the absence of Fe(III)–oxalate complexes. Consequently, aqueous OH production is substantially enhanced via the Fenton reaction and sustained by the influx of HO2 from the gas phase. This mechanism is potentially the major mechanism for sustaining soluble Fe(II) in ambient aerosols and can be tested by a combination of modeling and aerosol Fe speciation measurements. We hypothesize that this mechanism may also be relevant to mineral Fe dissolution in dust aerosols.
Morgenstern, Olaf, Michaela I Hegglin, Eugene Rozanov, Fiona M O'Connor, N Luke Abraham, Hideharu Akiyoshi, Alexander T Archibald, S Bekki, Neal Butchart, M P Chipperfield, Makoto Deushi, S S Dhomse, R R Garcia, Steven C Hardiman, Larry W Horowitz, Patrick Jöckel, Beatrice Josse, Douglas Kinnison, and Meiyun Lin, et al., February 2017: Review of the global models used within phase 1 of the Chemistry–Climate Model Initiative (CCMI). Geoscientific Model Development Discussion, 10(2), DOI:10.5194/gmd-10-639-2017. Abstract
We present an overview of state-of-the-art chemistry-climate and -transport models that are used within the Chemistry-Climate Model Initiative (CCMI). CCMI aims to conduct a detailed evaluation of participating models using process-oriented diagnostics derived from observations in order to gain confidence in the models' projections of the stratospheric ozone layer, air quality, where applicable global climate change, and the interactions between them. Interpretation of these diagnostics requires detailed knowledge of the radiative, chemical, dynamical, and physical processes incorporated in the models. Also an understanding of the degree to which CCMI recommendations for simulations have been followed is necessary to understand model response to anthropogenic and natural forcing and also to explain inter-model differences. This becomes even more important given the ongoing development and the ever-growing complexity of these models. This paper also provides an overview of the available CCMI simulations with the aim to inform CCMI data users.
We use transient GFDL-CM3 chemistry-climate model simulations over the 2006-2100 period to show how the influence of volcanic aerosols on the extent and timing of ozone recovery varies with a) future greenhouse gas scenarios (RCP4.5 and RCP8.5) and b) halogen loading. Current understanding is that elevated volcanic aerosols reduce ozone under high halogen loading, but increase ozone under low halogen loading when the chemistry is more NOx dominated. With extremely low aerosol loadings (designated here as ‘background’), global stratospheric ozone burden is simulated to return to 1980 levels around 2050 in the RCP8.5 scenario, but remains below 1980 levels throughout the 21st century in the RCP4.5 scenario. In contrast, with elevated volcanic aerosols, ozone column recovers more quickly to 1980 levels, with recovery dates ranging from the mid-2040s in RCP8.5 to the mid-2050s to early 2070s in RCP4.5. The ozone response in both future emission scenarios increases with enhanced volcanic aerosols. By 2100, the 1980-baseline adjusted global stratospheric ozone column is projected to be 20-40% greater in RCP8.5 and 110-200% greater in RCP4.5 with elevated volcanic aerosols compared to simulations with the extremely low background aerosols. The weaker ozone enhancement at 2100 in RCP8.5 than in RCP4.5 in response to elevated volcanic aerosols is due to a factor of 2.5 greater methane in RCP8.5 compared with RCP4.5. Our results demonstrate the substantial uncertainties in stratospheric ozone projections and expected recovery dates induced by volcanic aerosol perturbations that need to be considered in future model ozone projections.
Paulot, Fabien, Songmiao Fan, and Larry W Horowitz, January 2017: Contrasting seasonal responses of sulfate aerosols to declining SO2 emissions in the Eastern US: implications for the efficacy of SO2 emission controls. Geophysical Research Letters, 44(1), DOI:10.1002/2016GL070695. Abstract
Stringent controls have reduced US SO2 emissions by over 60% since the late 1990s. These controls have been more effective at reducing surface [ inline image] in summer (JJA) than in winter (DJF), a seasonal contrast that is not robustly captured by CMIP5 global models. We use the GFDL AM3 chemistry-climate model to show that oxidant limitation during winter causes [ inline image] (DJF) to be sensitive to primary inline image emissions, in-cloud titration of H2O2, and in-cloud oxidation by O3. The observed contrast in the seasonal response of [ inline image] to decreasing SO2 emissions is best explained by the O3 reaction, whose rate coefficient has increased over the past decades as a result of increasing NH3 emissions and decreasing SO2 emissions, both of which lower cloud water acidity. The fraction of SO2 oxidized to inline image is projected to keep increasing in future decades, delaying improvements in wintertime air quality.
Paulot, Fabien, David J Paynter, Paul Ginoux, Vaishali Naik, S Whitburn, M Van Damme, L Clarisse, P-F Coheur, and Larry W Horowitz, August 2017: Gas-aerosol partitioning of ammonia in biomass burning plumes: implications for the interpretation of spaceborne observations of ammonia and the radiative forcing of ammonium nitrate. Geophysical Research Letters, 44(15), DOI:10.1002/2017GL074215. Abstract
Satellite–derived enhancement ratios of NH3 relative to CO column burden ( math formula) in fires over Alaska, the Amazon, and South Equatorial Africa are 35, 45, and 70% lower than the corresponding ratio of their emissions factors ( math formula) from biomass burning derived from in-situ observations. Simulations performed using the GFDL AM3 global chemistry–climate model show that these regional differences may not entirely stem from an overestimate of NH3 emissions but rather from changes in the gas-aerosol partitioning of NH3 to NH math formula. Differences between math formula and math formula are largest in regions where EFNOx/NH3 is high, consistent with the production of NH4NO3. Biomass burning is estimated to contribute 13–24% of the global burden and direct radiative effect (DRE) of NH4NO3(-15 – -28 mWm−2), despite accounting for less than 6% of the global source of NH3. Production of NH4NO3 is largely concentrated over the Amazon and South Equatorial Africa, where its DRE can reach -1.9Wm−2 during the biomass burning season.
Prather, Michael J., X Zhu, C Flynn, Sarah A Strode, J Rodriguez, S Steenrod, J Liu, Jean-Francois Lamarque, Arlene M Fiore, Larry W Horowitz, and Jingqiu Mao, et al., June 2017: Global Atmospheric Chemistry – Which Air Matters. Atmospheric Chemistry and Physics, 17(14), DOI:10.5194/acp-17-9081-2017. Abstract
An approach for analysis and modeling of global atmospheric chemistry is developed for application to measurements that provide a tropospheric climatology of those heterogeneously distributed, reactive species that control the loss of methane and the production and loss of ozone. We identify key species (e.g., O3, NOx, HNO3, HNO4, C2H3NO5, H2O, HOOH, CH3OOH, HCHO, CO, CH4, C2H6, acetaldehyde, acetone), and presume that they can be measured simultaneously in air parcels on the scale of a few km horizontally and a few tenths vertically. Six global models have prepared such climatologies (at model resolution) for August with emphasis on the vast central Pacific and Atlantic Ocean basins. We show clear differences in model generated reactivities as well as species covariances that could readily be discriminated with an unbiased climatology. A primary tool is comparison of multi-dimensional probability densities of key species weighted by frequency of occurrence as well as by the reactivity of the parcels with respect to methane and ozone. The reactivity-weighted probabilities tell us which parcels matter in this case. Testing 100-km scale models with 2-km measurements using these tools also addresses a core question about model resolution and whether fine-scale atmospheric structures matter to the overall ozone and methane budget. A new method enabling these six global chemistry-climate models to ingest an externally-sourced climatology and then compute air parcel reactivity is demonstrated. Such an observed climatology is anticipated from the NASA Atmospheric Tomography (ATom) aircraft mission (2015–2020), measuring the key species, executing profiles over the Pacific and Atlantic Ocean basins. This work is a core part of the design of ATom.
Saikawa, E, Hankyul Kim, M Zhong, Greet Janssens-Manehout, J Kurokawa, Zbigniew Klimont, F Wagner, Vaishali Naik, Larry W Horowitz, and Q Zhang, May 2017: Comparison of Emissions Inventories of Anthropogenic Air Pollutants in China. Atmospheric Chemistry and Physics Discussions, 17(10), DOI:10.5194/acp-17-6393-2017. Abstract
Anthropogenic air pollutant emissions have been increasing rapidly in China. Modelers use emissions inventories to assess temporal and spatial distribution of these emissions to estimate their impacts on regional and global air quality. However, large uncertainties exist in emissions estimates and assessing discrepancies in these inventories is essential for better understanding of the trends in air pollution over China. We compare five different emissions inventories estimating emissions of carbon monoxide (CO), nitrogen oxides (NOx), sulfur dioxide (SO2), particulate matter with an aerodynamic diameter of 10 um or less (PM10) from China. The emissions inventories analyzed in this paper include Regional Emissions inventory in ASia v2.1 (REAS); Multi-resolution Emission Inventory for China (MEIC); Emission Database for Global Atmospheric Research v4.2 (EDGAR); the inventory by Yu Zhao (ZHAO); and the Greenhouse Gas and Air Pollution Interactions and Synergies (GAINS). We focus on the period between 2000 and 2008 during which the Chinese economic activities have more than doubled. In addition to the national total, we also analyzed emissions from four source sectors (industry, transportation, power, and residential) and within seven regions in China (East, North, Northeast, Central, Southwest, Northwest, and South) and found that large disagreements (~ seven fold) exist among the five inventories at disaggregated levels. These discrepancies lead to differences of 67 ug/m3, 15 ppbv, and 470 ppbv for monthly mean PM10, O3, and CO, respectively, in modelled regional concentrations in China. We also find that MEIC inventory emissions estimates create a VOC-limited environment that produces much lower O3 mixing ratio in the East and Central China compared to the simulations using REAS and EDGAR estimates. Our results illustrate that a better understanding of Chinese emissions at more disaggregated levels is essential for finding an effective mitigation measure for reducing national and regional air pollution in China.
Arctic haze has a distinct seasonal cycle with peak concentrations in winter but pristine conditions in summer. It is demonstrated that the Geophysical Fluid Dynamics Laboratory (GFDL) atmospheric general circulation model AM3 can reproduce the observed seasonality of Arctic black carbon (BC), an important component of Arctic haze. We use the model to study how large-scale circulation and removal drive the seasonal cycle of Arctic BC. It is found that despite large seasonal shifts in the general circulation pattern, the transport of BC into the Arctic varies little throughout the year. The seasonal cycle of Arctic BC is attributed mostly to variations in the controlling factors of wet removal, namely the hydrophilic fraction of BC and wet deposition efficiency of hydrophilic BC. Specifically, a confluence of low hydrophilic fraction and weak wet deposition, owing to slower aging process and less efficient mixed-phase cloud scavenging, respectively, is responsible for the wintertime peak of BC. The transition to low BC in summer is the consequence of a gradual increase in the wet deposition efficiency, while the increase of BC in late fall can be explained by a sharp decrease in the hydrophilic fraction. The results presented here suggest that future changes in the aging and wet deposition processes can potentially alter the concentrations of Arctic aerosols and their climate effects.
Silva, R A., J Jason West, Jean-Francois Lamarque, Drew Shindell, William J Collins, G Faluvegi, Gerd Folberth, Larry W Horowitz, T Nagashima, and Vaishali Naik, et al., September 2017: Future global mortality from changes in air pollution attributable to climate change. Nature Climate Change, 7(9), DOI:10.1038/nclimate3354. Abstract
Ground-level ozone and fine particulate matter (PM 2.5) are associated with premature human mortality1, 2, 3, 4; their future concentrations depend on changes in emissions, which dominate the near-term5, and on climate change6, 7. Previous global studies of the air-quality-related health effects of future climate change8, 9 used single atmospheric models. However, in related studies, mortality results differ among models10, 11, 12. Here we use an ensemble of global chemistry–climate models13 to show that premature mortality from changes in air pollution attributable to climate change, under the high greenhouse gas scenario RCP8.5 (ref. 14), is probably positive. We estimate 3,340 (−30,300 to 47,100) ozone-related deaths in 2030, relative to 2000 climate, and 43,600 (−195,000 to 237,000) in 2100 (14% of the increase in global ozone-related mortality). For PM 2.5, we estimate 55,600 (−34,300 to 164,000) deaths in 2030 and 215,000 (−76,100 to 595,000) in 2100 (countering by 16% the global decrease in PM 2.5-related mortality). Premature mortality attributable to climate change is estimated to be positive in all regions except Africa, and is greatest in India and East Asia. Most individual models yield increased mortality from climate change, but some yield decreases, suggesting caution in interpreting results from a single model. Climate change mitigation is likely to reduce air-pollution-related mortality.
West, J J., Y Zhang, Steven J Smith, R A Silva, J H Bowden, Vaishali Naik, Y Li, D Gilfillan, Z Adelman, M Fry, S C Anenberg, Larry W Horowitz, and Jean-Francois Lamarque, April 2017: Cobenefits of global and domestic greenhouse gas emissions for air quality and human health. The Lancet, 389(S23), DOI:10.1016/S0140-6736(17)31135-2.
Westervelt, Daniel M., A J Conley, Arlene M Fiore, Jean-Francois Lamarque, Drew Shindell, M Previdi, G Faluvegi, G Correa, and Larry W Horowitz, May 2017: Multi-model precipitation responses to removal of U.S. sulfur dioxide emissions. Journal of Geophysical Research: Atmospheres, 122(9), DOI:10.1002/2017JD026756. Abstract
Emissions of aerosols and their precursors are declining due to policies enacted to protect human health, yet we currently lack a full understanding of the magnitude, spatio-temporal pattern, statistical significance, and physical mechanisms of precipitation responses to aerosol reductions. We quantify the global and regional precipitation response to U.S. SO2 emissions reductions using three fully coupled chemistry-climate models: Community Earth System Model (CESM1), Geophysical Fluid Dynamics Laboratory Coupled Model 3 (GFDL-CM3), and Goddard Institute for Space Studies ModelE2 (GISS-E2). We contrast 200-year (or longer) simulations in which anthropogenic U.S. sulfur dioxide (SO2) emissions are set to zero with present-day control simulations to assess the aerosol, cloud, and precipitation response to U.S. SO2 reductions. In all three models, reductions in aerosol optical depth up to 70% and cloud droplet number column concentration up to 60% occur over the eastern U.S. and extend over the Atlantic Ocean. Precipitation responses occur both locally and remotely, with the models consistently showing an increase in most regions considered. We find a northward shift of the tropical rain belt location of up to 0.35° latitude especially near the Sahel, where the rainy season length and intensity are significantly enhanced in two of the three models. This enhancement is the result of greater warming in the Northern versus Southern Hemisphere, which acts to shift the Intertropical Convergence Zone (ITCZ) northward, delivering additional wet season rainfall to the Sahel. Two of our three models thus imply a previously unconsidered benefit of continued U.S. SO2 reductions for Sahel precipitation.
Barnes, Elizabeth A., Arlene M Fiore, and Larry W Horowitz, May 2016: Detection of trends in surface ozone in the presence of climate variability. Journal of Geophysical Research: Atmospheres, 121(10), DOI:10.1002/2015JD024397. Abstract
Trends in trace atmospheric constituents can be driven by trends in their (precursor) emissions but also by trends in meteorology. Here, we use ground-level ozone as an example to highlight the extent to which unforced, low-frequency climate variability can drive multi-decadal trends. Using output from six experiments of the GFDL chemistry-climate model (CM3), we demonstrate that 20-year trends in surface ozone driven by climate variability alone can be as large as those forced by changes in ozone precursor emissions or by anthropogenic climate change. We highlight regions and seasons where surface ozone is strongly influenced by climate variability, and thus, where a given forced trend may be more difficult to detect. A corollary is that this approach identifies regions and seasons of low variability, where measurement sites may be most effectively deployed to detect a particular trend driven by changing precursor emissions. We find that the RCP4.5 and RCP8.5 forced surface ozone trends in most locations emerge over background variability during the first half of the 21st century. Ozone trends are found to respond mostly to changes in emissions of ozone precursors and unforced climate variability, with a comparatively small impact from anthropogenic climate change. Thus, attempts to attribute observed trends to regional emissions changes require consideration of internal climate variability, particularly for short record lengths and small forced trends.
Global mean temperature over 1998 to 2015 increased at a slower rate (0.1 K decade−1) compared with the ensemble mean (forced) warming rate projected by Coupled Model Intercomparison Project 5 (CMIP5) models (0.2 K decade−1). Here we investigate the prospects for this slower rate to persist for a decade or more. The slower rate could persist if the transient climate response is overestimated by CMIP5 models by a factor of two, as suggested by recent low-end estimates. Alternatively, using CMIP5 models’ warming rate, the slower rate could still persist due to strong multidecadal internal variability cooling. Combining the CMIP5 ensemble warming rate with internal variability episodes from a single climate model—having the strongest multidecadal variability among CMIP5 models—we estimate that the warming slowdown (<0.1 K decade−1 trend beginning in 1998) could persist, due to internal variability cooling, through 2020, 2025 or 2030 with probabilities 16%, 11% and 6%, respectively.
Li, J, Jingqiu Mao, K-E Min, R A Washenfelder, Steven S Brown, J Kaiser, F N Keutsch, R Volkamer, G M Wolfe, T F Hanisco, I B Pollack, Marta Abalos, M Graus, J B Gilman, B M Lerner, C Warneke, J A de Gouw, Ann M Middlebrook, J Liao, A Welti, B H Henderson, V Faye McNeill, S R Hall, K Ullmann, Leo J Donner, Fabien Paulot, and Larry W Horowitz, August 2016: Observational constraints on glyoxal production from isoprene oxidation and its contribution to organic aerosol over the Southeast United States. Journal of Geophysical Research: Atmospheres, 121(16), DOI:10.1002/2016JD025331. Abstract
We use a 0-D photochemical box model and a 3-D global chemistry-climate model, combined with observations from the NOAA Southeast Nexus (SENEX) aircraft campaign, to understand the sources and sinks of glyoxal over the Southeast United States. Box model simulations suggest a large difference in glyoxal production among three isoprene oxidation mechanisms (AM3ST, AM3B, and MCM v3.3.1). These mechanisms are then implemented into a 3-D global chemistry-climate model. Comparison with field observations shows that the average vertical profile of glyoxal is best reproduced by AM3ST with an effective reactive uptake coefficient γglyx of 2 × 10-3, and AM3B without heterogeneous loss of glyoxal. The two mechanisms lead to 0-0.8 µg m-3 secondary organic aerosol (SOA) from glyoxal in the boundary layer of the Southeast U.S. in summer. We consider this to be the lower limit for the contribution of glyoxal to SOA, as other sources of glyoxal other than isoprene are not included in our model. In addition, we find that AM3B shows better agreement on both formaldehyde and the correlation between glyoxal and formaldehyde (RGF = [GLYX]/[HCHO]), resulting from the suppression of δ-isoprene peroxy radicals (δ-ISOPO2). We also find that MCM v3.3.1 may underestimate glyoxal production from isoprene oxidation, in part due to an underestimated yield from the reaction of IEPOX peroxy radicals (IEPOXOO) with HO2. Our work highlights that the gas-phase production of glyoxal represents a large uncertainty in quantifying its contribution to SOA.
Liu, H, D B Considine, and Larry W Horowitz, et al., April 2016: Using beryllium-7 to assess cross-tropopause transport in global models. Atmospheric Chemistry and Physics, 16(7), DOI:10.5194/acp-16-4641-2016. Abstract
We use the Global Modeling Initiative (GMI)
modeling framework to assess the utility of cosmogenic
beryllium-7 (7Be), a natural aerosol tracer, for evaluating
cross-tropopause transport in global models. The GMI
chemical transport model (CTM) was used to simulate atmospheric
7Be distributions using four different meteorological
data sets (GEOS1-STRAT DAS, GISS II0 GCM,
fvGCM, and GEOS4-DAS), featuring significantly different
stratosphere–troposphere exchange (STE) characteristics.
The simulations were compared with the upper troposphere
and/or lower stratosphere (UT/LS) 7Be climatology
constructed from 25 years of aircraft and balloon data, as
well as climatological records of surface concentrations and
deposition fluxes. Comparison of the fraction of surface air
of stratospheric origin estimated from the 7Be simulations
with observationally derived estimates indicates excessive
cross-tropopause transport at mid-latitudes in simulations using
GEOS1-STRAT and at high latitudes using GISS II0 meteorological
data. These simulations also overestimate 7Be
deposition fluxes at mid-latitudes (GEOS1-STRAT) and at
high latitudes (GISS II0), respectively. We show that excessive
cross-tropopause transport of 7Be corresponds to overestimated
stratospheric contribution to tropospheric ozone.
Our perspectives on STE in these meteorological fields based
on 7Be simulations are consistent with previous modeling
studies of tropospheric ozone using the same meteorological
fields. We conclude that the observational constraints for
7Be and observed 7Be total deposition fluxes can be used routinely
as a first-order assessment of cross-tropopause transport
in global models.
Parrish, David D., Ian E Galbally, Jean-Francois Lamarque, Vaishali Naik, and Larry W Horowitz, et al., January 2016: Seasonal cycles of O3 in the marine boundary layer: Observation and model simulation comparisons. Journal of Geophysical Research: Atmospheres, 121(1), DOI:10.1002/2015JD024101. Abstract
We present a two-step approach for quantitatively comparing modeled and measured seasonal cycles of O3: 1) fitting sine functions to monthly averaged measurements and model results (i.e. deriving a Fourier series expansion of these results), and 2) comparing the phase and amplitude of the statistically significant terms between the models and measurements. Two and only two sine terms are sufficient to quantify the O3 seasonal cycle in the marine boundary layer (MBL) in both the measurements and the model results. In addition to the expected fundamental (one sine cycle per year), a 2nd harmonic term (i.e. two sine cycles per year) is identified as a ubiquitous feature of O3 in the MBL. Three chemistry climate models (CAM-chem, GFDL-CM3 and GISS-E2-R) approximately reproduce many features of the measured seasonal cycles at MBL surface sites throughout the globe, with some notable quantitative disagreements, but give divergent results that do not agree with O3 sonde measurements above the MBL. This disagreement and divergence of results between models indicate that the treatment of the MBL dynamics in the CCMs is not adequate to reproduce the isolation of the MBL indicated by the observations. Within the MBL the models more accurately reproduce the second harmonic term than the fundamental term. We attribute the second harmonic term to the second harmonic of opposite phase in the photolysis rate of O3, while the fundamental term evidently has many influences. The parameters derived from the Fourier series expansion of the measurements are quantitative metrics that can serve as the basis for future model-measurement comparisons.
We update and evaluate the treatment of nitrate aerosols in the Geophysical Fluid Dynamics Laboratory (GFDL) atmospheric model (AM3). Accounting for the radiative effects of nitrate aerosols generally improves the simulated aerosol optical depth, although nitrate concentrations at the surface are biased high. This bias can be reduced by increasing the deposition of nitrate to account for the near-surface volatilization of ammonium nitrate or by neglecting the heterogeneous production of nitric acid to account for the inhibition of N2O5 reactive uptake at high nitrate concentrations. Globally, uncertainties in these processes can impact the simulated nitrate optical depth by up to 25 %, much more than the impact of uncertainties in the seasonality of ammonia emissions (6 %) or in the uptake of nitric acid on dust (13 %). Our best estimate for present-day fine nitrate optical depth at 550 nm is 0.006 (0.005–0.008). We only find a modest increase of nitrate optical depth (< 30 %) in response to the projected changes in the emissions of SO2 (−40 %) and ammonia (+38 %) from 2010 to 2050. Nitrate burden is projected to increase in the tropics and in the free troposphere, but to decrease at the surface in the midlatitudes because of lower nitric acid concentrations. Our results suggest that better constraints on the heterogeneous chemistry of nitric acid on dust, on tropical ammonia emissions, and on the transport of ammonia to the free troposphere are needed to improve projections of aerosol optical depth.
The effect of future climate change on surface ozone over North America, Europe, and East Asia is evaluated using present-day (2000s) and future (2100s) hourly surface ozone simulated by four global models. Future climate follows RCP8.5, while methane and anthropogenic ozone precursors are fixed at year-2000 levels. Climate change shifts the seasonal surface ozone peak to earlier in the year and increases the amplitude of the annual cycle. Increases in mean summertime and high-percentile ozone are generally found in polluted environments, while decreases are found in clean environments. We propose climate change augments the efficiency of precursor emissions to generate surface ozone in polluted regions, thus reducing precursor export to neighboring downwind locations. Even with constant biogenic emissions, climate change causes the largest ozone increases at high percentiles. In most cases, air quality extreme episodes become larger and contain higher ozone levels relative to the rest of the distribution.
Silva, R A., J Jason West, Jean-Francois Lamarque, Drew Shindell, William J Collins, S Dalsoren, G Faluvegi, Gerd Folberth, Larry W Horowitz, T Nagashima, and Vaishali Naik, et al., August 2016: The effect of future ambient air pollution on human premature mortality to 2100 using output from the ACCMIP model ensemble. Atmospheric Chemistry and Physics, 16(15), DOI:10.5194/acp-16-9847-2016. Abstract
Ambient air pollution from ground-level ozone and fine particulate matter (PM2.5) is associated with premature mortality. Future concentrations of these air pollutants will be driven by natural and anthropogenic emissions and by climate change. Using anthropogenic and biomass burning emissions projected in the four Representative Concentration Pathway scenarios (RCPs), the ACCMIP ensemble of chemistry-climate models simulated future concentrations of ozone and PM2.5 at selected decades between 2000 and 2100. We use output from the ACCMIP ensemble, together with projections of future population and baseline mortality rates, to quantify the human premature mortality impacts of future ambient air pollution. Future air pollution-related premature mortality in 2030, 2050 and 2100 is estimated for each scenario and for each model using a health impact function based on changes in concentrations of ozone and PM2.5 relative to 2000 and projected future population and baseline mortality rates. Additionally, the global mortality burden of ozone and PM2.5 in 2000 and each future period is estimated relative to 1850 concentrations, using present-day and future population and baseline mortality rates. The change in future ozone concentrations relative to 2000 is associated with excess global premature mortality in some scenarios/periods, particularly in RCP8.5 in 2100 (316 thousand deaths/year), likely driven by the large increase in methane emissions and by the net effect of climate change projected in this scenario, but it leads to considerable avoided premature mortality for the three other RCPs. However, the global mortality burden of ozone markedly increases from less than 0.4 million deaths/year in 2000 to between 1.09 and 2.36 million deaths/year in 2100, across RCPs, mostly due to the effect of increases in population and baseline mortality rates. Decreases in PM2.5 concentrations relative to 2000 are associated with avoided premature mortality in all scenarios, particularly in 2100: between −2.39 and −1.31 million deaths/year for the four RCPs due to the reductions in emissions projected in these scenarios. The global mortality burden of PM2.5 is estimated to decrease from 1.7 million deaths/year in 2000 to between 0.95 and 1.55 million deaths/year in 2100 for the four RCPs, due to the combined effect of decreases in PM2.5 concentrations and changes in population and baseline mortality rates. Trends in future air pollution-related mortality vary regionally across scenarios, reflecting assumptions for economic growth and air pollution control specific to each RCP and region. Mortality estimates differ among chemistry-climate models due to differences in simulated pollutant concentrations, and is the greatest contributor to overall mortality uncertainty for most cases assessed here, supporting the use of model ensembles to characterize uncertainty. Increases in exposed population and baseline mortality rates of respiratory diseases magnify the impact on premature mortality of changes in future air pollutant concentrations and explain why the future global mortality burden of air pollution can exceed the current burden, even where air pollutant concentrations decrease.
Climate change can influence fine particulate matter concentrations (PM2.5) through changes in air pollution meteorology. Knowledge of the extent to which climate change can exacerbate or alleviate air pollution in the future is needed for robust climate and air pollution policy decision-making. To examine the influence of climate on PM2.5, we use the Geophysical Fluid Dynamics Laboratory Coupled Model version 3 (GFDL CM3), a fully-coupled chemistry-climate model, combined with future emissions and concentrations provided by the four Representative Concentration Pathways (RCPs). For each of the RCPs, we conduct future simulations in which emissions of aerosols and their precursors are held at 2005 levels while other climate forcing agents evolve in time, such that only climate (and thus meteorology) can influence PM2.5 surface concentrations. We find a small increase in global, annual mean PM2.5 of about 0.21 μg m−3 (5%) for RCP8.5, a scenario with maximum warming. Changes in global mean PM2.5 are at a maximum in the fall and are mainly controlled by sulfate followed by organic aerosol with minimal influence of black carbon. RCP2.6 is the only scenario that projects a decrease in global PM2.5 with future climate changes, albeit only by −0.06 μg m−3 (1.5%) by the end of the 21st century. Regional and local changes in PM2.5 are larger, reaching upwards of 2 μg m−3 for polluted (eastern China) and dusty (western Africa) locations on an annually averaged basis in RCP8.5. Using multiple linear regression, we find that future PM2.5 concentrations are most sensitive to local temperature, followed by surface wind and precipitation. PM2.5 concentrations are robustly positively associated with temperature, while negatively related with precipitation and wind speed. Present-day (2006–2015) modeled sensitivities of PM2.5 to meteorological variables are evaluated against observations and found to agree reasonably well with observed sensitivities (within 10–50% over the eastern United States for several variables), although the modeled PM2.5 is less sensitive to precipitation than in the observations due to weaker convective scavenging. We conclude that the hypothesized “climate penalty” of future increases in PM2.5 is relatively minor on a global scale compared to the influence of emissions on PM2.5 concentrations.
Wolfe, G M., J Kaiser, T F Hanisco, F N Keutsch, J A de Gouw, J B Gilman, M Graus, C D Hatch, J Holloway, Larry W Horowitz, B H Lee, B M Lerner, Felipe D Lopez-Hilfiker, and Jingqiu Mao, et al., March 2016: Formaldehyde production from isoprene oxidation across NOx regimes. Atmospheric Chemistry and Physics, 16(4), DOI:10.5194/acp-16-2597-2016. Abstract
The chemical link between isoprene and formaldehyde (HCHO) is a strong, nonlinear function of NOx (i.e., NO + NO2). This relationship is a linchpin for top-down isoprene emission inventory verification from orbital HCHO column observations. It is also a benchmark for overall photochemical mechanism performance with regard to VOC oxidation. Using a comprehensive suite of airborne in situ observations over the southeast US, we quantify HCHO production across the urban–rural spectrum. Analysis of isoprene and its major first-generation oxidation products allows us to define both a "prompt" yield of HCHO (molecules of HCHO produced per molecule of freshly emitted isoprene) and the background HCHO mixing ratio (from oxidation of longer-lived hydrocarbons). Over the range of observed NOx values (roughly 0.1–2 ppbv), the prompt yield increases by a factor of 3 (from 0.3 to 0.9 ppbv ppbv−1), while background HCHO increases by a factor of 2 (from 1.6 to 3.3 ppbv). We apply the same method to evaluate the performance of both a global chemical transport model (AM3) and a measurement-constrained 0-D steady-state box model. Both models reproduce the NOx dependence of the prompt HCHO yield, illustrating that models with updated isoprene oxidation mechanisms can adequately capture the link between HCHO and recent isoprene emissions. On the other hand, both models underestimate background HCHO mixing ratios, suggesting missing HCHO precursors, inadequate representation of later-generation isoprene degradation and/or underestimated hydroxyl radical concentrations. Detailed process rates from the box model simulation demonstrate a 3-fold increase in HCHO production across the range of observed NOx values, driven by a 100 % increase in OH and a 40 % increase in branching of organic peroxy radical reactions to produce HCHO.
Zhang, Y, J H Bowden, Z Adelman, Vaishali Naik, Larry W Horowitz, Steven J Smith, and J Jason West, August 2016: Co-benefits of global and regional greenhouse gas mitigation on U.S. air quality in 2050. Atmospheric Chemistry and Physics, 16(15), DOI:10.5194/acp-16-9533-2016. Abstract
Policies to mitigate greenhouse gas (GHG) emissions will not only slow climate change, but can also have ancillary benefits of improved air quality. Here we examine the co-benefits of both global and regional GHG mitigation on U.S. air quality in 2050 at fine resolution, using dynamical downscaling methods, building on a previous global co-benefits study (West et al., 2013). The co-benefits for U.S. air quality are quantified via two mechanisms: through reductions in co-emitted air pollutants from the same sources, and by slowing climate change and its influence on air quality, following West et al. (2013). Additionally, we separate the total co-benefits into contributions from domestic GHG mitigation versus mitigation in foreign countries. We use the WRF model to dynamically downscale future global climate to the regional scale, the SMOKE program to directly process global anthropogenic emissions into the regional domain, and we provide dynamical boundary conditions from global simulations to the regional CMAQ model. The total co-benefits of global GHG mitigation from the RCP4.5 scenario compared with its reference are estimated to be higher in the eastern U.S. (ranging from 0.6–1.0 μg m-3) than the west (0–0.4 μg m-3) for PM2.5, with an average of 0.47 μg m-3 over U.S.; for O3, the total co-benefits are more uniform at 2–5 ppb with U.S. average of 3.55 ppb. Comparing the two mechanisms of co-benefits, we find that reductions of co-emitted air pollutants have a much greater influence on both PM2.5 (96 % of the total co-benefits) and O3 (89 % of the total) than the second co-benefits mechanism via slowing climate change, consistent with West et al. (2013). GHG mitigation from foreign countries contributes more to the U.S. O3 reduction (76 % of the total) than that from domestic GHG mitigation only (24 %), highlighting the importance of global methane reductions and the intercontinental transport of air pollutants. For PM2.5, the benefits of domestic GHG control are greater (74 % of total). Since foreign contributions to the co-benefits are comparable to that from the domestic reductions, especially for O3, previous studies that focus on local or regional co-benefits may greatly underestimate the total co-benefits of global GHG reductions. We conclude that the U.S. can gain significantly greater domestic air quality co-benefits by engaging with other nations to control GHGs.
Zhong, M, E Saikawa, Y Liu, Vaishali Naik, and Larry W Horowitz, et al., April 2016: Air Quality Modeling with WRF-Chem v3.5 in East and South Asia: sensitivity to emissions and evaluation of simulated air quality. Geoscientific Model Development, 9(3), DOI:10.5194/gmd-9-1201-2016. Abstract
We conducted simulations using the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) version 3.5 to study air quality in East and South Asia at a spatial resolution of 20 km × 20 km. We find large discrepancies between two existing emissions inventories: the Regional Emission Inventory in Asia version 2 (REAS) and the Emissions Database for Global Atmospheric Research version 4.2 (EDGAR) at the provincial level in China, with maximum differences up to 500 % for CO emissions, 190 % for NO, and 160 % for primary PM10. Such differences in the magnitude and the spatial distribution of emissions for various species lead to 40–70 % difference in surface PM10 concentrations, 16–20 % in surface O3 mixing ratios, and over 100 % in SO2 and NO2 mixing ratios in the polluted areas of China. Our sensitivity run shows WRF-Chem is sensitive to emissions, with the REAS-based simulation reproducing observed concentrations and mixing ratios better than the EDGAR-based simulation for July 2007. We conduct further model simulations using REAS emissions for January, April, July, and October in 2007 and evaluate simulations with available ground-level observations. The model results show clear regional variations in the seasonal cycle of surface PM10 and O3 over East and South Asia. The model meets the air quality model performance criteria for both PM10 (mean fractional bias, MFB ≤ ± 60 %) and O3 (MFB ≤ ± 15 %) in most of the observation sites, although the model underestimates PM10 over Northeast China in January. The model predicts the observed SO2 well at sites in Japan, while it tends to overestimate SO2 in China in July and October. The model underestimates most observed NO2 in all four months. These findings suggest that future model development and evaluation of emission inventories and models are needed for particulate matter and gaseous pollutants in East and South Asia.
The Arctic troposphere has warmed faster than the global average over the last several decades. It was suggested that atmospheric northward energy transport (ANET) into the Arctic had contributed to tropospheric warming in the Arctic. Here we calculate ANET based on the NCEP/NCAR reanalysis data from 1979 to 2012. During this period the zonally integrated energy flux into the Arctic has decreased rather than increased in all seasons. However, the trends are statistically insignificant except for the winter and annual mean fluxes. For the winter season, the transient eddy flux of energy increases over Greenland and the Greenland Sea and decreases over west-central Siberia (WCS). Trends in meridional wind variance and vorticity also indicate increasing transient eddy activity over Northern Canada, the Greenland Sea and the Norwegian Sea and decreasing activity over WCS. Inter-winter variations in local vorticity over the WCS are significantly anti-correlated with the Arctic climate.
Lin, Meiyun, Arlene M Fiore, Larry W Horowitz, Andrew O Langford, S J Oltmans, D W Tarasick, and H E Rieder, May 2015: Climate variability modulates western US ozone air quality in spring via deep stratospheric intrusions. Nature Communications, 6, 7105, DOI:10.1038/ncomms8105. Abstract
Evidence suggests deep stratospheric intrusions can elevate western US surface ozone to unhealthy levels during spring. These intrusions can be classified as ‘exceptional events’, which are not counted towards non-attainment determinations. Understanding the factors driving the year-to-year variability of these intrusions is thus relevant for effective implementation of the US ozone air quality standard. Here we use observations and model simulations to link these events to modes of climate variability. We show more frequent late spring stratospheric intrusions when the polar jet meanders towards the western United States, such as occurs following strong La Niña winters (Niño3.4<−1.0 °C). While El Niño leads to enhancements of upper tropospheric ozone, we find this influence does not reach surface air. Fewer and weaker intrusion events follow in the two springs after the 1991 volcanic eruption of Mt. Pinatubo. The linkage between La Niña and western US stratospheric intrusions can be exploited to provide a few months of lead time during which preparations could be made to deploy targeted measurements aimed at identifying these exceptional events.
Lin, Meiyun, and Larry W Horowitz, et al., October 2015: Revisiting the evidence of increasing springtime ozone mixing ratios in the free troposphere over western North America. Geophysical Research Letters, 42(20), DOI:10.1002/2015GL065311. Abstract
We present a 20-year time series of in-situ free tropospheric ozone observations above western North America during springtime and interpret results using the GFDL-AM3 chemistry-climate model hindcast simulations (1980–2014). Revisiting the analysis of Cooper et al. [2010], we show that sampling biases can substantially influence calculated trends. AM3 co-sampled in space and time with observations reproduces the observed ozone trend (0.65±0.32 ppbv year−1) over 1995–2008 (in simulations either with or without time-varying emissions), whereas AM3 ‘true median’ with continuous temporal and spatial sampling indicates an insignificant trend (0.25±0.32 ppbv year−1). Extending this analysis to 1995–2014, we find a weaker ozone trend of 0.31±0.21 ppbv year−1 from observations and 0.36±0.18 ppbv year−1 from AM3 ‘true median’. Rising Asian emissions and global methane contribute to this increase. While interannual variability complicates the attribution of ozone trends, multi-decadal hindcasts can aid in the estimation of robust confidence limits for trends based on sparse observational records.
Rieder, H E., Arlene M Fiore, Larry W Horowitz, and Vaishali Naik, January 2015: Projecting policy-relevant metrics for high summertime ozone pollution events over the eastern United States due to climate and emission changes during the 21st century. Journal of Geophysical Research: Atmospheres, 120(2), DOI:10.1002/2014JD022303. Abstract
Over the eastern United States (EUS), nitrogen oxides (NOx) emission controls have led to improved air quality over the past two decades, but concerns have been raised that climate warming may offset some of these gains. Here we analyze the effect of changing emissions and climate, in isolation and combination, on EUS summertime surface ozone (O3) over the recent past and the 21st century in an ensemble of simulations performed with the Geophysical Fluid Dynamics Laboratory CM3 chemistry-climate model. The simulated summertime EUS O3 is biased high but captures the structure of observed changes in regional O3 distributions following NOx emission reductions. We introduce a statistical bias correction, which allows derivation of policy-relevant statistics by assuming a stationary mean state bias in the model, but accurate simulation of changes at each quantile of the distribution. We contrast two different 21st century scenarios: (i) representative concentration pathway (RCP) 4.5 and (ii) simulations with well-mixed greenhouse gases (WMGG) following RCP4.5 but with emissions of air pollutants and precursors held fixed at 2005 levels (RCP4.5_WMGG). We find under RCP4.5 no exceedance of maximum daily 8 hour average ozone above 75 ppb by mid-21st century, reflecting the U.S. NOx emissions reductions projected in RCP4.5, while more than half of the EUS exceeds this level by the end of the 21st century under RCP4.5_WMGG. Further, we find a simple relationship between the changes in estimated 1 year return levels and regional NOx emission changes, implying that our results can be generalized to estimate changes in the frequency of EUS pollution events under different regional NOx emission scenarios.
We test the current generation of global chemistry-climate models in their ability to simulate observed, present-day surface ozone. Models are evaluated against hourly surface ozone from 4217 stations in North America and Europe that are averaged over 1° × 1° grid cells, allowing commensurate model-measurement comparison. Models are generally biased high during all hours of the day and in all regions. Most models simulate the shape of regional summertime diurnal and annual cycles well, correctly matching the timing of hourly (~ 15:00) and monthly (mid-June) peak surface ozone abundance. The amplitude of these cycles is less successfully matched. The observed summertime diurnal range (~ 25 ppb) is underestimated in all regions by about 7 ppb, and the observed seasonal range (~ 21 ppb) is underestimated by about 5 ppb except in the most polluted regions where it is overestimated by about 5 ppb. The models generally match the pattern of the observed summertime ozone enhancement, but they overestimate its magnitude in most regions. Most models capture the observed distribution of extreme episode sizes, correctly showing that about 80% of individual extreme events occur in large-scale, multi-day episodes of more than 100 grid cells. The observed linear relationship showing increases in ozone by up to 6 ppb for larger-sized episodes is also matched.
Westervelt, Daniel M., Larry W Horowitz, Vaishali Naik, and D L Mauzerall, November 2015: Radiative forcing and climate response to projected 21st century aerosol decreases. Atmospheric Chemistry and Physics, 15(22), DOI:10.5194/acp-15-12681-2015. Abstract
It is widely expected that global emissions of atmospheric aerosols and their precursors will decrease strongly throughout the remainder of the 21st century, due to emission reduction policies enacted to protect human health. For instance, global emissions of aerosols and their precursors are projected to decrease by as much as 80% by the year 2100, according to the four Representative Concentration Pathway (RCP) scenarios. The removal of aerosols will cause unintended climate consequences, including an unmasking of global warming from long-lived greenhouse gases. We use the Geophysical Fluid Dynamics Laboratory Climate Model version 3 (GFDL CM3) to simulate future climate over the 21st century with and without the aerosol emission changes projected by each of the RCPs in order to isolate the radiative forcing and climate response resulting from the aerosol reductions. We find that the projected global radiative forcing and climate response due to aerosol decreases do not vary significantly across the four RCPs by 2100, although there is some mid-century variation, especially in cloud droplet effective radius, that closely follows the RCP emissions and energy consumption projections. Up to 1 W m−2 of radiative forcing may be unmasked globally from 2005 to 2100 due to reductions in aerosol and precursor emissions, leading to average global temperature increases up to 1 K and global precipitation rate increases up to 0.09 mm d−1. Regionally and locally, climate impacts can be much larger, with a 2.1 K warming projected over China, Japan, and Korea due to the reduced aerosol emissions in RCP8.5, as well as nearly a 0.2 mm d−1 precipitation increase, a 7 g m−2 LWP decrease, and a 2 μm increase in cloud droplet effective radius. Future aerosol decreases could be responsible for 30–40% of total climate warming by 2100 in East Asia, even under the high greenhouse gas emissions scenario (RCP8.5). The expected unmasking of global warming caused by aerosol reductions will require more aggressive greenhouse gas mitigation policies than anticipated in order to meet desired climate targets.
Changing emissions can alter the surface O3 seasonal cycle, as detected from Northeastern U.S. (NE) observations during recent decades. Under continued regional precursor emission controls (-72% NE NOx by 2100), the NE surface O3 seasonal cycle reverses (to a winter maximum) in 21st Century transient chemistry-climate simulations. Over polluted regions, regional NOx largely controls the shape of surface O3 seasonal cycles. In the absence of regional NOx controls, climate warming contributes to a higher surface O3 summertime peak over the NE. A doubling of the global CH4 abundance by 2100 partially offsets summertime surface O3 decreases attained via NOx reductions and contributes to raising surface O3 during December-March when the O3 lifetime is longer. The similarity between surface O3 seasonal cycles over the NE and the InterMountain West by 2100 indicates a NE transition to a region representative of baseline surface O3 conditions.
Cooper, Owen R., David D Parrish, J R Ziemke, N V Balashov, M Cupeiro, Ian E Galbally, S Gilge, Larry W Horowitz, N R Jensen, Jean-Francois Lamarque, and Vaishali Naik, et al., July 2014: Global distribution and trends of tropospheric ozone: An observation-based review. Elementa: Science of the Anthropocene, 2, 000029, DOI:10.12952/journal.elementa.000029. Abstract
Tropospheric ozone plays a major role in Earth’s atmospheric chemistry processes and also acts as an air pollutant and greenhouse gas. Due to its short lifetime, and dependence on sunlight and precursor emissions from natural and anthropogenic sources, tropospheric ozone’s abundance is highly variable in space and time on seasonal, interannual and decadal time-scales. Recent, and sometimes rapid, changes in observed ozone mixing ratios and ozone precursor emissions inspired us to produce this up-to-date overview of tropospheric ozone’s global distribution and trends. Much of the text is a synthesis of in situ and remotely sensed ozone observations reported in the peer-reviewed literature, but we also include some new and extended analyses using well-known and referenced datasets to draw connections between ozone trends and distributions in different regions of the world. In addition, we provide a brief evaluation of the accuracy of rural or remote surface ozone trends calculated by three state-of-the-science chemistry-climate models, the tools used by scientists to fill the gaps in our knowledge of global tropospheric ozone distribution and trends.
Accurate estimates for North American background (NAB) ozone (O3) in surface air over the United States are needed for setting and implementing an attainable national O3 standard. These estimates rely on simulations with atmospheric chemistry-transport models that set North American anthropogenic emissions to zero, and to date have relied heavily on one global model. We examine NAB estimates for spring and summer 2006 with two independent global models (GEOS-Chem and GFDL AM3). We evaluate the base simulations, which include North American anthropogenic emissions, with mid-tropospheric O3 retrieved from space and ground-level O3 measurements. The models often bracket the observed values, implying value in developing a multi-model approach to estimate NAB O3. Consistent with earlier studies, the models robustly simulate the largest nation-wide NAB levels at high-altitude western U.S. sites (seasonal average maximum daily 8-h values of ∼40–50 ppb in spring and ∼25–40 ppb in summer) where it correlates with observed O3. At these sites, a 27-year GFDL AM3 simulation simulates observed O3 events above 60 ppb and indicates that year-to-year variations in NAB O3 influence their annual frequency (with NAB contributing 50–60 ppb or more during individual events). During summer over the eastern United States (EUS), when photochemical production from regional anthropogenic emissions peaks, NAB is largely uncorrelated with observed values and it is lower than at high-altitude sites (average values of ∼20–30 ppb). Four processes contribute substantially to model differences in specific regions and seasons: lightning NOx, biogenic isoprene emissions and chemistry, wildfires, and stratosphere-to-troposphere transport. Differences in the representations of these processes within the GFDL AM3 and GEOS-Chem models contribute more to uncertainty in NAB estimates, particularly in spring when NAB is highest, than the choice of horizontal resolution within a single model (GEOS-Chem). We propose that future efforts seek to constrain these processes with targeted analysis of multi-model simulations evaluated with observations of O3 and related species from multiple platforms, and thereby reduce the error on NAB estimates needed for air quality planning.
Li, X, Junfeng Liu, D L Mauzerall, Louisa K Emmons, S Walters, Larry W Horowitz, and X Tao, November 2014: Effects of trans-Eurasian transport of air pollutants on surface ozone concentrations over Western China. Journal of Geophysical Research: Atmospheres, 119(21), DOI:10.1002/2014JD021936. Abstract
Due to a lack of industrialization in western China, surface air there was, until recently, believed to be relatively unpolluted. However, recent measurements and modeling studies have found high levels of ozone (O3) there. Based on the state-of-the-science global chemical transport model MOZART-4, we identify the origin, pathway, and mechanism of trans-Eurasian transport of air pollutants to Western China in 2000. MOZART-4 generally simulates well the observed surface O3 over inland areas of China. Simulations find surface ozone concentrations over Western China on average to be about 10 ppbv higher than Eastern China. Using sensitivity studies, we find that anthropogenic emissions from all Eurasian regions except China contribute 10–15 ppbv surface O3 over western China, superimposed upon a 35–40 ppbv natural background. Transport from European anthropogenic sources to Northwestern China results in 2–6 ppbv O3 enhancements in spring and summer. Indian anthropogenic sources strongly influence O3 over the Tibetan Plateau during the summer monsoon. Transport of O3 originating from emissions in the Middle East occasionally reach Western China and increase surface ozone there by about 1–4 ppbv. These influences are of similar magnitude as trans-Pacific and trans-Atlantic transport of O3 and its precursors, indicating the significance of trans-Eurasian ozone transport in hemispheric transport of air pollution. Our study further indicates that mitigation of anthropogenic emissions from Europe, the Indian Subcontinent and the Middle East could benefit public health and agricultural productivity in Western China.
A potent greenhouse gas and biological irritant, tropospheric ozone is also the primary source of atmospheric hydroxyl radicals, which remove numerous hazardous trace gases from the atmosphere. Tropospheric ozone levels have increased in spring at remote sites in the mid-latitudes of the Northern Hemisphere over the past few decades; this increase has been attributed to a growth in Asian precursor emissions. In contrast, 40 years of continuous measurements at Mauna Loa Observatory in Hawaii reveal little change in tropospheric ozone levels during spring (March–April), but a rise in autumn (September–October). Here we examine the contribution of decadal shifts in atmospheric circulation patterns to decadal variability in tropospheric ozone levels at Mauna Loa using a suite of chemistry–climate model simulations. We show that the flow of ozone-rich air from Eurasia towards Hawaii during spring weakened in the 2000s as a result of La-Niña-like decadal cooling in the eastern equatorial Pacific Ocean. During autumn, in contrast, the flow of ozone-rich air from Eurasia to Hawaii strengthened in the mid-1990s onwards, coincident with the positive phase of the Pacific–North American pattern. We suggest that these shifts in atmospheric circulation patterns can reconcile observed trends in tropospheric ozone levels at Mauna Loa and the northern mid-latitudes in recent decades. We conclude that decadal variability in atmospheric circulation patterns needs to be considered when attributing observed changes in tropospheric ozone levels to human-induced trends in precursor emissions.
Coupled climate model simulations of volcanic eruptions and abrupt changes in CO2 concentration are compared in multiple realizations of Geophysical Fluid Dynamics Laboratory’s (GFDL) CM2.1. The change in global-mean surface temperature (GMST) is analyzed to determine whether a fast component of the climate sensitivity of relevance to the transient climate response (TCR, defined with the 1% yr−1 CO2-increase scenario) can be estimated from shorter-timescale climate changes. The fast component of the climate sensitivity estimated from the response of the climate model to volcanic forcing is similar to that of the simulations forced by abrupt CO2 changes, but is 5–15% smaller than the TCR. In addition, the partition between the top-of-atmosphere radiative restoring and ocean heat uptake is similar across radiative forcing agents. The possible asymmetry between warming and cooling climate perturbations, which may affect the utility of volcanic eruptions for estimating the TCR, is assessed by comparing simulations of abrupt CO2 doubling to abrupt CO2 halving. There is slightly less (~5%) GMST change in 0.5 × CO2 simulations than in 2 × CO2 simulations on the short (~10-yr) timescales relevant to the fast component of the volcanic signal. However, inferring the TCR from volcanic eruptions is more sensitive to uncertainties from internal climate variability and the estimation procedure.
The response of the GMST to volcanic eruptions is similar in GFDL’s CM2.1 and CM3, even though the latter has a higher TCR associated with a multidecadal timescale in its response. This is consistent with the expectation that the fast component of the climate sensitivity inferred from volcanic eruptions is a lower bound for the TCR.
Parrish, David D., Jean-Francois Lamarque, Vaishali Naik, and Larry W Horowitz, et al., May 2014: Long-term changes in lower tropospheric baseline ozone concentrations: Comparing chemistry-climate models and observations at northern mid-latitudes. Journal of Geophysical Research: Atmospheres, 119(9), DOI:10.1002/2013JD021435. Abstract
Two recent papers have quantified long-term ozone (O3) changes observed at northern mid-latitude sites that are believed to represent baseline (here understood as representative of continental to hemispheric scales) conditions. Three chemistry climate models (NCAR CAM-chem, GFDL-CM3 and GISS-E2-R) have calculated retrospective tropospheric O3 concentrations as part of the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP) and Coupled Model Intercomparison Project Phase 5 (CMIP5) model intercomparisons. We present an approach for quantitative comparisons of model results with measurements for seasonally averaged O3 concentrations. There is considerable qualitative agreement between the measurements and the models, but there are also substantial and consistent quantitative disagreements. Most notably models 1) overestimate absolute O3 mixing ratios, on average by ~5 to 17 ppbv in the year 2000, 2) capture only ~50% of O3 changes observed over the past five to six decades, and little of observed seasonal differences, and 3) capture ~25 to 45% of the rate of change of the long-term changes. These disagreements are significant enough to indicate that only limited confidence can be placed on estimates of present-day radiative forcing of tropospheric O3 derived from modeled historic concentration changes and on predicted future O3 concentrations. Evidently our understanding of tropospheric O3, or the incorporation of chemistry and transport processes into current chemical climate models, is incomplete. Modeled O3 trends approximately parallel estimated trends in anthropogenic emissions of NOX, an important O3 precursor, while measured O3 changes increase more rapidly than these emission estimates.
Rotstayn, L D., E L Plymin, Mark A Collier, Olivier Boucher, J-L Dufresne, J-J Luo, K von Salzen, S J Jeffrey, M-A Foujols, Yi Ming, and Larry W Horowitz, September 2014: Declining aerosols in CMIP5 projections: effects on atmospheric temperature structure and midlatitude jets. Journal of Climate, 27(18), DOI:10.1175/JCLI-D-14-00258.1. Abstract
We assess the effects of declining anthropogenic aerosols in Representative Concentration Pathway 4.5 (RCP4.5) in four models from the Coupled Model Intercomparison Project Phase 5 (CMIP5), with a focus on annual, zonal-mean atmospheric temperature structure and zonal winds. For each model, we diagnose the effect of declining aerosols from the difference between a projection forced by RCP4.5 for 2006–2100 and another that has identical forcing, except that anthropogenic aerosols are fixed at early 21st century levels. The response to declining aerosols is interpreted in terms of the meridional structure of aerosol radiative forcing, which peaks near 40°N and vanishes at the South Pole.
Increasing greenhouse gases cause amplified warming in the tropical upper troposphere and strengthening midlatitude jets in both hemispheres. However, for declining aerosols the vertically averaged tropospheric temperature response peaks near 40°N, rather than in the tropics. This implies that for declining aerosols the tropospheric meridional temperature gradient generally increases in the Southern Hemisphere (SH), but in the Northern Hemisphere (NH) it decreases in the tropics and subtropics. Consistent with thermal wind balance, the NH jet then strengthens on its poleward side and weakens on its equatorward side, whereas the SH jet strengthens more than the NH jet. The asymmetric response of the jets is thus consistent with the meridional structure of aerosol radiative forcing and the associated tropospheric warming: in the NH the latitude of maximum warming is roughly collocated with the jet, whereas in the SH warming is strongest in the tropics and weakest at high latitudes.
Long-range transport of black carbon (BC) is a growing concern as a result of the efficiency of BC in warming the climate and its adverse impact on human health. We study transpacific transport of BC during HIPPO-3 using a combination of inverse modeling and sensitivity analysis. We use the GEOS-Chem chemical transport model and its adjoint to constrain Asian BC emissions and estimate the source of BC over the North Pacific. We find that different sources of BC dominate the transport to the North Pacific during the southbound (29 March 2010) and northbound (13 April 2010) measurements in HIPPO-3. While biomass burning in Southeast Asia (SE) contributes about 60% of BC in March, more than 90% of BC comes from fossil fuel and biofuel combustion in East Asia (EA) during the April mission. GEOS-Chem simulations generally resolve the spatial and temporal variation of BC concentrations over the North Pacific, but are unable to reproduce the low and high tails of the observed BC distribution. We find that the optimized BC emissions derived from inverse modeling fail to improve model simulations significantly. This failure indicates that uncertainties in BC transport, rather than in emissions, account for the major biases in GEOS-Chem simulations of BC.
The aging process, transforming BC from hydrophobic into hydrophilic form, is one of the key factors controlling wet scavenging and remote concentrations of BC. Sensitivity tests on BC aging suggest that the aging time scale of anthropogenic BC from EA is several hours, faster than assumed in most global models, while the aging process of biomass burning BC from SE may occur much slower, with a time scale of a few days. To evaluate the effects of BC aging and wet deposition on transpacific transport of BC, we develop an idealized model of BC transport. We find that the mid-latitude air masses sampled during HIPPO-3 may have experienced a series of precipitation events, particularly near the EA and SE source region. Transpacific transport of BC is sensitive to BC aging when the aging rate is fast; this sensitivity peaks when the aging time scale is in the range of 1–1.5 d. Our findings indicate that BC aging close to the source must be simulated accurately at a process level in order to simulate better the global abundance and climate forcing of BC.
Results from the simulation of a coupled chemistry–climate model are presented for the period 1860 to 2005 using the observed greenhouse gas (GHG) and halocarbon concentrations. The model is coupled to a simulated ocean and uniquely includes both detailed tropospheric chemistry and detailed middle atmosphere chemistry, seamlessly from the surface to the model top layer centered at 0.02 hPa. It is found that there are only minor changes in simulated stratospheric temperature and ozone prior to the year 1960. As the halocarbon amounts increase after 1970, the model stratospheric ozone decreases approximately continuously until about 2000. The steadily increasing GHG concentrations cool the stratosphere from the beginning of the twentieth century at a rate that increases with height. During the early period the cooling leads to increased stratospheric ozone. The model results show a strong, albeit temporary, response to volcanic eruptions. While chlorofluorocarbon (CFC) concentrations remain low, the effect of eruptions is shown to increase the amount of HNO3, reducing ozone destruction by the NOx catalytic cycle. In the presence of anthropogenic chlorine, after the eruption of El Chichón and Mt. Pinatubo, chlorine radicals increased and the chlorine reservoirs decreased. The net volcanic effect on nitrogen and chlorine chemistry depends on altitude and, for these two volcanoes, leads to an ozone increase in the middle stratosphere and a decrease in the lower stratosphere. Model lower-stratospheric temperatures are also shown to increase during the last three major volcanic eruptions, by about 0.6 K in the global and annual average, consistent with observations.
Bowman, K W., Larry W Horowitz, and Vaishali Naik, April 2013: Evaluation of ACCMIP outgoing longwave radiation from tropospheric ozone using TES satellite observations. Atmospheric Chemistry and Physics, 13(8), DOI:10.5194/acp-13-4057-2013. Abstract
We use simultaneous observations of tropospheric ozone and outgoing longwave radiation (OLR) sensitivity to tropospheric ozone from the Tropospheric Emission Spectrometer (TES) to evaluate model tropospheric ozone and its effect on OLR simulated by a suite of chemistry-climate models that participated in the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP). The ensemble mean of ACCMIP models show a persistent but modest tropospheric ozone low bias (5–20 ppb) in the Southern Hemisphere (SH) and modest high bias (5–10 ppb) in the Northern Hemisphere (NH) relative to TES ozone for 2005–2010. These ozone biases have a significant impact on the OLR. Using TES instantaneous radiative kernels (IRK), we show that the ACCMIP ensemble mean tropospheric ozone low bias leads up to 120 mW m−2 OLR high bias locally but zonally compensating errors reduce the global OLR high bias to 39 ± 41 m Wm−2 relative to TES data. We show that there is a correlation (R2 = 0.59) between the magnitude of the ACCMIP OLR bias and the deviation of the ACCMIP preindustrial to present day (1750–2010) ozone radiative forcing (RF) from the ensemble ozone RF mean. However, this correlation is driven primarily by models whose absolute OLR bias from tropospheric ozone exceeds 100 m Wm−2. Removing these models leads to a mean ozone radiative forcing of 394 ± 42 m Wm−2. The mean is about the same and the standard deviation is about 30% lower than an ensemble ozone RF of 384 ± 60 m Wm−2 derived from 14 of the 16 ACCMIP models reported in a companion ACCMIP study. These results point towards a profitable direction of combining satellite observations and chemistry-climate model simulations to reduce uncertainty in ozone radiative forcing.
Eyring, Veronika, and Larry W Horowitz, et al., May 2013: Long-term ozone changes and associated climate impacts in CMIP5 simulations. Journal of Geophysical Research: Atmospheres, 118(10), DOI:10.1002/jgrd.50316. Abstract
Ozone changes and associated climate impacts in the Coupled Model Intercomparison Project Phase 5 (CMIP5) simulations are analyzed over the historical (1960-2005) and future (2006-2100) period under four Representative Concentration Pathways (RCP). In contrast to CMIP3, where half of the models prescribed constant stratospheric ozone, CMIP5 models all consider past ozone depletion and future ozone recovery. Multimodel mean climatologies and long-term changes in total and tropospheric column ozone calculated from CMIP5 models with either interactive or prescribed ozone are in reasonable agreement with observations. However, some large deviations from observations exist for individual models with interactive chemistry, and these models are excluded in the projections. Stratospheric ozone projections forced with a single halogen but four greenhouse gas (GHG) scenarios show largest differences in the northern midlatitudes and in the Arctic in spring (~20 and 40 DU by 2100, respectively). By 2050 these differences are much smaller and negligible over Antarctica in austral spring. Differences in future tropospheric column ozone are mainly caused by differences in methane concentrations and stratospheric input, leading to ~10 DU increases compared to 2000 in RCP 8.5. Large variations in stratospheric ozone particularly in CMIP5 models with interactive chemistry drive correspondingly large variations in lower stratospheric temperature trends. The results also illustrate that future SH summer-time circulation changes are controlled by both the ozone recovery rate and the rate of GHG increases, emphasizing the importance of simulating and taking into account ozone forcings when examining future climate projections.
Fang, Y, Vaishali Naik, Larry W Horowitz, and D L Mauzerall, February 2013: Air pollution and associated human mortality: the role of air pollutant emissions, climate change and methane concentration increases from the preindustrial period to present. Atmospheric Chemistry and Physics, 13(3), DOI:10.5194/acp-13-1377-2013. Abstract
Increases in surface ozone (O3) and fine particulate matter (≤2.5 μm} aerodynamic diameter, PM2.5) are associated with excess premature human mortalities. Here we estimate changes in surface O3 and PM2.5 since preindustrial (1860) times and the global present-day (2000) premature human mortalities associated with these changes. We go beyond previous work to analyze and differentiate the contribution of three factors: changes in emissions of short-lived air pollutants, climate change, and increased methane (CH4) concentrations, to air pollution levels and the associated premature mortalities. We use a coupled chemistry-climate model in conjunction with global population distributions in 2000 to estimate exposure attributable to concentration changes since 1860 from each factor. Attributable mortalities are estimated using health impact functions of long-term relative risk estimates for O3 and PM2.5 from the epidemiology literature. We find global mean surface PM2.5 and health-relevant O3 (defined as the maximum 6-month mean of 1-h daily maximum O3 in a year) have increased by 8 ± 0.16 μg m−3 and 30 ± 0.16 ppbv, respectively, over this industrial period as a result of combined changes in emissions of air pollutants (EMIS), climate (CLIM) and CH4 concentrations (TCH4). EMIS, CLIM and TCH4 cause global average PM2.5(O3) to change by +7.5 ± 0.19 μg m−3 (+25 ± 0.30 ppbv), +0.4 ± 0.17 μg m−3 (+0.5 ± 0.28 ppbv), and −0.02 ± 0.01 μg m−3 (+4.3 ± 0.33 ppbv), respectively. Total changes in PM2.5 are associated with 1.5 (95% confidence interval, CI, 1.0–2.5) million all-cause mortalities annually and in O3 are associated with 375 (95% CI, 129–592) thousand respiratory mortalities annually. Most air pollution mortality is driven by changes in emissions of short-lived air pollutants and their precursors (95% and 85% of mortalities from PM2.5 and O3, respectively). However, changing climate and increasing CH4 concentrations also increased premature mortality associated with air pollution globally up to 5% and 15%, respectively. In some regions, the contribution of climate change and increased CH4 together are responsible for more than 20% of the respiratory mortality associated with O3 exposure. We find the interaction between climate change and atmospheric chemistry has influenced atmospheric composition and human mortality associated with industrial air pollution. In addition to driving 13% of the total historical changes in surface O3 and 15% of the associated mortalities, CH4 is the dominant factor driving changes in atmospheric OH and H2O2 since preindustrial time. Our study highlights the benefits to air quality and human health of CH4 mitigation as a component of future air pollution control policy.
Fang, Y, Arlene M Fiore, Jean-Francois Lamarque, Larry W Horowitz, and Meiyun Lin, February 2013: Using synthetic tracers as a proxy for summertime PM2.5 air quality over the Northeastern United States in physical climate models. Geophysical Research Letters, 40(4), DOI:10.1002/GRL.50162. Abstract
Fine particulate matter (PM2.5) is a criteria pollutant. Its sensitivity to meteorology implies its distribution will likely change with climate shifts. Limited availability of global climate models with full chemistry complicates efforts to assess rigorously the uncertainties in the PM2.5 response to a warming climate. We evaluate the potential for PM2.5 distributions in a chemistry-climate model under current-day and warmer climate conditions over the Northeastern United States to be represented by a Synthetic Aerosol tracer (SAt). The SAt implemented into the GFDL chemistry-climate model (AM3) follows the protocol of a recent multi-model community effort (HTAP), with CO emissions, 25-day chemical lifetime and wet deposition rate of sulfate. Over the Northeastern United States, the summer daily time series of SAt correlates strongly with that of PM2.5, with similar cumulative density functions, under both present and future climate conditions. With a linear regression model derived from PM2.5 and SAt in the current-day simulation, we reconstruct both the current-day and future PM2.5 daily time series from the simulated SAt. This reconstruction captures the summer mean PM2.5, the incidence of days above the 24-h mean PM2.5 NAAQS, as well as PM2.5 responses to climate change. This reconstruction also works over other polluted Northern Hemispheric regions and in spring. Our proof-of-concept study demonstrates that simple tracers can be developed to mimic PM2.5, including its response to climate change, as an easy-to-implement and low-cost addition to physical climate models that should help air quality managers to reap the benefits of climate models that have no chemistry.
Climate change modulates surface concentrations of fine particulate matter (PM2.5) and ozone (O3), indirectly affecting premature mortality attributed to air pollution. We estimate the change in global premature mortality and years of life lost (YLL) associated with changes in surface O3 and PM2.5 over the 21st century as a result of climate change. We use a global coupled chemistry-climate model to simulate current and future climate and the effect of changing climate on air quality. Epidemiological concentration-response relationships are applied to estimate resulting changes in premature mortality and YLL. The effect of climate change on air quality is isolated by holding emissions of air pollutants constant while allowing climate to evolve over the 21st century according to a moderate projection of greenhouse gas emissions (A1B scenario). Resulting changes in 21st century climate alone lead to an increase in simulated PM2.5 concentrations globally, and to higher (lower) O3 concentrations over populated (remote) regions. Global annual premature mortality associated with chronic exposure to PM2.5 increases by approximately 100 thousand deaths (95 % confidence interval, CI, of 66–130 thousand) with corresponding YLL increasing by nearly 900 thousand (95 % CI, 576–1,128 thousand) years. The annual premature mortality due to respiratory disease associated with chronic O3 exposure increases by +6,300 deaths (95 % CI, 1,600–10,400). This climate penalty indicates that stronger emission controls will be needed in the future to meet current air quality standards and to avoid higher health risks associated with climate change induced worsening of air quality over populated regions.
[1] Climate models incorporate a number of adjustable parameters in their cloud formulations. They arise from uncertainties in cloud processes. These parameters are tuned to achieve a desired radiation balance and to best reproduce the observed climate. A given radiation balance can be achieved by multiple combinations of parameters.
[2] We investigate the impact of cloud tuning in the CMIP5 GFDL CM3 coupled climate model by constructing two alternate configurations. They achieve the desired radiation balance using different, but plausible, combinations of parameters. The present-day climate is nearly indistinguishable among all configurations. However, the magnitude of the aerosol indirect effects differs by as much as 1.2 Wm− 2, resulting in significantly different temperature evolution over the 20th century.
Secondary organic aerosols (SOA) exert a significant influence on ambient air quality and regional climate. Recent field, laboratorial and modeling studies have confirmed that in-cloud processes contribute to a large fraction of SOA production. This study evaluates the key factors that govern the production of cloud-process SOA (SOAcld) in a global scale based on the GFDL coupled chemistry-climate model AM3 in which full cloud chemistry is employed. The association between SOAcld production rate and six factors (i.e. liquid water content (LWC), total carbon chemical loss rate (TCloss), temperature, VOC/NOx, OH, and O3) is examined. We find that LWC alone determines the spatial pattern of SOAcld production, particularly over the tropical, subtropical and temperate forest regions, and is strongly correlated with SOAcld production. TCloss ranks the second and mainly represents the seasonal variability of vegetation growth. Other individual factors are essentially uncorrelated to SOAcld production. We find that the rate of SOAcld production is simultaneously determined by both LWC and TCloss, but responds linearly to LWC and nonlinearly (or concavely) to TCloss. A parameterization based on LWC and TCloss can capture well the spatial and temporal variability of the process-based SOAcld formation (R2=0.5) and can be easily applied to global three dimensional models to represent the SOA production from cloud processes.
Lamarque, Jean-Francois, Larry W Horowitz, and Vaishali Naik, et al., February 2013: The Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP): overview and description of models, simulations and climate diagnostics. Geoscientific Model Development, DOI:10.5194/gmd-6-179-2013. Abstract
The Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP) consists of a series of timeslice experiments targeting the long-term changes in atmospheric composition between 1850 and 2100, with the goal of documenting radiative forcing and the associated composition changes. Here we introduce the various simulations performed under ACCMIP and the associated model output. The ACCMIP models have a wide range of horizontal and vertical resolutions, vertical extent, chemistry schemes and interaction with radiation and clouds. While anthropogenic and biomass burning emissions were specified for all time slices in the ACCMIP protocol, it is found that the natural emissions lead to a significant range in emissions, mostly for ozone precursors. The analysis of selected present-day climate diagnostics (precipitation, temperature, specific humidity and zonal wind) reveals biases consistent with state-of-the-art climate models. The model-to-model comparison of changes in temperature, specific humidity and zonal wind between 1850 and 2000 and between 2000 and 2100 indicates mostly consistent results, but with outliers different enough to possibly affect their representation of climate impact on chemistry.
Lee, Y-H, Larry W Horowitz, and Vaishali Naik, et al., March 2013: Evaluation of preindustrial to present-day black carbon and its albedo forcing from ACCMIP (Atmospheric Chemistry and Climate Model Intercomparison Project). Atmospheric Chemistry and Physics, 13(5), DOI:10.5194/acp-13-2607-2013. Abstract
As part of the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP), we evaluate the historical black carbon (BC) aerosols simulated by 8 ACCMIP models against observations including 12 ice core records, long-term surface mass concentrations and recent Arctic BC snowpack measurements. We also estimate BC albedo forcing by performing additional simulations using offline models with prescribed meteorology from 1996–2000. We evaluated the vertical profile of BC snow concentrations from these offline simulations using the recent BC snowpack measurements.
Despite using the same BC emissions, the global BC burden differs by approximately a factor of 3 among models due to differences in aerosol removal parameterizations and simulated meteorology: 34 Gg to 103 Gg in 1850 and 82 Gg to 315 Gg in 2000. However, the global BC burden from preindustrial to present-day increases by 2.5–3 times with little variation among models, roughly matching the 2.5-fold increase in total BC emissions during the same period. We find a large divergence among models at both Northern Hemisphere (NH) and Southern Hemisphere (SH) high latitude regions for BC burden and at SH high latitude regions for deposition fluxes. The ACCMIP simulations match the observed BC surface mass concentrations well in Europe and North America except at Jungfraujoch and Ispra. However, the models fail to predict the Arctic BC seasonality due to severe underestimations during winter and spring. The simulated vertically resolved BC snow concentrations are, on average, within a factor of 2–3 of the BC snowpack measurements except for Greenland and the Arctic Ocean.
For the ice core evaluation, models tend to capture both the observed temporal trends and the magnitudes well at Greenland sites. However, models fail to predict the decreasing trend of BC depositions/ice-core concentrations from the 1950s to the 1970s in most Tibetan Plateau ice cores. The distinct temporal trend at the Tibetan Plateau ice cores indicates a strong influence from Western Europe, but the modeled BC increases in that period are consistent with the emission changes in Eastern Europe, the Middle East, South and East Asia. At the Alps site, the simulated BCsuggests a strong influence from Europe, which agrees with the Alps ice core observations. Models successfully simulate higher BC concentrations observed at Zuoqiupu during the non-monsoon season than monsoon season, but models underpredict BC in both seasons. Despite a large divergence in BC deposition at two Antarctic ice core sites, models are able to capture the relative increase from preindustrial to present-day seen in the ice cores.
In 2000 relative to 1850, globally annually averaged BC surface albedo forcing from the offline simulations ranges from 0.014 to 0.019 W m−2 among the ACCMIP models. Comparing offline and online BC albedo forcings computed by some of the same models, we find that the global annual mean can vary by up to a factor of two because of different aerosol models or different BC-snow parameterizations and snow cover. The spatial distributions of the offline BC albedo forcing in 2000 show especially high BC forcing (i.e. over 0.1 W m−2) over Manchuria, Karakoram, and most of the Former USSR. Models predict the highest global annual mean BC forcing in 1980 rather than 2000, mostly driven by the high fossil fuel and biofuel emissions in the Former USSR in 1980.
Employing the Geophysical Fluid Dynamics Laboratory (GFDL)'s fully-coupled chemistry-climate (ocean/atmosphere/land/sea ice) model (CM3) with an explicit physical representation of aerosol indirect effects (cloud-water droplet activation), we find that the dramatic emission reductions (35–80%) in anthropogenic aerosols and their precursors projected by Representative Concentration Pathway (RCP) 4.5 result in ~1°C of additional warming and ~0.1 mm day−1 of additional precipitation, both globally averaged, by the end of the 21st century. The impact of these reductions in aerosol emissions on simulated global mean surface temperature and precipitation becomes apparent by mid-21st century. Furthermore, we find that the aerosol emission reductions cause precipitation to increase in East and South Asia by ~1.0 mm day−1 through the 2nd half of the 21st century. Both the simulated temperature and precipitation responses in CM3 are significantly stronger than the previously simulated responses in our earlier climate model (CM2.1) that only considered direct radiative forcing by aerosols. We conclude that sulfate aerosol indirect effects greatly enhance the impacts of aerosols on surface temperature in CM3, while both direct and indirect effects from sulfate aerosols dominate the strong precipitation response, possibly with a small contribution from carbonaceous aerosols. Just as we found with the previous GFDL model, CM3 produces surface warming patterns that are uncorrelated with the spatial distribution of 21stcentury changes in aerosol loading. However, the largest precipitation increases in CM3 are co-located with the region of greatest aerosol decrease, in and downwind of Asia.
Biomass burning is one of the largest sources of trace gases and aerosols to the atmosphere, and has profound influence on tropospheric oxidants and radiative forcing. Using a fully coupled chemistry-climate model (GFDL AM3), we find that co-emission of trace gases and aerosol from present-day biomass burning increases the global tropospheric ozone burden by 5.1%, and decreases global mean OH by 6.3%. Gas and aerosol emissions combine to increase CH4 lifetime non-linearly. Heterogeneous processes are shown to contribute partly to the observed lower ΔO3/ΔCO ratios in northern high latitudes versus tropical regions. The radiative forcing from biomass burning is shown to vary non-linearly with biomass burning strength. At present-day emission levels, biomass burning produces a net radiative forcing of −0.19 W/m2 (−0.29 from short-lived species, mostly aerosol direct and indirect effects, +0.10 from CH4 and CH4-induced changes in O3 and stratospheric H2O), but increasing emissions to over 5 times present levels would result in a positive net forcing.
Mao, Jingqiu, Fabien Paulot, D J Jacob, R C Cohen, John D Crounse, P O Wennberg, Christoph A Keller, R Hudman, M P Barkley, and Larry W Horowitz, October 2013: Ozone and organic nitrates over the eastern United States: sensitivity to isoprene chemistry. Journal of Geophysical Research: Atmospheres, 118(19), DOI:10.1002/jgrd.50817. Abstract
We implement a new isoprene oxidation mechanism in a global 3-D chemical transport model (GEOS-Chem). Model results are evaluated with observations for ozone, isoprene oxidation products, and related species from the ICARTT aircraft campaign over the eastern United States in summer 2004. The model achieves an unbiased simulation of ozone in the boundary layer and the free troposphere, reflecting canceling effects from recent model updates for isoprene chemistry, bromine chemistry, and HO2 loss to aerosols. Simulation of the ozone-CO correlation is improved relative to previous versions of the model and this is attributed to a lower and reversible yield of isoprene nitrates, increasing the ozone production efficiency (OPE) per unit of nitrogen oxides (NOx ≡ NO + NO2). The model successfully reproduces the observed concentrations of organic nitrates (∑ANs) and their correlations with HCHO and ozone. ∑ANs in the model is principally composed of secondary isoprene nitrates, including a major contribution from nighttime isoprene oxidation. The correlations of ∑ANs with HCHO and ozone then provide sensitive tests of isoprene chemistry and argue in particular against a fast isomerization channel for isoprene peroxy radicals. ∑ANs can provide an important reservoir for exporting NOx from the US boundary layer. We find that the dependence of surface ozone on isoprene emission is positive throughout the US, even if NOx emissions are reduced by a factor of 4. Previous models showed negative dependences that we attribute to erroneous titration of OH by isoprene.
Nabat, Pierre, Larry W Horowitz, and Vaishali Naik, et al., May 2013: A 4-D climatology (1979–2009) of the monthly tropospheric aerosol optical depth distribution over the Mediterranean region from a comparative evaluation and blending of remote sensing and model products. Atmospheric Measurement Techniques, 6(5), DOI:10.5194/amt-6-1287-2013. Abstract
Since the 1980s several spaceborne sensors have been used to retrieve the aerosol optical depth (AOD) over the Mediterranean region. In parallel, AOD climatologies coming from different numerical model simulations are now also available, permitting to distinguish the contribution of several aerosol types to the total AOD. In this work, we perform a comparative analysis of this unique multi-year database in terms of total AOD and of its apportionment by the five main aerosol types (soil dust, sea-salt, sulfate, black and organic carbon). We use 9 different satellite-derived monthly AOD products: NOAA/AVHRR, SeaWiFS (2 products), TERRA/MISR, TERRA/MODIS, AQUA/MODIS, ENVISAT/MERIS, PARASOL/POLDER and MSG/SEVIRI, as well as 3 more historical datasets: NIMBUS7/CZCS, TOMS (onboard NIMBUS7 and Earth-Probe) and METEOSAT/MVIRI. Monthly model datasets include the aerosol climatology from Tegen et al. (1997), the climate-chemistry models LMDz-OR-INCA and RegCM-4, the multi-model mean coming from the ACCMIP exercise, and the reanalyses GEMS and MACC. Ground-based Level-2 AERONET AOD observations from 47 stations around the basin are used here to evaluate the model and satellite data. The sensor MODIS (on AQUA and TERRA) has the best average AOD scores over this region, showing a relevant spatio-temporal variability and highlighting high dust loads over Northern Africa and the sea (spring and summer), and sulfate aerosols over continental Europe (summer). The comparison also shows limitations of certain datasets (especially MERIS and SeaWiFS standard products). Models reproduce the main patterns of the AOD variability over the basin. The MACC reanalysis is the closest to AERONET data, but appears to underestimate dust over Northern Africa, where RegCM-4 is found closer to MODIS thanks to its interactive scheme for dust emissions. The vertical dimension is also investigated using the CALIOP instrument. This study confirms differences of vertical distribution between dust aerosols showing a large vertical spread, and other continental and marine aerosols which are confined in the boundary layer. From this compilation, we propose a 4-D blended product from model and satellite data, consisting in monthly time series of 3-D aerosol distribution at a 50 km horizontal resolution over the Euro-Mediterranean marine and continental region for the 2003–2009 period. The product is based on the total AOD from AQUA/MODIS, apportioned into sulfates, black and organic carbon from the MACC reanalysis, and into dust and sea-salt aerosols from RegCM-4 simulations, which are distributed vertically based on CALIOP climatology. We extend the 2003–2009 reconstruction to the past up to 1979 using the 2003–2009 average and applying the decreasing trend in sulfate aerosols from LMDz-OR-INCA, whose AOD trends over Europe and the Mediterranean are median among the ACCMIP models. Finally optical properties of the different aerosol types in this region are proposed from Mie calculations so that this reconstruction can be included in regional climate models for aerosol radiative forcing and aerosol-climate studies.
Naik, Vaishali, A Voulgarakis, Arlene M Fiore, Larry W Horowitz, Jean-Francois Lamarque, and Meiyun Lin, et al., May 2013: Preindustrial to present day changes in tropospheric hydroxyl radical and methane lifetime from the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP). Atmospheric Chemistry and Physics, 13(10), DOI:10.5194/acp-13-5277-2013. Abstract
We have analysed results from 17 global models, participating in the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP), to explore trends in hydroxyl radical concentration (OH) and methane (CH4) lifetime since preindustrial times (1850) and gain a better understanding of their key drivers. For the present day (2000), the models tend to simulate higher OH abundances in the Northern Hemisphere versus Southern Hemisphere. Evaluation of simulated carbon monoxide concentrations, the primary sink for OH, against observations suggests low biases in the Northern Hemisphere that may contribute to the high north-south OH asymmetry in the models. A comparison of modelled and observed methyl chloroform lifetime suggests that the present day global multi-model mean OH concentration is slightly overestimated. Despite large regional changes, the modelled global mean OH concentration is roughly constant over the past 150 yr, due to concurrent increases in OH sources (humidity, tropospheric ozone, and NOx emissions), together with decreases in stratospheric ozone and increase in tropospheric temperature, compensated by increases in OH sinks (methane abundance, carbon monoxide and non-methane volatile organic carbon (NMVOC) emissions). The large intermodel diversity in the sign and magnitude of OH and methane lifetime changes over this period reflects differences in the relative importance of chemical and physical drivers of OH within each model. For the 1980 to 2000 period, we find that climate warming and a slight increase in mean OH leads to a 4.3 ± 1.9% decrease in the methane lifetime. Analysing sensitivity simulations performed by 10 models, we find that preindustrial to present day climate change decreased the methane lifetime by about 4 months, representing a negative feedback on the climate system. Further, using a subset of the models, we find that global mean OH increased by 46.4 ± 12.2% in response to preindustrial to present day anthropogenic NOx emission increases, and decreased by 17.3 ± 2.3%, 7.6 ± 1.5%, and 3.1 ± 3.0% due to methane burden, and anthropogenic CO, and NMVOC emissions increases, respectively.
We describe and evaluate atmospheric chemistry in the newly developed Geophysical Fluid Dynamics Laboratory chemistry-climate model (GFDL AM3) and apply it to investigate the net impact of preindustrial (PI) to present (PD) changes in short-lived pollutant emissions (ozone precursors, sulfur dioxide, and carbonaceous aerosols) and methane concentration on atmospheric composition and climate forcing. The inclusion of online troposphere-stratosphere interactions, gas-aerosol chemistry, and aerosol-cloud interactions (including direct and indirect aerosol radiative effects) in AM3 enables a more complete representation of interactions among short-lived species, and thus their net climate impact, than was considered in previous climate assessments. The base AM3 simulation, driven with observed sea surface temperature (SST) and sea ice cover (SIC) over the period 1981–2007, generally reproduces the observed mean magnitude, spatial distribution, and seasonal cycle of tropospheric ozone and carbon monoxide. The global mean aerosol optical depth in our base simulation is within 5% of satellite measurements over the 1982–2006 time period. We conduct a pair of simulations in which only the short-lived pollutant emissions and methane concentrations are changed from PI (1860) to PD (2000) levels (i.e., SST, SIC, greenhouse gases, and ozone depleting substances are held at PD levels). From the PI to PD, we find that changes in short-lived pollutant emissions and methane have caused the tropospheric ozone burden to increase by 39% and the global burdens of sulfate, black carbon and organic carbon to increase by factors of 3, 2.4 and 1.4, respectively. Tropospheric hydroxyl concentration decreases by 7%, showing that increases in OH sinks (methane, carbon monoxide, non-methane volatile organic compounds, and sulfur dioxide) dominate over sources (ozone and nitrogen oxides) in the model. Combined changes in tropospheric ozone and aerosols cause a net negative top-of-the-atmosphere radiative forcing perturbation (−1.05 Wm-2) indicating that the negative forcing (direct plus indirect) from aerosol changes dominates over the positive forcing due to ozone increases, thus masking nearly half of the PI to PD positive forcing from long-lived greenhouse gases globally, consistent with other current generation chemistry-climate models.
The Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP) examined the short-lived drivers of climate change in current climate models. Here we evaluate the 10 ACCMIP models that included aerosols, 8 of which also participated in the Coupled Model Intercomparison Project phase 5 (CMIP5).
The models reproduce present-day total aerosol optical depth (AOD) relatively well, though many are biased low. Contributions from individual aerosol components are quite different, however, and most models underestimate east Asian AOD. The models capture most 1980–2000 AOD trends well, but underpredict increases over the Yellow/Eastern Sea. They strongly underestimate absorbing AOD in many regions.
We examine both the direct radiative forcing (RF) and the forcing including rapid adjustments (effective radiative forcing; ERF, including direct and indirect effects). The models' all-sky 1850 to 2000 global mean annual average total aerosol RF is (mean; range) −0.26 W m−2; −0.06 to −0.49 W m−2. Screening based on model skill in capturing observed AOD yields a best estimate of −0.42 W m−2; −0.33 to −0.50 W m−2, including adjustment for missing aerosol components in some models. Many ACCMIP and CMIP5 models appear to produce substantially smaller aerosol RF than this best estimate. Climate feedbacks contribute substantially (35 to −58%) to modeled historical aerosol RF. The 1850 to 2000 aerosol ERF is −1.17 W m−2; −0.71 to −1.44 W m−2. Thus adjustments, including clouds, typically cause greater forcing than direct RF. Despite this, the multi-model spread relative to the mean is typically the same for ERF as it is for RF, or even smaller, over areas with substantial forcing. The largest 1850 to 2000 negative aerosol RF and ERF values are over and near Europe, south and east Asia and North America. ERF, however, is positive over the Sahara, the Karakoram, high Southern latitudes and especially the Arctic.
Global aerosol RF peaks in most models around 1980, declining thereafter with only weak sensitivity to the Representative Concentration Pathway (RCP). One model, however, projects approximately stable RF levels, while two show increasingly negative RF due to nitrate (not included in most models). Aerosol ERF, in contrast, becomes more negative during 1980 to 2000. During this period, increased Asian emissions appear to have a larger impact on aerosol ERF than European and North American decreases due to their being upwind of the large, relatively pristine Pacific Ocean. There is no clear relationship between historical aerosol ERF and climate sensitivity in the CMIP5 subset of ACCMIP models. In the ACCMIP/CMIP5 models, historical aerosol ERF of about −0.8 to −1.5 W m−2 is most consistent with observed historical warming. Aerosol ERF masks a large portion of greenhouse forcing during the late 20th and early 21st century at the global scale. Regionally, aerosol ERF is so large that net forcing is negative over most industrialized and biomass burning regions through 1980, but remains strongly negative only over east and southeast Asia by 2000. Net forcing is strongly positive by 1980 over most deserts, the Arctic, Australia, and most tropical oceans. Both the magnitude of and area covered by positive forcing expand steadily thereafter.
Silva, R A., Larry W Horowitz, and Vaishali Naik, et al., September 2013: Global premature mortality due to anthropogenic outdoor air pollution and the contribution of past climate change. Environmental Research Letters, 8(3), DOI:10.1088/1748-9326/8/3/034005. Abstract
Increased concentrations of ozone and fine particulate matter (PM2.5) since preindustrial times reflect increased emissions, but also contributions of past climate change. Here we use modeled concentrations from an ensemble of chemistry–climate models to estimate the global burden of anthropogenic outdoor air pollution on present-day premature human mortality, and the component of that burden attributable to past climate change. Using simulated concentrations for 2000 and 1850 and concentration–response functions (CRFs), we estimate that, at present, 470 000 (95% confidence interval, 140 000 to 900 000) premature respiratory deaths are associated globally and annually with anthropogenic ozone, and 2.1 (1.3 to 3.0) million deaths with anthropogenic PM2.5-related cardiopulmonary diseases (93%) and lung cancer (7%). These estimates are smaller than ones from previous studies because we use modeled 1850 air pollution rather than a counterfactual low concentration, and because of different emissions. Uncertainty in CRFs contributes more to overall uncertainty than the spread of model results. Mortality attributed to the effects of past climate change on air quality is considerably smaller than the global burden: 1500 (−20 000 to 27 000) deaths yr−1 due to ozone and 2200 (−350 000 to 140 000) due to PM2.5. The small multi-model means are coincidental, as there are larger ranges of results for individual models, reflected in the large uncertainties, with some models suggesting that past climate change has reduced air pollution mortality.
Stevenson, David S., Vaishali Naik, and Larry W Horowitz, et al., March 2013: Tropospheric ozone changes, radiative forcing and attribution to emissions in the Atmospheric Chemistry and Climate Model Inter-comparison Project (ACCMIP). Atmospheric Chemistry and Physics, 13(6), DOI:10.5194/acp-13-3063-2013. Abstract
Ozone (O3) from 17 atmospheric chemistry models taking part in the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP) has been used to calculate tropospheric ozone radiative forcings (RFs). We calculate a~value for the pre-industrial (1750) to present-day (2010) tropospheric ozone RF of 0.40 W m−2. The model range of pre-industrial to present-day changes in O3 produces a spread (±1 standard deviation) in RFs of ±17%. Three different radiation schemes were used – we find differences in RFs between schemes (for the same ozone fields) of ±10%. Applying two different tropopause definitions gives differences in RFs of ±3%. Given additional (unquantified) uncertainties associated with emissions, climate-chemistry interactions and land-use change, we estimate an overall uncertainty of ±30% for the tropospheric ozone RF. Experiments carried out by a subset of six models attribute tropospheric ozone RF to increased emissions of methane (47%), nitrogen oxides (29%), carbon monoxide (15%) and non-methane volatile organic compounds (9%); earlier studies attributed more of the tropospheric ozone RF to methane and less to nitrogen oxides. Normalising RFs to changes in tropospheric column ozone, we find a global mean normalised RF of 0.042 W m−2 DU−1, a value similar to previous work. Using normalised RFs and future tropospheric column ozone projections we calculate future tropospheric ozone RFs (W m−2; relative to 1850 – add 0.04 W m−2 to make relative to 1750) for the Representative Concentration Pathways in 2030 (2100) of: RCP2.6: 0.31 (0.16); RCP4.5: 0.38 (0.26); RCP6.0: 0.33 (0.24); and RCP8.5: 0.42 (0.56). Models show some coherent responses of ozone to climate change: decreases in the tropical lower troposphere, associated with increases in water vapour; and increases in the sub-tropical to mid-latitude upper troposphere, associated with increases in lightning and stratosphere-to-troposphere transport.
Su, H, Leo J Donner, Larry W Horowitz, and Charles J Seman, et al., April 2013: Diagnosis of regime-dependent cloud simulation errors in CMIP5 models using “A-Train” satellite observations and reanalysis data. Journal of Geophysical Research: Atmospheres, 118(7), DOI:10.1029/2012JD018575. Abstract
The vertical distributions of cloud water content (CWC) and cloud fraction (CF) over
the tropical oceans, produced by 13 coupled atmosphere-ocean models submitted to the Phase 5
of Coupled Model Intercomparison Project (CMIP5), are evaluated against CloudSat/CALIPSO
observations as a function of large-scale parameters. Available CALIPSO simulator CF outputs
are also examined. A diagnostic framework is developed to decompose the cloud simulation
errors into large-scale errors, cloud parameterization errors and co-variation errors. We find that
the cloud parameterization errors contribute predominantly to the total errors for all models. The
errors associated with large-scale temperature and moisture structures are relatively greater than
those associated with large-scale mid-tropospheric vertical velocity and lower-level divergence.
All models capture the separation of deep and shallow clouds in distinct large-scale regimes;
however, the vertical structures of high/low clouds and their variations with large-scale
parameters differ significantly from the observations. The CWCs associated with deep
convective clouds simulated in most models do not reach as high in altitude as observed, and
their magnitudes are generally weaker than CloudSat total CWC, which includes the contribution
of precipitating condensates, but are close to CloudSat non-precipitating CWC. All models
reproduce maximum CF associated with convective detrainment, but CALIPSO simulator CFs
generally agree better with CloudSat/CALIPSO combined retrieval than the model CFs,
especially in the mid-troposphere. Model simulated low clouds tend to have little variation with
large-scale parameters except lower-troposphere stability, while the observed low cloud CWC,
CF and cloud top height vary consistently in all large-scale regimes.
Turner, A J., Arlene M Fiore, Larry W Horowitz, and M Bauer, January 2013: Summertime cyclones over the Great Lakes Storm Track from 1860–2100: variability, trends, and association with ozone pollution. Atmospheric Chemistry and Physics, 13(2), DOI:10.5194/acp-13-565-2013. Abstract
Prior work indicates that the frequency of summertime mid-latitude cyclones tracking across the Great Lakes Storm Track (GLST, bounded by: 70° W, 90° W, 40° N, and 50° N) are strongly anticorrelated with ozone (O3) pollution episodes over the Northeastern United States (US). We apply the MAP Climatology of Mid-latitude Storminess (MCMS) algorithm to 6-hourly sea level pressure fields from over 2500 yr of simulations with the GFDL CM3 global coupled chemistry-climate model. These simulations include (1) 875 yr with constant 1860 emissions and forcings (Pre-industrial Control), (2) five ensemble members for 1860–2005 emissions and forcings (Historical), and (3) future (2006–2100) scenarios following the Representative Concentration Pathways (RCP 8.5 (one member; extreme warming); RCP 4.5 (three members; moderate warming); RCP 4.5* (one member; a variation on RCP 4.5 in which only well-mixed greenhouse gases evolve along the RCP 4.5 trajectory)). The GFDL CM3 Historical simulations capture the mean and variability of summertime cyclones traversing the GLST within the range determined from four reanalysis datasets. Over the 21st century (2006–2100), the frequency of summertime mid-latitude cyclones in the GLST decreases under the RCP 8.5 scenario (m = −0.06 a−1, p < 0.01) and in the RCP 4.5 ensemble mean (m = −0.03 a−1, p < 0.01). These trends are significant when assessed relative to the variability in the Pre-industrial Control simulation (p > 0.06 for 100-yr sampling intervals; −0.01 a−1 < m < 0.02 a−1). In addition, the RCP 4.5* scenario enables us to determine the relationship between summertime GLST cyclones and high-O3 events (>95th percentile) in the absence of emission changes. The summertime GLST cyclone frequency explains less than 10% of the variability in high-O3 events over the Northeastern US in the model. Our findings imply that careful study is required prior to applying the strong relationship noted in earlier work to changes in storm counts.
Results from simulations performed for the Atmospheric Chemistry and Climate Modeling Intercomparison Project (ACCMIP) are analysed to examine how OH and methane lifetime may change from present-day to the future, under different climate and emissions scenarios. Present-day (2000) mean tropospheric chemical lifetime derived from the ACCMIP multi-model mean is 9.8 ± 1.6 yr, lower than a recent observationally-based estimate, but with a similar range to previous multi-model estimates. Future model projections are based on the four Representative Concentration Pathways (RCPs), and the results also exhibit a~large range. Decreases in global methane lifetime of 4.5 ± 9.1% are simulated for the scenario with lowest radiative forcing by 2100 (RCP 2.6), while increases of 8.5 ± 10.4% are simulated for the scenario with highest radiative forcing (RCP 8.5). In this scenario, the key driver of the evolution of OH and methane lifetime is methane itself, since its concentration more than doubles by 2100, and it consumes much of the OH that exists in the troposphere. Stratospheric ozone recovery, which drives tropospheric OH decreases through photolysis modifications, also plays a~partial role. In the other scenarios, where methane changes are less drastic, the interplay between various competing drivers leads to smaller and more diverse OH and methane lifetime responses, which are difficult to attribute. For all scenarios, regional OH changes are even more variable, with the most robust feature being the large decreases over the remote oceans in RCP 8.5. Through a~regression analysis, we suggest that differences in emissions of non-methane volatile organic compounds and in the simulation of photolysis rates may be the main factors causing the differences in simulated present-day OH and methane lifetime. Diversity in predicted changes between present-day and future was found to be associated more strongly with differences in modelled climate changes, specifically global temperature and humidity. Finally, through perturbation experiments we calculated an OH feedback factor (F) of 1.29 from present-day conditions (1.65 from 2100 RCP 8.5 conditions) and a~climate feedback on methane lifetime of 0.33 ± 0.13 yr K−1, on average.
West, J J., Vaishali Naik, and Larry W Horowitz, et al., October 2013: Co-benefits of mitigating global greenhouse gas emissions for future air quality and human health. Nature Climate Change, 3(10), DOI:10.1038/nclimate2009. Abstract
Actions to reduce greenhouse gas (GHG) emissions often reduce co-emitted air pollutants, bringing co-benefits for air quality and human health. Past studies typically evaluated near-term and local co-benefits, neglecting the long-range transport of air pollutants, long-term demographic changes, and the influence of climate change on air quality. Here we simulate the co-benefits of global GHG reductions on air quality and human health using a global atmospheric model and consistent future scenarios, via two mechanisms: reducing co-emitted air pollutants, and slowing climate change and its effect on air quality. We use new relationships between chronic mortality and exposure to fine particulate matter and ozone, global modelling methods and new future scenarios. Relative to a reference scenario, global GHG mitigation avoids 0.5±0.2, 1.3±0.5 and 2.2±0.8 million premature deaths in 2030, 2050 and 2100. Global average marginal co-benefits of avoided mortality are US$50–380 per tonne of CO2, which exceed previous estimates, exceed marginal abatement costs in 2030 and 2050, and are within the low range of costs in 2100. East Asian co-benefits are 10–70 times the marginal cost in 2030. Air quality and health co-benefits, especially as they are mainly local and near-term, provide strong additional motivation for transitioning to a low-carbon future.
We examine the influence of alternative ocean and atmosphere subcomponents on climate model simulation of transient sensitivities by comparing three GFDL climate models used for the CMIP5. The base model ESM2M is closely related to GFDL's CMIP3 climate model CM2.1, and makes use of a depth coordinate ocean component. The second model, ESM2G, is identical to ESM2M but makes use of an isopycnal coordinate ocean model. We compare the impact of this "ocean swap" with an "atmosphere swap" that produces the CM3 climate model by replacing the AM2 atmosphere with AM3 while retaining a depth coordinate ocean model. The atmosphere swap is found to have much larger influence on sensitivities of global surface temperature and Northern Hemisphere sea ice cover. The atmosphere swap also introduces a multi-decadal response timescale through its indirect influence on heat uptake. Despite significant differences in their interior ocean mean states, the ESM2M and ESM2G simulations of these metrics of climate change are very similar, except for an enhanced high latitude salinity response accompanied by temporarily advancing sea ice in ESM2G. In the ESM2G historical simulation this behavior results in the establishment of a strong halocline in the subpolar North Atlantic during the early 20th century and an associated cooling which are counter to observations in that region. The Atlantic meridional overturning declines comparably in all three models.
Young, Paul J., Vaishali Naik, and Larry W Horowitz, et al., February 2013: Pre-industrial to end 21st century projections of tropospheric ozone from the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP). Atmospheric Chemistry and Physics, 13(4), DOI:10.5194/acp-13-2063-2013. Abstract
Present day tropospheric ozone and its changes between 1850 and 2100 are considered, analysing 15 global models that participated in the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP). The multi-model mean compares well against present day observations. The seasonal cycle correlates well, except for some locations in the tropical upper troposphere. Most (75%) of the models are encompassed with a range of global mean tropospheric ozone column estimates from satellite data, although there is a suggestion of a high bias in the Northern Hemisphere and a low bias in the Southern Hemisphere. Compared to the present day multi-model mean tropospheric ozone burden of 337 Tg, the multi-model mean burden for 1850 time slice is ~ 30% lower. Future changes were modelled using emissions and climate projections from four Representative Concentration Pathways (RCPs). Compared to 2000, the relative changes for the tropospheric ozone burden in 2030 (2100) for the different RCPs are: −5% (−22%) for RCP2.6, 3% (−8%) for RCP4.5, 0% (−9%) for RCP6.0, and 5% (15%) for RCP8.5. Model agreement on the magnitude of the change is greatest for larger changes. Reductions in precursor emissions are common across the RCPs and drive ozone decreases in all but RCP8.5, where doubled methane and a larger stratospheric influx increase ozone. Models with high ozone abundances for the present day also have high ozone levels for the other time slices, but there are no models consistently predicting large or small changes. Spatial patterns of ozone changes are well correlated across most models, but are notably different for models without time evolving stratospheric ozone concentrations. A unified approach to ozone budget specifications is recommended to help future studies attribute ozone changes and inter-model differences more clearly.
Fan, Songmiao, J P Schwarz, Junfeng Liu, D W Fahey, Paul Ginoux, Larry W Horowitz, Hiram Levy II, Yi Ming, and J R Spackman, December 2012: Inferring ice formation processes from global-scale black carbon profiles observed in the remote atmosphere and model simulations. Journal of Geophysical Research: Atmospheres, 117, D23205, DOI:10.1029/2012JD018126. Abstract
Black carbon (BC) aerosol absorbs solar radiation and can act as cloud condensation nucleus and ice formation nucleus. The current generation of climate models have difficulty in accurately predicting global-scale BC concentrations. Previously, an ensemble of such models was compared to measurements, revealing model biases in the tropical troposphere and in the polar troposphere. Here, global aerosol distributions are simulated using different parameterizations of wet removal and model results are compared to BC profiles observed in the remote atmosphere to explore the possible sources of these biases. The model-data comparison suggests a slow removal of BC aerosol during transport to the Arctic in winter and spring, because ice crystal growth causes evaporation of liquid cloud via the Bergron process and, hence, release of BC aerosol back to ambient air. By contrast, more efficient model wet removal is needed in the cold upper troposphere over the tropical Pacific. Parcel model simulations with detailed droplet and ice nucleation and growth processes suggest that ice formation in this region may be suppressed due to a lack of ice nuclei (mainly insoluble dust particles) in the remote atmosphere, allowing liquid and mixed-phase clouds to persist under freezing temperatures, and forming liquid precipitation capable of removing aerosol incorporated in cloud water. Falling ice crystals can scavenge droplets in lower clouds, which also results in efficient removal of cloud condensation nuclei. The combination of models with global-scale BC measurements in this study has provided new, latitude-dependent information on ice formation processes in the atmosphere, and highlights the importance of a consistent treatment of aerosol and moist physics in climate models.
Emissions of air pollutants and their precursors determine regional air quality and can alter climate. Climate change can perturb the long-range transport, chemical processing, and local meteorology that influence air pollution. We review the implications of projected changes in methane (CH4), ozone precursors (O3), and aerosols for climate (expressed in terms of the radiative forcing metric or changes in global surface temperature) and hemispheric-to-continental scale air quality. Reducing the O3 precursor CH4 would slow near-term warming by decreasing both CH4 and tropospheric O3. Uncertainty remains as to the net climate forcing from anthropogenic nitrogen oxide (NOx) emissions, which increase tropospheric O3 (warming) but also increase aerosols and decrease CH4 (both cooling). Anthropogenic emissions of carbon monoxide (CO) and non-CH4 volatile organic compounds (NMVOC) warm by increasing both O3 and CH4. Radiative impacts from secondary organic aerosols (SOA) are poorly understood. Black carbon emission controls, by reducing the absorption of sunlight in the atmosphere and on snow and ice, have the potential to slow near-term warming, but uncertainties in coincident emissions of reflective (cooling) aerosols and poorly constrained cloud indirect effects confound robust estimates of net climate impacts. Reducing sulfate and nitrate aerosols would improve air quality and lessen interference with the hydrologic cycle, but lead to warming. A holistic and balanced view is thus needed to assess how air pollution controls influence climate; a first step towards this goal involves estimating net climate impacts from individual emission sectors. Modeling and observational analyses suggest a warming climate degrades air quality (increasing surface O3 and particulate matter) in many populated regions, including during pollution episodes. Prior Intergovernmental Panel on Climate Change (IPCC) scenarios (SRES) allowed unconstrained growth, whereas the Representative Concentration Pathway (RCP) scenarios assume uniformly an aggressive reduction, of air pollutant emissions. New estimates from the current generation of chemistry–climate models with RCP emissions thus project improved air quality over the next century relative to those using the IPCC SRES scenarios. These two sets of projections likely bracket possible futures. We find that uncertainty in emission-driven changes in air quality is generally greater than uncertainty in climate-driven changes. Confidence in air quality projections is limited by the reliability of anthropogenic emission trajectories and the uncertainties in regional climate responses, feedbacks with the terrestrial biosphere, and oxidation pathways affecting O3 and SOA.
Using NASA's A-Train satellite measurements, we evaluate the accuracy of cloud water content (CWC) and water vapor mixing ratio (H2O) outputs from 19 climate models submitted to the Phase 5 of Coupled Model Intercomparison Project (CMIP5), and assess improvements relative to their counterparts for the earlier CMIP3. We find more than half of the models show improvements from CMIP3 to CMIP5 in simulating column-integrated cloud amount, while changes in water vapor simulation are insignificant. For the 19 CMIP5 models, the model spreads and their differences from the observations are larger in the upper troposphere (UT) than in the lower or mid-troposphere (L/MT). The modeled mean CWCs over tropical oceans range from ~3% to ~15× observations in the UT and 40% to 2× observations in the L/MT. For modeled H2Os, the mean values over tropical oceans range from ~1% to 2× of the observations in the UT and within 10% of the observations in the L/MT. The spatial distributions of clouds at 215 hPa are relatively well-correlated with observations, noticeably better than those for the L/MT clouds. Although both water vapor and clouds are better simulated in the L/MT than in the UT, there is no apparent correlation between the model biases in clouds and water vapor. Numerical scores are used to compare different model performances in regards to spatial mean, variance and distribution of CWC and H2O over tropical oceans. Model performances at each pressure level are ranked according to the average of all the relevant scores for that level.
With a more-than-doubling in the atmospheric abundance of the potent greenhouse gas methane (CH4) since preindustrial times, and indications of renewed growth following a leveling off in recent years, questions arise as to future trends and resulting climate and public health impacts from continued growth without mitigation. Changes in atmospheric methane lifetime are determined by factors which regulate the abundance of OH, the primary methane removal mechanism, including changes in CH4 itself. We investigate the role of emissions of short-lived species and climate in determining the evolution of tropospheric methane lifetime in a suite of historical (1860�2005) and Representative Concentration Pathway (RCP) simulations (2006�2100), conducted with the Geophysical Fluid Dynamics Laboratory (GFDL) fully coupled chemistry-climate model (CM3). From preindustrial to present, CM3 simulates an overall 5% increase in CH4 lifetime due to a doubling of the methane burden which offsets coincident increases in nitrogen oxide (NOx) emissions. Over the last two decades, however, the methane lifetime declines steadily, coinciding with the most rapid climate warming and observed slow-down in CH4 growth rates, reflecting a possible negative feedback through the CH4 sink. The aerosol indirect effect plays a significant role in the CM3 climate and thus in the future evolution of the methane lifetime, due to the rapid projected decline of aerosols under all four RCPs. In all scenarios, the methane lifetime decreases (by 5�13%) except for the most extreme warming case (RCP8.5), where it increases by 4% due to the near-doubling of the CH4 abundance, reflecting a positive feedback on the climate system. In the RCP4.5 scenario changes in short-lived climate forcing agents reinforce climate warming and enhance OH, leading to a more-than-doubling of the decrease in methane lifetime from 2006 to 2100 relative to a simulation in which only well-mixed greenhouse gases are allowed to change along the RCP4.5 scenario (13% vs. 5%) Future work should include process-based studies to better understand and elucidate the individual mechanisms controlling methane lifetime.
Koffi, B, Paul Ginoux, and Larry W Horowitz, et al., May 2012: Application of the CALIOP Layer Product to evaluate the vertical distribution of aerosols estimated by global models: Part 1. AeroCom phase I results. Journal of Geophysical Research: Atmospheres, 117, D10201, DOI:10.1029/2011JD016858. Abstract
The CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) Layer product is used for a multi-model evaluation of the vertical distribution of aerosols. Annual and seasonal aerosol extinction profiles are analysed over 13 sub-continental regions representative of industrial, dust, and biomass burning pollution, from CALIOP 2007-2009 observations and from AeroCom (Aerosol Comparisons between Observations and Models) 2000 simulations. An extinction mean height diagnostic (Zα) is defined to quantitatively assess the models' performance. It is calculated over the 0-6 km and 0-10 km altitude ranges by weighting the altitude of each 100 m altitude layer by its aerosol extinction coefficient. The mean extinction profiles derived from CALIOP layer products provide consistent regional and seasonal specificities and a low inter-annual variability. While the outputs from most models are significantly correlated with the observed Zα climatologies, some do better than others, and 2 of the 12 models perform particularly well in all seasons. Over industrial and maritime regions, most models show higher Zα than observed by CALIOP, whereas over the African and Chinese dust source regions, Zα is underestimated during Northern Hemisphere Spring and Summer. The positive model bias in Zα is mainly due to an overestimate of the extinction above 6 km. Potential CALIOP and model limitations, and methodological factors that might contribute to the differences are discussed.
Li, J-L, D E Waliser, W-T Chen, B Guan, T Kubar, Graeme L Stephens, Hsi-Yen Ma, M Deng, Leo J Donner, Charles J Seman, and Larry W Horowitz, August 2012: An observationally based evaluation of cloud ice water in CMIP3 and CMIP5 GCMs and contemporary reanalyses using contemporary satellite data. Journal of Geophysical Research: Atmospheres, 117, D16105, DOI:10.1029/2012JD017640. Abstract
We perform an observationally based evaluation of the cloud ice water content
(CIWC) and path (CIWP) of present-day GCMs, notably 20th century CMIP5 simulations,
and compare these results to CMIP3 and two recent reanalyses. We use three different
CloudSat + CALIPSO ice water products and two methods to remove the contribution
from the convective core ice mass and/or precipitating cloud hydrometeors with variable
sizes and falling speeds so that a robust observational estimate can be obtained for
model evaluations. The results show that for annual mean CIWP, there are factors
of 2–10 in the differences between observations and models for a majority of the GCMs
and for a number of regions. However, there are a number of CMIP5 models, including
CNRM-CM5, MRI, CCSM4 and CanESM2, as well as the UCLA CGCM, that perform
well compared to our past evaluations. Systematic biases in CIWC vertical structure occur
below the mid-troposphere where the models overestimate CIWC, with this bias arising
mostly from the extratropics. The tropics are marked by model differences in the level of
maximum CIWC (250–550 hPa). Based on a number of metrics, the ensemble behavior
of CMIP5 has improved considerably relative to CMIP3, although neither the CMIP5
ensemble mean nor any individual model performs particularly well, and there are still a
number of models that exhibit very large biases despite the availability of relevant
observations. The implications of these results on model representations of the Earth
radiation balance are discussed, along with caveats and uncertainties associated with the
observational estimates, model and observation representations of the precipitating and
cloudy ice components, relevant physical processes and parameterizations.
Many prior studies clearly document episodic Asian pollution in the western U.S.
free troposphere. Here, we examine the mechanisms involved in the transport of Asian
pollution plumes into western U.S. surface air through an integrated analysis of in situ
and satellite measurements in May–June 2010 with a new global high-resolution
(50 50 km2) chemistry-climate model (GFDL AM3). We find that AM3 with
full stratosphere-troposphere chemistry nudged to reanalysis winds successfully
reproduces observed sharp ozone gradients above California, including the interleaving
and mixing of Asian pollution and stratospheric air associated with complex interactions of
midlatitude cyclone air streams. Asian pollution descends isentropically behind cold fronts;
at 800 hPa a maximum enhancement to ozone occurs over the southwestern U.S.,
including the densely populated Los Angeles Basin. During strong episodes, Asian
emissions can contribute 8–15 ppbv ozone in the model on days when observed daily
maximum 8-h average ozone (MDA8 O3) exceeds 60 ppbv. We find that in the absence
of Asian anthropogenic emissions, 20% of MDA8 O3 exceedances of 60 ppbv in the model
would not have occurred in the southwestern USA. For a 75 ppbv threshold, that
statistic increases to 53%. Our analysis indicates the potential for Asian emissions to
contribute to high-O3 episodes over the high-elevation western USA, with implications
for attaining more stringent ozone standards in this region. We further demonstrate a
proof-of-concept approach using satellite CO column measurements as a qualitative early
warning indicator to forecast Asian ozone pollution events in the western U.S. with
lead times of 1–3 days.
The published literature debates the extent to which naturally occurring stratospheric ozone intrusions reach the surface and contribute to exceedances of the U.S. National Ambient Air Quality Standard (NAAQS) for ground-level ozone (75 ppbv implemented in 2008). Analysis of ozonesondes, lidar, and surface measurements over the western U.S. from April to June 2010 show that a global high-resolution (~50x50 km2) chemistry-climate model (GFDL AM3) captures the observed layered features and sharp ozone gradients of deep stratospheric intrusions, representing a major improvement over previous chemical transport models. Thirteen intrusions enhanced total daily maximum 8-hour average (MDA8) ozone to ~70-86 ppbv at surface sites. With a stratospheric ozone tracer defined relative to a dynamically-varying tropopause, we find that stratospheric intrusions can episodically increase surface MDA8 ozone by 20-40 ppbv (all model estimates are bias corrected), including on days when observed ozone exceeds the NAAQS threshold. These stratospheric intrusions elevated background ozone concentrations (estimated by turning off North American anthropogenic emissions in the model) to MDA8 values of 60-75 ppbv. At high-elevation western U.S. sites, the 25th-75th percentile of the stratospheric contribution is 15-25 ppbv when observed MDA8 ozone is 60-70 ppbv, and increases to ~17-40 ppbv for the 70-85 ppbv range. These estimates, up to 2-3 times greater than previously reported, indicate a major role for stratospheric intrusions in contributing to springtime high-O3 events over the high-altitude western U.S., posing a challenge for staying below the ozone NAAQS threshold, particularly if a value in the 60-70 ppbv range were to be adopted.
Liu, Junfeng, Larry W Horowitz, Songmiao Fan, Hiram Levy II, and A G Carlton, August 2012: Global in-cloud production of secondary organic aerosols: Implementation of a detailed chemical mechanism in the GFDL atmospheric model AM3. Journal of Geophysical Research: Atmospheres, 117, D15303, DOI:10.1029/2012JD017838. Abstract
Secondary organic aerosols (SOA) constitute a significant fraction of ambient aerosols, but their global source is only beginning to be understood. Substantial evidence has shown that oxidation of water-soluble organic species in the liquid cloud leads to the formation of SOA. To evaluate this global source and explore its sensitivity to various assumptions concerning cloud properties, we simulate in-cloud SOA (IC-SOA) formation based on detailed multi-phase chemistry incorporated into the newly developed Geophysical Fluid Dynamics Laboratory (GFDL) coupled chemistry-climate model AM3. We find global IC-SOA production is around 20-30 Tg∙yr-1 between 1999 and 2001. Depending on season and location, oxalic acid accounts for 40-90% of the total IC-SOA source (particularly between 800hPa - 400hPa), and glyoxylic acid and oligomers (formed by glyoxal and methylglyoxal in evaporating clouds) each contribute an additional 10-20%. Besides glyoxal and methylglyoxal (extensively studied by previous research), glycolaldehyde and acetic acid are among the most important precursors leading to the formation of IC-SOA, particularly oxalic acid. Different implementations of cloud fraction or cloud lifetime in global climate models could potentially modify estimates of IC-SOA mass production by 20-30%. Dense IC-SOA production occurs in the tropical and mid-latitude regions of the lower troposphere (surface to 500hPa). In DJF, IC-SOA production is concentrated over the western Amazon and southern Africa. In JJA, substantial IC-SOA production occurs over southern China and boreal forest regions. This study confirms a significant in-cloud source of SOA, which will directly and indirectly influence global radiation balance and regional climate.
The direct radiative forcing of the climate system includes effects due to scattering and absorbing aerosols. This study explores how important physical climate characteristics contribute to the magnitudes of the direct radiative forcings (DRF) from anthropogenic sulfate, black carbon, and organic carbon. For this purpose, we employ the GFDL CM2.1 global climate model, which has reasonable aerosol concentrations and reconstruction of 20th Century climate change. Sulfate and carbonaceous aerosols constitute the most important anthropogenic aerosol perturbations to the climate system, and provide striking contrasts between primarily scattering (sulfate and organic carbon) and primarily absorbing (black carbon) species. The quantitative roles of cloud coverage, surface albedo, and relative humidity in governing the sign and magnitude of all-sky top-of-atmosphere forcings are examined. Clouds reduce the global-mean sulfate TOA DRF by almost 50%, reduce the global-mean organic carbon TOA DRF by more than 30%, and increase the global-mean black carbon TOA DRF by almost 80%. Sulfate forcing is increased by over 50% as a result of hygroscopic growth, while high-albedo surfaces are found to have only a minor (less than 10%) impact on all global-mean forcings. Although the radiative forcing magnitudes are subject to uncertainties in the state of mixing of the aerosol species, it is clear that fundamental physical climate characteristics play a large role in governing aerosol direct radiative forcing magnitudes.
We use long-term, coincident O3 and temperature measurements at the regionally representative US Environmental Protection Agency Clean Air Status and Trends Network (CASTNet) over the eastern US from 1988 through 2009 to characterize the surface O3 response to year-to-year fluctuations in weather, for the purpose of evaluating global chemistry-climate models. We first produce a monthly climatology for each site over all available years, defined as the slope of the best-fit line (mO3-T) between monthly average values of maximum daily 8-hour average (MDA8) O3 and monthly average values of daily maximum surface temperature (Tmax). Applying two distinct statistical approaches to aggregate the site-specific measurements to the regional scale, we find that summertime mO3-T is 3–6 ppb K−1 (r = 0.5–0.8) over the Northeast, 3–4 ppb K−1 (r = 0.5–0.9) over the Great Lakes, and 3–6 ppb K−1 (r = 0.2–0.8) over the Mid-Atlantic. The Geophysical Fluid Dynamics Laboratory (GFDL) Atmospheric Model version 3 (AM3) global chemistry-climate model generally captures the seasonal variations in correlation coefficients and mO3-T despite biases in both monthly mean summertime MDA8 O3 (up to +10 to +30 ppb) and daily Tmax (up to +5 K) over the eastern US. During summer, GFDL AM3 reproduces mO3-T over the Northeast (mO3-T = 2–6 ppb K−1; r = 0.6–0.9), but underestimates mO3-Tby 4 ppb K−1 over the Mid-Atlantic, in part due to excessively warm temperatures above which O3 production saturates in the model. Combining Tmax biases in GFDL AM3 with an observation-based mO3-T estimate of 3 ppb K−1 implies that temperature biases could explain up to 5–15 ppb of the MDA8 O3 bias in August and September though correcting for excessively cool temperatures would worsen the O3 bias in June. We underscore the need for long-term, coincident measurements of air pollution and meteorological variables to develop process-level constraints for evaluating chemistry-climate models used to project air quality responses to climate change.
Methane emissions contribute to global baseline surface ozone concentrations; therefore reducing methane to address climate change has significant co-benefits for air quality and human health. We analyze the costs of reducing methane from 2005 to 2030, as might be motivated to reduce climate forcing, and the resulting benefits from lower surface ozone to 2060. We construct three plausible scenarios of methane emission reductions, relative to a base scenario, ranging from 75 to 180 Mton CH4 yr−1 decreased in 2030. Using compilations of the global availability of methane emission reductions, the least aggressive scenario (A) does not incur any positive marginal costs to 2030, while the most aggressive (C) requires discovery of new methane abatement technologies. The present value of implementation costs for Scenario B are nearly equal to Scenario A, as it implements cost-saving options more quickly, even though it adopts positive cost measures. We estimate the avoided premature human mortalities due to surface ozone decreases by combining transient full-chemistry simulations of these scenarios in a global atmospheric chemical transport model, with concentration-mortality relationships from a short-term epidemiologic study and projected global population. An estimated 38,000 premature mortalities are avoided globally in 2030 under Scenario B. As benefits of methane reduction are positive but costs are negative for Scenario A, it is justified regardless of how avoided mortalities are valued. The incremental benefits of Scenario B also far outweigh the incremental costs. Scenario C has incremental costs that roughly equal benefits, only when technological learning is assumed. Benefits within industrialized nations alone also exceed costs in Scenarios A and B, assuming that the lowest-cost emission reductions, including those in developing nations, are implemented. Monetized co-benefits of methane mitigation for human health are estimated to be $13–17 per ton CO2eq, with a wider range possible under alternative assumptions. Methane mitigation can be a cost-effective means of long-term and international air quality management, with concurrent benefits for climate.
Anenberg, S C., J Jason West, Larry W Horowitz, and D Q Tong, April 2011: The Global Burden of Air Pollution on Mortality: Anenberg et al. Respond. Environmental Health Perspectives, 119(4), DOI:10.1289/ehp.1003276R.
Avnery, S, D L Mauzerall, J Liu, and Larry W Horowitz, April 2011: Global crop yield reductions due to surface ozone exposure: 1. Year 2000 crop production losses and economic damage. Atmospheric Environment, 45(13), DOI:10.1016/j.atmosenv.2010.11.045. Abstract
Exposure to elevated concentrations of surface ozone (O3) causes substantial reductions in the agricultural yields of many crops. As emissions of O3 precursors rise in many parts of the world over the next few decades, yield reductions from O3 exposure appear likely to increase the challenges of feeding a global population projected to grow from 6 to 9 billion between 2000 and 2050. This study estimates year 2000 global yield reductions of three key staple crops (soybean, maize, and wheat) due to surface ozone exposure using hourly O3 concentrations simulated by the Model for Ozone and Related Chemical Tracers version 2.4 (MOZART-2). We calculate crop losses according to two metrics of ozone exposure – seasonal daytime (08:00–19:59) mean O3 (M12) and accumulated O3 above a threshold of 40 ppbv (AOT40) – and predict crop yield losses using crop-specific O3 concentration:response functions established by field studies. Our results indicate that year 2000 O3-induced global yield reductions ranged, depending on the metric used, from 8.5–14% for soybean, 3.9–15% for wheat, and 2.2–5.5% for maize. Global crop production losses totaled 79–121 million metric tons, worth $11–18 billion annually (USD2000). Our calculated yield reductions agree well with previous estimates, providing further evidence that yields of major crops across the globe are already being substantially reduced by exposure to surface ozone – a risk that will grow unless O3-precursor emissions are curbed in the future or crop cultivars are developed and utilized that are resistant to O3.
Avnery, S, D L Mauzerall, J Liu, and Larry W Horowitz, April 2011: Global crop yield reductions due to surface ozone exposure: 2. Year 2030 potential crop production losses and economic damage under two scenarios of O3 pollution. Atmospheric Environment, 45(13), DOI:10.1016/j.atmosenv.2011.01.002. Abstract
We examine the potential global risk of increasing surface ozone (O3) exposure to three key staple crops (soybean, maize, and wheat) in the near future (year 2030) according to two trajectories of O3 pollution: the Intergovernmental Panel on Climate Change Special Report on Emissions Scenarios (IPCC SRES) A2 and B1 storylines, which represent upper- and lower-boundary projections, respectively, of most O3 precursor emissions in 2030. We use simulated hourly O3 concentrations from the Model for Ozone and Related Chemical Tracers version 2.4 (MOZART-2), satellite-derived datasets of agricultural production, and field-based concentration:response relationships to calculate crop yield reductions resulting from O3 exposure. We then calculate the associated crop production losses and their economic value. We compare our results to the estimated impact of O3 on global agriculture in the year 2000, which we assessed in our companion paper [Avnery et al., 2011]. In the A2 scenario we find global year 2030 yield loss of wheat due to O3 exposure ranges from 5.4 to 26% (a further reduction in yield of +1.5–10% from year 2000 values), 15–19% for soybean (reduction of +0.9–11%), and 4.4–8.7% for maize (reduction of +2.1–3.2%) depending on the metric used, with total global agricultural losses worth $17–35 billion USD2000 annually (an increase of +$6–17 billion in losses from 2000). Under the B1 scenario, we project less severe but still substantial reductions in yields in 2030: 4.0–17% for wheat (a further decrease in yield of +0.1–1.8% from 2000), 9.5–15% for soybean (decrease of +0.7–1.0%), and 2.5–6.0% for maize (decrease of + 0.3–0.5%), with total losses worth $12–21 billion annually (an increase of +$1–3 billion in losses from 2000). Because our analysis uses crop data from the year 2000, which likely underestimates agricultural production in 2030 due to the need to feed a population increasing from approximately 6 to 8 billion people between 2000 and 2030, our calculations of crop production and economic losses are highly conservative. Our results suggest that O3 pollution poses a growing threat to global food security even under an optimistic scenario of future ozone precursor emissions. Further efforts to reduce surface O3 concentrations thus provide an excellent opportunity to increase global grain yields without the environmental degradation associated with additional fertilizer application or land cultivation.
The Geophysical Fluid Dynamics Laboratory (GFDL) has developed a coupled general circulation model (CM3) for atmosphere, oceans, land, and sea ice. The goal of CM3 is to address emerging issues in climate change, including aerosol-cloud interactions, chemistry-climate interactions, and coupling between the troposphere and stratosphere. The model is also designed to serve as the physical-system component of earth-system models and models for decadal prediction in the near-term future, for example, through improved simulations in tropical land precipitation relative to earlier-generation GFDL models. This paper describes the dynamical core, physical parameterizations, and basic simulation characteristics of the atmospheric component (AM3) of this model.
Relative to GFDL AM2, AM3 includes new treatments of deep and shallow cumulus convection, cloud-droplet activation by aerosols, sub-grid variability of stratiform vertical velocities for droplet activation, and atmospheric chemistry driven by emissions with advective, convective, and turbulent transport. AM3 employs a cubed-sphere implementation of a finite-volume dynamical core and is coupled to LM3, a new land model with eco-system dynamics and hydrology.
Most basic circulation features in AM3 are simulated as realistically, or more so, than in AM2. In particular, dry biases have been reduced over South America. In coupled mode, the simulation of Arctic sea ice concentration has improved. AM3 aerosol optical depths, scattering properties, and surface clear-sky downward shortwave radiation are more realistic than in AM2. The simulation of marine stratocumulus decks and the intensity distributions of precipitation remain problematic, as in AM2.
The last two decades of the 20th century warm in CM3 by .32°C relative to 1881-1920. The Climate Research Unit (CRU) and Goddard Institute for Space Studies analyses of observations show warming of .56°C and .52°C, respectively, over this period. CM3 includes anthropogenic cooling by aerosol cloud interactions, and its warming by late 20th century is somewhat less realistic than in CM2.1, which warmed .66°C but did not include aerosol cloud interactions. The improved simulation of the direct aerosol effect (apparent in surface clear-sky downward radiation) in CM3 evidently acts in concert with its simulation of cloud-aerosol interactions to limit greenhouse gas warming in a way that is consistent with observed global temperature changes.
Air pollution (ozone and particulate matter in surface air) is strongly linked to synoptic weather and thus is likely sensitive to climate change. In order to isolate the responses of air pollutant transport and wet removal to a warming climate, we examine a simple carbon monoxide (CO)–like tracer (COt) and a soluble version (SAt), both with the 2001 CO emissions, in simulations with the GFDL chemistry-climate model (AM3) for present (1981-2000) and future (2081-2100) climates. In 2081-2100, projected reductions in lower tropospheric ventilation and wet deposition exacerbate surface air pollution as evidenced by higher surface COt and SAt concentrations. However, the average horizontal general circulation patterns in 2081-2100 are similar to 1981-2000, so the spatial distribution of COt changes little. Precipitation is an important factor controlling soluble pollutant wet removal, but the total global precipitation change alone does not necessarily indicate the sign of the soluble pollutant response to climate change. Over certain latitudinal bands, however, the annual wet deposition change can be explained mainly by the simulated changes in large-scale (LS) precipitation. In regions such as North America, differences in the seasonality of LS precipitation and tracer burdens contribute to an apparent inconsistency of changes in annual wet deposition versus annual precipitation. As a step towards an ultimate goal of developing a simple index that can be applied to infer changes in soluble pollutants directly from changes in precipitation fields as projected by physical climate models, we explore here a “Diagnosed Precipitation Impact” (DPI) index. This index captures the sign and magnitude (within 50%) of the relative annual mean changes in the global wet deposition of the soluble pollutant. DPI can only be usefully applied in climate models in which LS precipitation dominates wet deposition and horizontal transport patterns change little as climate warms. Our findings support the need for tighter emission regulations, for both soluble and insoluble pollutants, to obtain a desired level of air quality as climate warms.
The recently developed GFDL Atmospheric Model version 3 (AM3), an atmospheric general circulation model (GCM), incorporates a prognostic treatment of cloud drop number to simulate the aerosol indirect effect. Since cloud drop activation depends on cloud-scale vertical velocities, which are not reproduced in present-day GCMs, additional assumptions on the subgrid variability are required to implement a local activation parameterization into a GCM.
This paper describes the subgrid activation assumptions in AM3 and explores sensitivities by constructing alternate configurations. These alternate model configurations exhibit only small differences in their present-day climatology. However, the total anthropogenic radiative flux perturbation (RFP) between present-day and preindustrial conditions varies by ±50% from the reference, because of a large difference in the magnitude of the aerosol indirect effect. The spread in RFP does not originate directly from the subgrid assumptions but indirectly through the cloud retuning necessary to maintain a realistic radiation balance. In particular, the paper shows a linear correlation between the choice of autoconversion threshold radius and the RFP.
Climate sensitivity changes only minimally between the reference and alternate configurations. If implemented in a fully coupled model, these alternate configurations would therefore likely produce substantially different warming from preindustrial to present day.
This paper documents time mean simulation characteristics from the ocean and sea ice components in a new coupled climate model developed at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL). The climate model, known as CM3, is formulated with effectively the same ocean and sea ice components as the earlier GFDL climate model, CM2.1, yet with extensive developments made to the atmosphere and land model components. Both CM2.1 and CM3 show stable mean climate indices, such as large scale circulation and sea surface temperatures (SSTs). There are notable improvements in the CM3 climate simulation relative to CM2.1, including a modified SST bias pattern and reduced biases in the Arctic sea ice cover. We anticipate SST differences between CM2.1 and CM3 in lower latitudes through analysis of the atmospheric fluxes at the ocean surface in corresponding Atmospheric Model Intercomparison Project (AMIP) simulations. In contrast, SST changes in the high latitudes are dominated by ocean and sea ice effects absent in AMIP simulations. The ocean interior simulation in CM3 is generally warmer than CM2.1, which adversely impacts the interior biases.
Desert dust plays an important role in the climate system through its impact on Earth's radiative budget and its role in the biogeochemical cycle as a source of iron in high-nutrient-low-chlorophyll regions. A large degree of diversity exists between the many global models that simulate the dust cycle to estimate its impact on climate. We present the results of a broad intercomparison of a total of 15 global aerosol models within the AeroCom project. Each model is compared to observations focusing on variables responsible for the uncertainties in estimating the direct radiative effect and the dust impact on the biogeochemical cycle, i.e., aerosol optical depth (AOD) and dust deposition. Additional comparisons to Angström Exponent (AE), coarse mode AOD and dust surface concentration are included to extend the assessment of model performance. These datasets form a benchmark data set which is proposed for model inspection and future dust model developments. In general, models perform better in simulating climatology of vertically averaged integrated parameters (AOD and AE) in dusty sites than they do with total deposition and surface concentration. Almost all models overestimate deposition fluxes over Europe, the Indian Ocean, the Atlantic Ocean and ice core data. Differences among the models arise when simulating deposition at remote sites with low fluxes over the Pacific and the Southern Atlantic Ocean. This study also highlights important differences in models ability to reproduce the deposition flux over Antarctica. The cause of this discrepancy could not be identified but different dust regimes at each site and issues with data quality should be considered. Models generally simulate better surface concentration at stations downwind of the main sources than at remote ones. Likewise, they simulate better surface concentration at stations affected by Saharan dust than at stations affected by Asian dust. Most models simulate the gradient in AOD and AE between the different dusty regions, however the seasonality and magnitude of both variables is better simulated at African stations than Middle East ones. The models also reproduce the dust transport across the Atlantic in terms of both AOD and AE; they simulate the offshore transport of West Africa throughout the year and limit the transport across the Atlantic to the summer months, yet overestimating the AOD and transporting too fine particles. However, most of the models do not reproduce the southward displacement of the dust cloud during the winter responsible of the transport of dust into South America. Based on the dependency of AOD on aerosol burden and size distribution we use model data bias with respect to AOD and AE and infer on the over/under estimation of the dust emissions. According to this we suggest the emissions in the Sahara be between 792 and 2271 Tg/yr and the one in the Middle East between 212 and 329 Tg/yr.
This study evaluates the sensitivity of long-range transport of black carbon (BC)
from mid- and high-latitude source regions to the Arctic to aging, dry deposition and
wet removal processes using the GFDL coupled chemistry and climate model (AM3).
We derive a simple parameterization for BC aging (i.e., coating with soluble materials)
which allows the rate of aging to vary diurnally and seasonally. Slow aging during
winter permits BC to remain largely hydrophobic throughout transport from
mid-latitude source regions to the Arctic. In addition, we apply surface-dependent dry
deposition velocities and reduce the wet removal efficiency of BC in ice clouds. The
inclusion of the above parameterizations significantly improves simulated magnitude,
seasonal cycle and vertical profile of BC over the Arctic compared with those in the
base model configuration. In particular, wintertime concentrations of BC in the Arctic
are increased by a factor of 100 throughout the tropospheric column. Based on
sensitivity tests involving each process, we find that the transport of BC to the Arctic
is a synergistic process. A comprehensive understanding of microphysics and
chemistry related to aging, dry and wet removal processes is thus essential to the
simulation of BC concentrations over the Arctic.
Saikawa, E, J Kurokawa, M Takigawa, J Borken-Kleefeld, D L Mauzerall, Larry W Horowitz, and T Ohara, September 2011: The impact of China's vehicle emissions on regional air quality in 2000 and 2020: a scenario analysis. Atmospheric Chemistry and Physics, 11(18), DOI:10.5194/acp-11-9465-2011. Abstract
The number of vehicles in China has been increasing rapidly. We evaluate the impact of current and possible future vehicle emissions from China on Asian air quality. We modify the Regional Emission Inventory in Asia (REAS) for China's road transport sector in 2000 using updated Chinese data for vehicle numbers, annual mileage and emission factors. We develop two scenarios for 2020: a scenario where emission factors remain the same as they were before any regulation was implemented (business-as-usual, BAU), and a scenario where Euro 3 vehicle emission standards are applied to all vehicles (except motorcycles and rural vehicles). The Euro 3 scenario is an approximation of what may be the case in 2020 as, starting in 2008, all new gasoline and diesel vehicles in China (except motorcycles) were required to meet the Euro 3 emission standards. Using the Weather Research and Forecasting model coupled with Chemistry (WRF/Chem), we examine the regional air quality response to China's vehicle emissions in 2000 and in 2020 for the BAU and Euro 3 scenarios. We evaluate the 2000 model results with observations in Japan, China, Korea, and Russia. Under BAU in 2020, emissions of carbon monoxide (CO), nitrogen oxides (NOx), non-methane volatile organic compounds (NMVOCs), black carbon (BC) and organic carbon (OC) from China's vehicles more than double compared to the 2000 baseline. If all vehicles meet the Euro 3 regulations in 2020, however, these emissions are reduced by more than 50% relative to BAU. The implementation of stringent vehicle emission standards leads to a large, simultaneous reduction of the surface ozone (O3) mixing ratios and particulate matter (PM2.5) concentrations. In the Euro 3 scenario, surface O3 is reduced by more than 10 ppbv and surface PM2.5 is reduced by more than 10 μg m−3 relative to BAU in Northeast China in all seasons. In spring, surface O3 mixing ratios and PM2.5 concentrations in neighboring countries are also reduced by more than 3 ppbv and 1 μg m−3, respectively. We find that effective regulation of China's road transport sector will be of significant benefit for air quality both within China and across East Asia as well.
Anenberg, S C., Larry W Horowitz, D Q Tong, and J Jason West, September 2010: An estimate of the global burden of anthropogenic ozone and fine particulate matter on premature human mortality using atmospheric modeling. Environmental Health Perspectives, 118(9), DOI:10.1289/ehp.0901220. Abstract
Background: Ground-level concentrations of ozone (O3) and fine particulate matter [≤ 2.5 µm in aerodynamic diameter (PM2.5)] have increased since preindustrial times in urban and rural regions and are associated with cardiovascular and respiratory mortality.
Objectives: We estimated the global burden of mortality due to O3 and PM2.5 from anthropogenic emissions using global atmospheric chemical transport model simulations of preindustrial and present-day (2000) concentrations to derive exposure estimates.
Methods: Attributable mortalities were estimated using health impact functions based on long-term relative risk estimates for O3 and PM2.5 from the epidemiology literature. Using simulated concentrations rather than previous methods based on measurements allows the inclusion of rural areas where measurements are often unavailable and avoids making assumptions for background air pollution.
Results: Anthropogenic O3 was associated with an estimated 0.7 ± 0.3 million respiratory mortalities (6.3 ± 3.0 million years of life lost) annually. Anthropogenic PM2.5 was associated with 3.5 ± 0.9 million cardiopulmonary and 220,000 ± 80,000 lung cancer mortalities (30 ± 7.6 million years of life lost) annually. Mortality estimates were reduced approximately 30% when we assumed low-concentration thresholds of 33.3 ppb for O3 and 5.8 µg/m3 for PM2.5. These estimates were sensitive to concentration thresholds and concentration–mortality relationships, often by > 50%.
Conclusions: Anthropogenic O3 and PM2.5 contribute substantially to global premature mortality. PM2.5 mortality estimates are about 50% higher than previous measurement-based estimates based on common assumptions, mainly because of methodologic differences. Specifically, we included rural populations, suggesting higher estimates; however, the coarse resolution of the global atmospheric model may underestimate urban PM2.5 exposures.
Fang, Y, Arlene M Fiore, Larry W Horowitz, Hiram Levy II, Y Hu, and A G. Russell, September 2010: Sensitivity of the NOy budget over the United States to anthropogenic and lightning NOx in summer. Journal of Geophysical Research: Atmospheres, 115, D18312, DOI:10.1029/2010JD014079. Abstract
We examine the implications of new estimates of the anthropogenic and lightning
nitrogen oxide (NOx) source for the budget of oxidized nitrogen (NOy) over the United
States in summer using a 3-D global chemical transport model (MOZART-4). As a result
of the EPA State Implementation (SIP) call, power plant NOx emissions over the eastern
United States decreased significantly, as reflected by a 23% decrease in summer surface
emissions from our 2004 inventory to the 1999 U.S. EPA National Emissions Inventory.
We increase the model lightning NOx source over northern mid-latitude continents (by a
factor of 10) and the fraction emitted into the free troposphere (FT, from 80% to 98%) to
better match the recent observation-based estimates. While these NOx source updates
improve the simulation of NOx and O3 compared to the INTEX-NA aircraft observations,
a bias in the partitioning between HNO3 and PAN remains especially above 8km,
suggesting gaps in the current understanding of upper tropospheric processes. We
estimate a model NOy export efficiency of 4-14% to the North Atlantic in the FT, within
the range of previous plume-based estimates (3%-20%) and lower than the 30% exported
directly from the continental boundary layer. Lightning NOx contributes 24-43% of the
FT NOy export from the U.S. to the North Atlantic and 28-34% to the NOy wet deposition
over the United States, with the ranges reflecting different assumptions. Increasing
lightning NOx decreases the fractional contribution of PAN to total NOy export, increases
the O3 production in the northern extratropical FT by 33%, and decreases the regional
mean ozone production efficiency per unit NOx (OPE) by 30%. If models underestimate
the lightning NOx source, they would overestimate the background OPE in the FT and the
fractional contribution of PAN to NOy export. Therefore, a model underestimate oflightning NOx would likely lead to an overestimate of the downwind O3 production due
to anthropogenic NOx export. Better constraints on the lightning NOx source are required
to more confidently assess the impacts of anthropogenic emissions and their changes on
air quality over downwind regions.
Energy security and climate change concerns have led to the promotion of biomass-derived ethanol, an oxygenated volatile organic compound (OVOC), as a substitute for fossil fuels. Although ethanol is ubiquitous in the troposphere, our knowledge of its current atmospheric budget and distribution is limited. Here, for the first time we use a global chemical transport model in conjunction with atmospheric observations to place constraints on the ethanol budget, noting that additional measurements of ethanol (and its precursors) are still needed to enhance confidence in our estimated budget. Global sources of ethanol in the model include 5.0 Tg yr−1 from industrial sources and biofuels, 9.2 Tg yr−1 from terrestrial plants, ~0.5 Tg yr−1 from biomass burning, and 0.05 Tg yr−1 from atmospheric reactions of the ethyl peroxy radical (C2H5O2) with itself and with the methyl peroxy radical (CH3O2). The resulting atmospheric lifetime of ethanol in the model is 2.8 days. Gas-phase oxidation by the hydroxyl radical (OH) is the primary global sink of ethanol in the model (65%), followed by dry deposition (25%), and wet deposition (10%). Over continental areas, ethanol concentrations predominantly reflect direct anthropogenic and biogenic emission sources. Uncertainty in the biogenic ethanol emissions, estimated at a factor of three, may contribute to the 50% model underestimate of observations in the North American boundary layer. Current levels of ethanol measured in remote regions are an order of magnitude larger than those in the model, suggesting a major gap in understanding. Stronger constraints on the budget and distribution of ethanol and OVOCs are a critical step towards assessing the impacts of increasing the use of ethanol as a fuel.
Fang, Y, Arlene M Fiore, Larry W Horowitz, Anand Gnanadesikan, Hiram Levy II, Y Hu, and A G. Russell, December 2009: Estimating the contribution of strong daily export events to total pollutant export from the United States in summer. Journal of Geophysical Research: Atmospheres, 114, D23302, DOI:10.1029/2008JD010946. Abstract
While the export of pollutants from the United States exhibits notable variability from day to day and is often considered to be “episodic,” the contribution of strong daily export events to total export has not been quantified. We use carbon monoxide (CO) as a tracer of anthropogenic pollutants in the Model of OZone And Related Tracers (MOZART) to estimate this contribution. We first identify the major export pathway from the United States to be through the northeast boundary (24–48°N along 67.5°W and 80–67.5°W along 48°N), and then analyze 15 summers of daily CO export fluxes through this boundary. These daily CO export fluxes have a nearly Gaussian distribution with a mean of 1100 Gg CO day−1 and a standard deviation of 490 Gg CO day−1. To focus on the synoptic variability, we define a “synoptic background” export flux equal to the 15 day moving average export flux and classify strong export days according to their fluxes relative to this background. As expected from Gaussian statistics, 16% of summer days are “strong export days,” classified as those days when the CO export flux exceeds the synoptic background by one standard deviation or more. Strong export days contributes 25% to the total export, a value determined by the relative standard deviation of the CO flux distribution. Regressing the anomalies of the CO export flux through the northeast U.S. boundary relative to the synoptic background on the daily anomalies in the surface pressure field (also relative to a 15 day running mean) suggests that strong daily export fluxes are correlated with passages of midlatitude cyclones over the Gulf of Saint Lawrence. The associated cyclonic circulation and Warm Conveyor Belts (WCBs) that lift surface pollutants over the northeastern United States have been shown previously to be associated with long-range transport events. Comparison with observations from the 2004 INTEX-NA field campaign confirms that our model captures the observed enhancements in CO outflow and resolves the processes associated with cyclone passages on strong export days. “Moderate export days,” defined as days when the CO flux through the northeast boundary exceeds the 15 day running mean by less than one standard deviation, represent an additional 34% of summer days and 40% of total export. These days are also associated with migratory midlatitude cyclones. The remaining 35% of total export occurs on “weak export days” (50% of summer days) when high pressure anomalies occur over the Gulf of Saint Lawrence. Our findings for summer also apply to spring, when the U.S. pollutant export is typically strongest, with similar contributions to total export and associated meteorology on strong, moderate and weak export days. Although cyclone passages are the primary driver for strong daily export events, export during days without cyclone passages also makes a considerable contribution to the total export and thereby to the global pollutant budget.
Understanding the surface O3
response over a “receptor” region to emission changes over a foreign
“source” region is key to evaluating the potential gains from an
international approach to abate ozone (O3) pollution. We apply an
ensemble of 21 global and hemispheric chemical transport models to estimate
the spatial average surface O3 response over east Asia (EA),
Europe (EU), North America (NA), and south Asia (SA) to 20% decreases in
anthropogenic emissions of the O3 precursors, NOx,
NMVOC, and CO (individually and combined), from each of these regions. We
find that the ensemble mean surface O3 concentrations in the base
case (year 2001) simulation matches available observations throughout the
year over EU but overestimates them by >10 ppb during summer and early fall
over the eastern United States and Japan. The sum of the O3
responses to NOx, CO, and NMVOC decreases separately is
approximately equal to that from a simultaneous reduction of all precursors.
We define a continental-scale “import sensitivity” as the ratio of the O3
response to the 20% reductions in foreign versus “domestic” (i.e., over the
source region itself) emissions. For example, the combined reduction of
emissions from the three foreign regions produces an ensemble spatial mean
decrease of 0.6 ppb over EU (0.4 ppb from NA), less than the 0.8 ppb from
the reduction of EU emissions, leading to an import sensitivity ratio of
0.7. The ensemble mean surface O3 response to foreign emissions
is largest in spring and late fall (0.7–0.9 ppb decrease in all regions from
the combined precursor reductions in the three foreign regions), with import
sensitivities ranging from 0.5 to 1.1 (responses to domestic emission
reductions are 0.8–1.6 ppb). High O3 values are much more
sensitive to domestic emissions than to foreign emissions, as indicated by
lower import sensitivities of 0.2 to 0.3 during July in EA, EU, and NA when
O3 levels are typically highest and by the weaker relative
response of annual incidences of daily maximum 8-h average O3
above 60 ppb to emission reductions in a foreign region (<10–20% of that to
domestic) as compared to the annual mean response (up to 50% of that to
domestic). Applying the ensemble annual mean results to changes in
anthropogenic emissions from 1996 to 2002, we estimate a Northern
Hemispheric increase in background surface O3 of about 0.1 ppb a−1,
at the low end of the 0.1–0.5 ppb a−1 derived from observations.
From an additional simulation in which global atmospheric methane was
reduced, we infer that 20% reductions in anthropogenic methane emissions
from a foreign source region would yield an O3 response in a
receptor region that roughly equals that produced by combined 20% reductions
of anthropogenic NOx, NMVOC, and CO emissions from the foreign
source region.
We evaluate black carbon (BC) model predictions from the AeroCom model intercomparison project by considering the diversity among year 2000 model simulations and comparing model predictions with available measurements. These model-measurement intercomparisons include BC surface and aircraft concentrations, aerosol absorption optical depth (AAOD) retrievals from AERONET and Ozone Monitoring Instrument (OMI) and BC column estimations based on AERONET. In regions other than Asia, most models are biased high compared to surface concentration measurements. However compared with (column) AAOD or BC burden retreivals, the models are generally biased low. The average ratio of model to retrieved AAOD is less than 0.7 in South American and 0.6 in African biomass burning regions; both of these regions lack surface concentration measurements. In Asia the average model to observed ratio is 0.7 for AAOD and 0.5 for BC surface concentrations. Compared with aircraft measurements over the Americas at latitudes between 0 and 50N, the average model is a factor of 8 larger than observed, and most models exceed the measured BC standard deviation in the mid to upper troposphere. At higher latitudes the average model to aircraft BC ratio is 0.4 and models underestimate the observed BC loading in the lower and middle troposphere associated with springtime Arctic haze. Low model bias for AAOD but overestimation of surface and upper atmospheric BC concentrations at lower latitudes suggests that most models are underestimating BC absorption and should improve estimates for refractive index, particle size, and optical effects of BC coating. Retrieval uncertainties and/or differences with model diagnostic treatment may also contribute to the model-measurement disparity. Largest AeroCom model diversity occurred in northern Eurasia and the remote Arctic, regions influenced by anthropogenic sources. Changing emissions, aging, removal, or optical properties within a single model generated a smaller change in model predictions than the range represented by the full set of AeroCom models. Upper tropospheric concentrations of BC mass from the aircraft measurements are suggested to provide a unique new benchmark to test scavenging and vertical dispersion of BC in global models.
Our objectives are to evaluate inter-continental source-receptor relationships for fine aerosols and to identify the regions whose emissions have dominant influence on receptor continents. We simulate sulfate, black carbon (BC), organic carbon (OC), and mineral dust aerosols using a global coupled chemistry-aerosol model (MOZART-2) driven with NCEP/NCAR reanalysis meteorology for 1997–2003 and emissions approximately representing year 2000. The concentrations of simulated aerosol species in general agree within a factor of 2 with observations, except that the model tends to overestimate sulfate over Europe in summer, underestimate BC and OC over the western and southeastern (SE) U.S. and Europe, and underestimate dust over the SE U.S. By tagging emissions from ten continental regions, we quantify the contribution of each region's emissions on surface aerosol concentrations (relevant for air quality) and aerosol optical depth (AOD, relevant for visibility and climate) globally. We find that domestic emissions contribute substantially to surface aerosol concentrations (57–95%) over all regions, but are responsible for a smaller fraction of AOD (26–76%). We define “background” aerosols as those aerosols over a region that result from inter-continental transport, DMS oxidation, and emissions from ships or volcanoes. Transport from other continental source regions accounts for a substantial portion of background aerosol concentrations: 36–97% for surface concentrations and 38–89% for AOD. We identify the Region of Primary Influence (RPI) as the source region with the largest contribution to the receptor's background aerosol concentrations (or AOD). We find that for dust Africa is the RPI for both aerosol concentrations and AOD over all other receptor regions. For non-dust aerosols (particularly for sulfate and BC), the RPIs for aerosol concentrations and AOD are identical for most receptor regions. These findings indicate that the reduction of the emission of non-dust aerosols and their precursors from an RPI will simultaneously improve both air quality and visibility over a receptor region.
Liu, J, D L Mauzerall, and Larry W Horowitz, September 2009: Evaluating inter-continental transport of fine aerosols:(2) Global health impact. Atmospheric Environment, 43(28), DOI:10.1016/j.atmosenv.2009.05.032. Abstract
In this second of two companion papers, we quantify for the first time the global impact on premature mortality of the inter-continental transport of fine aerosols (including sulfate, black carbon, organic carbon, and mineral dust) using the global modeling results of (Liu et al., 2009). Our objective is to estimate the number of premature mortalities in each of ten selected continental regions resulting from fine aerosols transported from foreign regions in approximately year 2000. Our simulated annual mean population-weighted (P-W) concentrations of total PM2.5 (aerosols with diameter less than 2.5 μm) are highest in East Asia (EA, 30 μg m−3) and lowest in Australia (3.6 μg m−3). Dust is the dominant component of PM2.5 transported between continents. We estimate global annual premature mortalities (for adults age 30 and up) due to inter-continental transport of PM2.5 to be nearly 380 thousand (K) in 2000. Approximately half of these deaths occur in the Indian subcontinent (IN), mostly due to aerosols transported from Africa and the Middle East (ME). Approximately 90K deaths globally are associated with exposure to foreign (i.e., originating outside a receptor region) non-dust PM2.5. More than half of the premature mortalities associated with foreign non-dust aerosols are due to aerosols originating from Europe (20K), ME (18K) and EA (15K); and nearly 60% of the 90K deaths occur in EA (21K), IN (19K) and Southeast Asia (16K). The lower and higher bounds of our estimated 95% confidence interval (considering uncertainties from the concentration–response relationship and simulated aerosol concentrations) are 18% and 240% of the estimated deaths, respectively, and could be larger if additional uncertainties were quantified. We find that in 2000 nearly 6.6K premature deaths in North America (NA) were associated with foreign PM2.5 exposure (5.5K from dust PM2.5). NA is least impacted by foreign PM2.5 compared to receptors on the Eurasian continent. However, the number of premature mortalities associated with foreign aerosols in NA (mostly occurring in the U.S.) is comparable to the reduction in premature mortalities expected to result from tightening the U.S. 8-h O3 standard from 0.08 ppmv to 0.075 ppmv. International efforts to reduce inter-continental transport of fine aerosol pollution would substantially benefit public health on the Eurasian continent and would also benefit public health in the United States.
Livingstone, P L., and Larry W Horowitz, et al., December 2009: Simulating PM concentration during a winter episode in a subtropical valley: Sensitivity simulations and evaluation methods. Atmospheric Environment, 43(37), DOI:10.1016/j.atmosenv.2009.07.033. Abstract
We investigated a two-week episode with high PM concentrations in California Central Valley during the Christmas–New Year of 2000–2001 using a modeling system that consists of a computationally efficient, 3-D photochemical–microphysical transport model, a mesoscale meteorological model, emission models, and an evaluation package. One hundred simulations were conducted with fine resolutions and observational constraints, to reproduce spatial and temporal features of observed PM concentrations and to understand the formation mechanism of the episode. Simulated PM concentrations consist of secondary inorganic components, mainly ammonium nitrate, and total carbon in areas with elevated concentrations in the accumulation mode, and consist of mainly dust and sea salt in the coarse mode. Simulated oxidants and nitrate were significantly elevated over the valley, and the latter showed much less amplitude than the former. Simulated PM concentrations were evaluated with observations systematically with spatially and temporally paired method, a more restrictive multivariate method (NMFROC), and a more flexible “gradient evaluation” method. The paired evaluation shows that high correlation coefficient (R = ~0.8) and low fractional error (FE = ~0.1) could be achieved at stations with elevated 24-h concentration of PM in the accumulation mode in some simulations. The NMFROC method was used to extract useful information from seemingly failed simulations. A “gradient evaluation” method is introduced here to extract additional information from simulations. We found that emission reductions of NOx and AVOC showed similar effects on percentage basis in different areas, and both are more effective than reducing NH3 for abating elevated concentrations of accumulation mode PM in California Central Valley during the winter episode.
Saikawa, E, Vaishali Naik, Larry W Horowitz, J Liu, and D L Mauzerall, June 2009: Present and potential future contributions of sulfate, black and organic carbon aerosols from China to global air quality, premature mortality and radiative forcing. Atmospheric Environment, 43(17), DOI:10.1016/j.atmosenv.2009.02.017. Abstract
Aerosols are harmful to human health and have both direct and indirect effects on climate. China is a major contributor to global emissions of sulfur dioxide (SO2), a sulfate (SO42−) precursor, organic carbon (OC), and black carbon (BC) aerosols. Although increasingly examined, the effect of present and potential future levels of these emissions on global premature mortality and climate change has not been well quantified. Through both direct radiative effects and indirect effects on clouds, SO42− and OC exert negative radiative forcing (cooling) while BC exerts positive forcing (warming). We analyze the effect of China's emissions of SO2, SO42−, OC and BC in 2000 and for three emission scenarios in 2030 on global surface aerosol concentrations, premature mortality, and radiative forcing (RF). Using global models of chemical transport (MOZART-2) and radiative transfer (GFDL RTM), and combining simulation results with gridded population data, mortality rates, and concentration–response relationships from the epidemiological literature, we estimate the contribution of Chinese aerosols to global annual premature mortality and to RF in 2000 and 2030. In 2000, we estimate these aerosols cause approximately 470 000 premature deaths in China and an additional 30 000 deaths globally. In 2030, aggressive emission controls lead to a 50% reduction in premature deaths from the 2000 level to 240 000 in China and 10 000 elsewhere, while under a high emissions scenario premature deaths increase 50% from the 2000 level to 720 000 in China and to 40 000 elsewhere. Because the negative RF from SO42− and OC is larger than the positive forcing from BC, Chinese aerosols lead to global net direct RF of −74 mW m−2 in 2000 and between −15 and −97 mW m−2 in 2030 depending on the emissions scenario. Our analysis indicates that increased effort to reduce greenhouse gases is essential to address climate change as China's anticipated reduction of aerosols will result in the loss of net negative radiative forcing.
Observations and models
demonstrate that ozone and its precursors can be transported between
continents and across oceans. We model the influences of 10% reductions in
anthropogenic nitrogen oxide (NOx) emissions from each of nine
world regions on surface ozone air quality in that region and all other
regions. In doing so, we quantify the relative importance of long-range
transport between all source-receptor pairs, for direct short-term ozone
changes. We find that for population-weighted concentrations during the
three-month "ozone-season", the strongest inter-regional influences are from
Europe to the Former Soviet Union, East Asia to Southeast Asia, and Europe
to Africa. The largest influences per unit of NOx reduced,
however, are seen for source regions in the tropics and Southern Hemisphere,
which we attribute mainly to greater sensitivity to changes in NOx
in the lower troposphere, and secondarily to increased vertical convection
to the free troposphere in tropical regions, allowing pollutants to be
transported further. Results show, for example, that NOx
reductions in North America are ~20% as effective per unit NOx in
reducing ozone in Europe during summer, as NOx reductions from
Europe itself. Reducing anthropogenic emissions of non-methane volatile
organic compounds (NMVOCs) and carbon monoxide (CO) by 10% in selected
regions, can have as large an impact on long-range ozone transport as NOx
reductions, depending on the source region. We find that for many
source-receptor pairs, the season of greatest long-range influence does not
coincide with the season when ozone is highest in the receptor region.
Reducing NOx emissions in most source regions causes a larger
decrease in export of ozone from the source region than in ozone production
outside of the source region.
West, J J., Vaishali Naik, Larry W Horowitz, and Arlene M Fiore, August 2009: Effect of regional precursor emission controls on long-range ozone transport – Part 2: Steady-state changes in ozone air quality and impacts on human mortality. Atmospheric Chemistry and Physics, 9(16), DOI:10.5194/acp-9-6095-2009. Abstract
Large-scale changes in ozone precursor emissions affect ozone directly in the short term, and also affect methane, which in turn causes long-term changes in ozone that affect surface ozone air quality. Here we assess the effects of changes in ozone precursor emissions on the long-term change in surface ozone via methane, as a function of the emission region, by modeling 10% reductions in anthropogenic nitrogen oxide (NOx) emissions from each of nine world regions. Reductions in NOx emissions from all world regions increase methane and long-term surface ozone. While this long-term increase is small compared to the intra-regional short-term ozone decrease, it is comparable to or larger than the short-term inter-continental ozone decrease for some source-receptor pairs. The increase in methane and long-term surface ozone per ton of NOx reduced is greatest in tropical and Southern Hemisphere regions, exceeding that from temperate Northern Hemisphere regions by roughly a factor of ten. We also assess changes in premature ozone-related human mortality associated with regional precursor reductions and long-range transport, showing that for 10% regional NOx reductions, the strongest inter-regional influence is for emissions from Europe affecting mortalities in Africa. Reductions of NOx in North America, Europe, the Former Soviet Union, and Australia are shown to reduce more mortalities outside of the source regions than within. Among world regions, NOx reductions in India cause the greatest number of avoided mortalities per ton, mainly in India itself. Finally, by increasing global methane, NOx reductions in one hemisphere tend to cause long-term increases in ozone concentration and mortalities in the opposite hemisphere. Reducing emissions of methane, and to a lesser extent carbon monoxide and non-methane volatile organic compounds, alongside NOx reductions would avoid this disbenefit.
Reducing methane (CH4) emissions is an attractive option for jointly addressing climate and ozone (O3) air quality goals. With multidecadal full-chemistry transient simulations in the MOZART-2 tropospheric chemistry model, we show that tropospheric O3 responds approximately linearly to changes in CH4 emissions over a range of anthropogenic emissions from 0–430 Tg CH4a−1 (0.11–0.16 Tg tropospheric O3 or ∼11–15 ppt global mean surface O3 decrease per Tg a−1 CH4 reduced). We find that neither the air quality nor climate benefits depend strongly on the location of the CH4 emission reductions, implying that the lowest cost emission controls can be targeted. With a series of future (2005–2030) transient simulations, we demonstrate that cost-effective CH4 controls would offset the positive climate forcing from CH4 and O3 that would otherwise occur (from increases in NOx and CH4 emissions in the baseline scenario) and improve O3 air quality. We estimate that anthropogenic CH4 contributes 0.7 Wm−2 to climate forcing and ∼4 ppb to surface O3 in 2030 under the baseline scenario. Although the response of surface O3 to CH4 is relatively uniform spatially compared to that from other O3 precursors, it is strongest in regions where surface air mixes frequently with the free troposphere and where the local O3 formation regime is NOx-saturated. In the model, CH4 oxidation within the boundary layer (below ∼2.5 km) contributes more to surface O3 than CH4 oxidation in the free troposphere. In NOx-saturated regions, the surface O3 sensitivity to CH4 can be twice that of the global mean, with >70% of this sensitivity resulting from boundary layer oxidation of CH4. Accurately representing the NOx distribution is thus crucial for quantifying the O3 sensitivity to CH4.
Heald, C L., C Henze, Larry W Horowitz, J Feddema, Jean-Francois Lamarque, A Guenther, Peter G Hess, F Vitt, J H Seinfeld, A H Goldstein, and I Y Fung, March 2008: Predicted change in global secondary organic aerosol concentrations in response to future climate, emissions, and land use change. Journal of Geophysical Research, 113, D05211, DOI:10.1029/2007JD009092. Abstract
The sensitivity of secondary organic aerosol (SOA) concentration to changes in climate and emissions is investigated using a coupled global atmosphere-land model driven by the year 2100 IPCC A1B scenario predictions. The Community Atmosphere Model (CAM3) is updated with recent laboratory determined yields for SOA formation from monoterpene oxidation, isoprene photooxidation and aromatic photooxidation. Biogenic emissions of isoprene and monoterpenes are simulated interactively using the Model of Emissions of Gases and Aerosols (MEGAN2) within the Community Land Model (CLM3). The global mean SOA burden is predicted to increase by 36% in 2100, primarily the result of rising biogenic and anthropogenic emissions which independently increase the burden by 26% and 7%. The later includes enhanced biogenic SOA formation due to increased emissions of primary organic aerosol (5–25% increases in surface SOA concentrations in 2100). Climate change alone (via temperature, removal rates, and oxidative capacity) does not change the global mean SOA production, but the global burden increases by 6%. The global burden of anthropogenic SOA experiences proportionally more growth than biogenic SOA in 2100 from the net effect of climate and emissions (67% increase predicted). Projected anthropogenic land use change for 2100 (A2) is predicted to reduce the global SOA burden by 14%, largely the result of cropland expansion. South America is the largest global source region for SOA in the present day and 2100, but Asia experiences the largest relative growth in SOA production by 2100 because of the large predicted increases in Asian anthropogenic aromatic emissions. The projected decrease in global sulfur emissions implies that SOA will contribute a progressively larger fraction of the global aerosol burden.
This study quantifies the seasonality and geographic variability of global pollutant inflow to Asia. Asia is often looked to as a major source of intercontinental air pollution transport with rising emissions and efficient pollutant export processes. However, the degree to which foreign emissions have been imported to Asia has not been thoroughly examined. The Model Inter-Comparison Study for Asia (MICS-Asia) is an international collaboration to study air pollution transport and chemistry in Asia. Using the global atmospheric chemistry Model of Ozone and Related Tracers (MOZART v. 2.4), and comparing results with a suite of regional models participating in MICS-Asia, we find that imported O3 contributes significantly throughout Asia. The choice of upper boundary condition is found to be particularly important for O3, even for surface concentrations. Both North America and Europe contribute to ground-level O3 concentrations throughout the region, though the seasonality of these two sources varies. North American contributions peak at over 10% of monthly mean O3 during winter months in East Asia, compared to Europe's spring- and autumn-maxima (5–8%). In comparison to observed data from the Acid Deposition Monitoring Network in East Asia (EANET), MOZART concentrations for O3 generally fall within the range of the MICS models, but MOZART is unable to capture the fine spatial variability of shorter-lived species as well as the regional models.
This study examines the impact of
projected changes (A1B “marker” scenario) in emissions of four short-lived
air pollutants (ozone, black carbon, organic carbon, and sulfate) on future
climate. Through year 2030, simulated climate is only weakly dependent on
the projected levels of short-lived air pollutants, primarily the result of
a near cancellation of their global net radiative forcing. However, by year
2100, the projected decrease in sulfate aerosol (driven by a 65% reduction
in global sulfur dioxide emissions) and the projected increase in black
carbon aerosol (driven by a 100% increase in its global emissions)
contribute a significant portion of the simulated A1B surface air warming
relative to the year 2000: 0.2°C (Southern Hemisphere), 0.4°C globally,
0.6°C (Northern Hemisphere), 1.5–3°C (wintertime Arctic), and 1.5–2°C (∼40%
of the total) in the summertime United States. These projected changes are
also responsible for a significant decrease in central United States late
summer root zone soil water and precipitation. By year 2100, changes in
short-lived air pollutants produce a global average increase in radiative
forcing of ∼1 W/m2; over east Asia it exceeds 5 W/m2.
However, the resulting regional patterns of surface temperature warming do
not follow the regional patterns of changes in short-lived species
emissions, tropospheric loadings, or radiative forcing (global pattern
correlation coefficient of −0.172). Rather, the regional patterns of warming
from short-lived species are similar to the patterns for well-mixed
greenhouse gases (global pattern correlation coefficient of 0.8) with the
strongest warming occurring over the summer continental United States,
Mediterranean Sea, and southern Europe and over the winter Arctic.
Parrington, M, D B A Jones, K W Bowman, Larry W Horowitz, A M Thompson, D W Tarasick, and J. C. Witte, 2008: Estimating the summertime tropospheric ozone distribution over North America through assimilation of observations from the Tropospheric Emission Spectrometer. Journal of Geophysical Research, 113, D18307, DOI:10.1029/2007JD009341. Abstract
We assimilate ozone and CO retrievals from the Tropospheric Emission Spectrometer (TES) for July and August 2006 into the GEOS-Chem and AM2-Chem models. We show that the spatiotemporal sampling of the TES measurements is sufficient to constrain the tropospheric ozone distribution in the models despite their different chemical and transport mechanisms. Assimilation of TES data reduces the mean differences in ozone between the models from almost 8 ppbv to 1.5 ppbv. Differences between the mean model profiles and ozonesonde data over North America are reduced from almost 30% to within 5% for GEOS-Chem, and from 40% to within 10% for AM2-Chem, below 200 hPa. The absolute biases are larger in the upper troposphere and lower stratosphere (UT/LS), increasing to 10% and 30% in GEOS-Chem and AM2-Chem, respectively, at 200 hPa. The larger bias in the UT/LS reflects the influence of the spatial sampling of TES, the vertical smoothing of the TES retrievals, and the coarse vertical resolution of the models. The largest discrepancy in ozone between the models is associated with the ozone maximum over the southeastern USA. The assimilation reduces the mean bias between the models from 26 to 16 ppbv in this region. In GEOS-Chem, there is an increase of about 11 ppbv in the upper troposphere, consistent with the increase in ozone obtained by a previous study using GEOS-Chem with an improved estimate of lightning NOx emissions over the USA. Our results show that assimilation of TES observations into models of tropospheric chemistry and transport provides an improved description of free tropospheric ozone.
Sanderson, M, Frank Dentener, Arlene M Fiore, C Cuvelier, T J Keating, A Zuber, C Atherton, D J Bergmann, T Diehl, R Doherty, B N Duncan, Peter G Hess, and Larry W Horowitz, et al., 2008: A multi-model study of the hemispheric transport and deposition of oxidised nitrogen. Geophysical Research Letters, 35, L17815, DOI:10.1029/2008GL035389. Abstract
Fifteen chemistry-transport models are
used to quantify, for the first time, the export of oxidised nitrogen (NOy)
to and from four regions (Europe, North America, South Asia, and East Asia),
and to estimate the uncertainty in the results. Between 12 and 24% of the NOx
emitted is exported from each region annually. The strongest impact of each
source region on a foreign region is: Europe on East Asia, North America on
Europe, South Asia on East Asia, and East Asia on North America. Europe
exports the most NOy, and East Asia the least. East Asia receives
the most NOy from the other regions. Between 8 and 15% of NOx
emitted in each region is transported over distances larger than 1000 km,
with 3–10% ultimately deposited over the foreign regions.
We use the GISS (Goddard Institute for Space Studies), GFDL (Geophysical Fluid Dynamics Laboratory) and NCAR (National Center for Atmospheric Research) climate models to study the climate impact of the future evolution of short-lived radiatively active species (ozone and aerosols). The models used mid-range A1B emission scenarios, independently calculated the resulting composition change, and then performed transient simulations to 2050 examining the response to projected changes in short-lived species and to changes in both long-lived and short-lived species together. By 2050, two models show that the global mean annual average warming due to long-lived GHGs (greenhouse gases) is enhanced by 20–25% due to the radiatively active short-lived species. One model shows virtually no effect from short-lived species. Intermodel differences are largely related to differences in emissions projections for short-lived species, which are substantial even for a particular storyline. For aerosols, these uncertainties are usually dominant, though for sulfate uncertainties in aerosol physics are also substantial. For tropospheric ozone, uncertainties in physical processes are more important than uncertainties in precursor emissions. Differences in future atmospheric burdens and radiative forcing for aerosols are dominated by divergent assumptions about emissions from South and East Asia. In all three models, the spatial distribution of radiative forcing is less important than that of climate sensitivity in predicting climate impact. Both short-lived and long-lived species appear to cause enhanced climate responses in the same regions of high sensitivity rather than short-lived species having an enhanced effect primarily near polluted areas. Since short-lived species can significantly influence climate, regional air quality emission control strategies for short-lived pollutants may substantially impact climate over large (e.g., hemispheric) scales.
Transport of radon-222 and methyl iodide by deep convection is analyzed in the Geophysical Fluid Dynamics Laboratory (GFDL) Atmospheric Model 2 (AM2) using two parameterizations for deep convection. One of these parameterizations represents deep convection as an ensemble of entraining plumes; the other represents deep convection as an ensemble of entraining plumes with associated mesoscale updrafts and downdrafts. Although precipitation patterns are generally similar in AM2 with both parameterizations, the deep convective mass fluxes are more than three times larger in the middle- to upper troposphere for the parameterization consisting only of entraining plumes, but do not extend across the tropopause, unlike the parameterization including mesoscale circulations. The differences in mass fluxes result mainly from a different partitioning between convective and stratiform precipitation; the parameterization including mesoscale circulations detrains considerably more water vapor in the middle troposphere and is associated with more stratiform rain. The distributions of both radon-222 and methyl iodide reflect the different mass fluxes. Relative to observations (limited by infrequent spatial and temporal sampling), AM2 tends to simulate lower concentrations of radon-222 and methyl iodide in the planetary boundary layer, producing a negative model bias through much of the troposphere, with both cumulus parameterizations. The shapes of the observed profiles suggest that the larger deep convective mass fluxes and associated transport in the parameterization lacking a mesoscale component are less realistic.
Gloor, M, Edward J Dlugokencky, C Brenninkmeijer, Larry W Horowitz, D Hurst, G Dutton, Cyril Crevoisier, Toshinobu Machida, and P P Tans, August 2007: Three-dimensional SF6 data and tropospheric transport simulations: Signals, modeling accuracy, and implications for inverse modeling. Journal of Geophysical Research, 112, D15112, DOI:10.1029/2006JD007973. Abstract
Surface emissions of SF6 are closely tied to human activity and thus fairly well known. They therefore can and have been used to evaluate tropospheric transport predicted by models. A range of new atmospheric SF6 data permit us to expand on earlier studies. The purpose of this first of two papers is to characterize known and new transport constraints provided by the data and to use them to quantify predictive skill of the MOZART-2 atmospheric chemistry and transport model. Main noteworthy observational constraints are (1) a well-known steep N-S gradient at the surface confined to an ≈40° wide latitude band in the tropics; (2) a fairly uniform N-S gradient in the upper troposphere; (3) an increase in the temporal variation in upper troposphere Northern Hemisphere records with increasing latitude; (4) a negative SF6 gradient in Northern Hemisphere vertical profiles from the surface to 8 km height, but a positive gradient in the Southern Hemisphere; and (5) a clear reflection in surface records of large-scale seasonal atmosphere movements like the undulations of the Intertropical Convergence Zone (ITCZ). Comparison of observations with simulations reveal excellent modeling skills with regards to (1) large-scale annual mean latitudinal gradients at remote surface sites (relative bias of N-S hemisphere difference ≤ 5%) and aloft (≈10 km, relative bias ≤ 25%); (2) seasonality in signals at remote sites caused by large-scale movements of the atmosphere; (3) time variation in upper troposphere records; (4) “faithfulness” of advective transport on timescales up to ≈1 week; and (5) the general shapes and seasonal variation of vertical profiles. The model (1) underestimates the variation in the vertical of profiles, particularly those from locations close to high emissions regions, and (2) overestimates the difference in SF6 between the planetary boundary layer (PBL) and free troposphere over North America, and thus likely Eurasia, during winter by approximately a factor of 2 (STD ≈ 100%). The comparisons permit estimating lower bounds on representation errors which are large for sites close to continental outflow regions. Given the magnitude of the signals and signal variance, SF6 provides a strong constraint on interhemispheric transport, PBL ventilation, dispersion pathways of northern midlatitude surface emissions through the upper troposphere, and large-scale movements of the atmosphere.
Horowitz, Larry W., Arlene M Fiore, G P Milly, R C Cohen, A Perring, P J Wooldridge, Peter G Hess, Louisa K Emmons, and Jean-Francois Lamarque, 2007: Observational constraints on the chemistry of isoprene nitrates over the eastern United States. Journal of Geophysical Research, 112, D12S08, DOI:10.1029/2006JD007747. Abstract
The
formation of organic nitrates during the oxidation of the biogenic
hydrocarbon isoprene can strongly affect boundary layer concentrations of
ozone and nitrogen oxides (NOx = NO + NO2). We
constrain uncertainties in the chemistry of these isoprene nitrates using
chemical transport model simulations in conjunction with observations over
the eastern United States from the International Consortium for Atmospheric
Research on Transport and Transformation (ICARTT) field campaign during
summer 2004. The model best captures the observed boundary layer
concentrations of organic nitrates and their correlation with ozone using a
4% yield of isoprene nitrate production from the reaction of isoprene
hydroxyperoxy radicals with NO, a recycling of 40% NOx when
isoprene nitrates react with OH and ozone, and a fast dry deposition rate of
isoprene nitrates. Simulated boundary layer concentrations are only weakly
sensitive to the rate of photochemical loss of the isoprene nitrates. An 8%
yield of isoprene nitrates degrades agreement with the observations
somewhat, but concentrations are still within 50% of observations and thus
cannot be ruled out by this study. Our results indicate that complete
recycling of NOx from the reactions of isoprene nitrates and slow
rates of isoprene nitrate deposition are incompatible with the observations.
We find that ~50% of the isoprene nitrate production in the model occurs via
reactions of isoprene (or its oxidation products) with the NO3
radical, but note that the isoprene nitrate yield from this pathway is
highly uncertain. Using recent estimates of rapid reaction rates with ozone,
20–24% of isoprene nitrates are lost via this pathway, implying that
ozonolysis is an important loss process for isoprene nitrates. Isoprene
nitrates are shown to have a major impact on the nitrogen oxide (NOx
= NO + NO2) budget in the summertime U.S. continental boundary
layer, consuming 15–19% of the emitted NOx , of which 4–6% is
recycled back to NOx and the remainder is exported as isoprene
nitrates (2–3%) or deposited (8–10%). Our constraints on reaction rates,
branching ratios, and deposition rates need to be confirmed through further
laboratory and field measurements. The model systematically underestimates
free tropospheric concentrations of organic nitrates, indicating a need for
future investigation of the processes controlling the observed distribution.
Mena-Carrasco, M, Y Tang, Gregory R Carmichael, T Chai, N Thongbongchoo, J E Campbell, S Kulkarni, Larry W Horowitz, Jeffrey Vukovich, M Avery, W Brune, Jack E Dibb, Louisa K Emmons, F Flocke, G W Sachse, D Tan, Rick Shetter, R W Talbot, D G Streets, G Frost, and Donald R Blake, June 2007: Improving regional ozone modeling through systematic evaluation of errors using the aircraft observations during the International Consortium for Atmospheric Research on Transport and Transformation. Journal of Geophysical Research, 112, D12S19, DOI:10.1029/2006JD007762. Abstract
During the operational phase of the ICARTT field experiment in 2004, the
regional air quality model STEM showed a strong positive surface bias and a
negative upper troposphere bias (compared to observed DC-8 and WP-3
observations) with respect to ozone. After updating emissions from NEI 1999
to NEI 2001 (with a 2004 large point sources inventory update), and
modifying boundary conditions, low-level model bias decreases from 11.21 to
1.45 ppbv for the NASA DC-8 observations and from 8.26 to -0.34 for the NOAA
WP-3. Improvements in boundary conditions provided by global models decrease
the upper troposphere negative ozone bias, while accounting for biomass
burning emissions improved model performance for CO. The covariances of
ozone bias were highly correlated to NOz, NOy, and HNO3
biases. Interpolation of bias information through kriging showed that
decreasing emissions in SE United States would reduce regional ozone model
bias and improve model correlation coefficients. The spatial distribution of
forecast errors was analyzed using kriging, which identified distinct
features, which when compared to errors in postanalysis simulations, helped
document improvements. Changes in dry deposition to crops were shown to
reduce substantially high bias in the forecasts in the Midwest, while
updated emissions were shown to account for decreases in bias in the eastern
United States. Observed and modeled ozone production efficiencies for the
DC-8 were calculated and shown to be very similar (7.8) suggesting that
recurring ozone bias is due to overestimation of NOx emissions.
Sensitivity studies showed that ozone formation in the United States is most
sensitive to NOx emissions, followed by VOCs and CO. PAN as a
reservoir of NOx can contribute to a significant amount of
surface ozone through thermal decomposition.
Ming, Yi, V Ramaswamy, Leo J Donner, Vaughan T J Phillips, Stephen A Klein, Paul Ginoux, and Larry W Horowitz, February 2007: Modeling the interactions between aerosols and liquid water clouds with a self-consistent cloud scheme in a general circulation model. Journal of the Atmospheric Sciences, 64(4), DOI:10.1175/JAS3874.1. Abstract
To model aerosol-cloud interactions in general circulation
models (GCMs), a prognostic cloud scheme of cloud liquid water and amount is expanded to include droplet number concentration (Nd) in a way that allows them to be calculated using the same large-scale and convective updraft velocity field. In the scheme, the evolution of droplets fully interacts with the model meteorology. An explicit treatment of cloud condensation nuclei (CCN) activation enables the scheme to take into account the contributions to Nd of multiple aerosol species (i.e., sulfate, organic, and sea-salt aerosols) and to consider kinetic limitations of the activation process. An implementation of the prognostic scheme in the Geophysical Fluid Dynamics Laboratory (GFDL) AM2 GCM yields a vertical distribution of Nd with a characteristic maximum in the lower troposphere; this feature differs from the profile that would be obtained if Ndis diagnosed from the sulfate mass concentration based on an often-used empirical relationship. Prognosticated Nd exhibits large variations with respect to the sulfate mass concentration. The mean values are generally consistent with the empirical relationship over ocean, but show negative biases over the Northern Hemisphere midlatitude land, perhaps owing to the neglect of subgrid variations of large-scale ascents and inadequate convective sources. The prognostic scheme leads to a substantial improvement in the agreement of model-predicted present-day liquid water path (LWP) and cloud forcing with satellite measurements compared to using the empirical relationship.
The simulations with preindustrial and present-day aerosols show that the
combined first and second indirect effects of anthropogenic sulfate and organic aerosols give rise to a steady-state global annual mean flux change of -1.8 W m-2, consisting of -2.0 W m-2 in shortwave and 0.2 W m-2 in longwave. The ratios of the flux changes in the Northern Hemisphere (NH) to that in Southern Hemisphere (SH) and of the flux changes over ocean to that over land are 2.9 and 0.73, respectively. These estimates are consistent with the averages of values from previous studies stated in a recent review. The model response to higher Nd alters the cloud field; LWP and total cloud amount increase by 19% and 0.6%, respectively. Largely owing to high sulfate concentrations from fossil fuel burning, the NH midlatitude land and oceans experience strong radiative cooling. So does the tropical land, which is dominated by biomass burning-derived organic aerosol. The computed annual, zonal-mean flux changes are determined to be statistically significant, exceeding the model's natural variations in the NH low and midlatitudes and in the SH low latitudes. This study reaffirms the major role of sulfate in providing CCN for cloud formation.
Biomass burning is a major source of air
pollutants, some of which are also climate forcing agents. We investigate
the sensitivity of direct radiative forcing due to tropospheric ozone and
aerosols (carbonaceous and sulfate) to a marginal reduction in their (or
their precursor) emissions from major biomass burning regions. We find that
the largest negative global forcing is for 10% emission reductions in
tropical regions, including Africa (−4.1 mWm−2 from gas and −4.1
mWm−2 from aerosols), and South America (−3.0 mWm−2
from gas and −2.8 mWm−2 from aerosols). We estimate that a unit
reduction in the amount of biomass burned in India produces the largest
negative ozone and aerosol forcing. Our analysis indicates that reducing
biomass burning emissions causes negative global radiative forcing due to
ozone and aerosols; however, regional differences need to be considered when
evaluating controls on biomass burning to mitigate global climate change.
Singh, H B., L Salas, D Herlth, R Kolyer, E Czech, M Avery, J H Crawford, R Bradley Pierce, G W Sachse, Donald R Blake, R C Cohen, T H Bertram, A Perring, P J Wooldridge, Jack E Dibb, L G Huey, R C Hudman, S Turquety, Louisa K Emmons, F Flocke, Y Tang, Gregory R Carmichael, and Larry W Horowitz, 2007: Reactive nitrogen distribution and partitioning in the North American troposphere and lowermost stratosphere. Journal of Geophysical Research, 112, D12S04, DOI:10.1029/2006JD007664. Abstract
A
comprehensive group of reactive nitrogen species (NO, NO2, HNO3,
HO2NO2, PANs, alkyl nitrates, and aerosol-NO3-) were measured over North America during July/August 2004 from
the NASA DC-8 platform (0.1–12 km). Nitrogen containing tracers of biomass
combustion (HCN and CH3CN) were also measured along with a host
of other gaseous (CO, VOC, OVOC, halocarbon) and aerosol tracers. Clean
background air as well as air with influences from biogenic emissions,
anthropogenic pollution, biomass combustion, convection, lightning, and the
stratosphere was sampled over the continental United States, the Atlantic,
and the Pacific. The North American upper troposphere (UT) was found to be
greatly influenced by both lightning NOx and surface pollution
lofted via convection and contained elevated concentrations of PAN, ozone,
hydrocarbons, and NOx. Observational data suggest that lightning
was a far greater contributor to NOx in the UT than previously
believed. PAN provided a dominant reservoir of reactive nitrogen in the UT
while nitric acid dominated in the lower troposphere (LT). Peroxynitric acid
(HO2NO2) was present in sizable concentrations peaking
at around 8 km. Aerosol nitrate appeared to be mostly contained in large
soil based particles in the LT. Plumes from Alaskan fires contained large
amounts of PAN and aerosol nitrate but little enhancement in ozone. A
comparison of observed data with simulations from four 3-D models shows
significant differences between observations and models as well as among
models. We investigate the partitioning and interplay of the reactive
nitrogen species within characteristic air masses and further examine their
role in ozone formation.
Tang, Y, Gregory R Carmichael, N Thongbongchoo, T Chai, Larry W Horowitz, R Bradley Pierce, J A Al-Saadi, G Pfister, Jeffrey Vukovich, M Avery, G W Sachse, Marta Abalos, J S Holloway, E L Atlas, F Flocke, R J Weber, L G Huey, Jack E Dibb, D G Streets, and W Brune, 2007: Influence of lateral and top boundary conditions on regional air quality prediction: A multiscale study coupling regional and global chemical transport models. Journal of Geophysical Research, 112, D10S18, DOI:10.1029/2006JD007515. Abstract
The
sensitivity of regional air quality model to various lateral and top
boundary conditions is studied at 2 scales: a 60 km domain covering the
whole USA and a 12 km domain over northeastern USA. Three global models
(MOZART-NCAR, MOZART-GFDL and RAQMS) are used to drive the STEM-2K3 regional
model with time-varied lateral and top boundary conditions (BCs). The
regional simulations with different global BCs are examined using ICARTT
aircraft measurements performed in the summer of 2004, and the simulations
are shown to be sensitive to the boundary conditions from the global models,
especially for relatively long-lived species, like CO and O3.
Differences in the mean CO concentrations from three different global-model
boundary conditions are as large as 40 ppbv, and the effects of the BCs on
CO are shown to be important throughout the troposphere, even near surface.
Top boundary conditions show strong effect on O3 predictions
above 4 km. Over certain model grids, the model's sensitivity to BCs is
found to depend not only on the distance from the domain's top and lateral
boundaries, downwind/upwind situation, but also on regional emissions and
species properties. The near-surface prediction over polluted area is
usually not as sensitive to the variation of BCs, but to the magnitude of
their background concentrations. We also test the sensitivity of model to
temporal and spatial variations of the BCs by comparing the simulations with
time-varied BCs to the corresponding simulations with time-mean and profile
BCs. Removing the time variation of BCs leads to a significant bias on the
variation prediction and sometime causes the bias in predicted mean values.
The effect of model resolution on the BC sensitivity is also studied.
Changes in emissions of ozone (O3) precursors affect both air
quality and climate. We first examine the sensitivity of surface O3
concentrations (O3srf) and net radiative forcing of
climate (RFnet) to reductions in emissions of four precursors -
nitrogen oxides (NOx), non-methane volatile organic
compounds, carbon monoxide, and methane (CH4). We show that
long-term CH4-induced changes in O3, known to be
important for climate, are also relevant for air quality; for example, NOxreductions increase CH4, causing a long-term O3
increase that partially counteracts the direct O3 decrease.
Second, we assess the radiative forcing resulting from actions to improve O3
air quality by calculating the ratio of ΔRFnet
to changes in metrics of O3srf. Decreases in CH4
emissions cause the greatest RFnet decrease per unit reduction
in O3srf, while NOxreductions
increase RFnet. Of the available means to improve O3
air quality, therefore, CH4 abatement best reduces climate
forcing.
Bates, T S., T L Anderson, T Baynard, T Bond, Olivier Boucher, Gregory R Carmichael, C Erlick, Huan Guo, Larry W Horowitz, S Howell, S Kulkarni, H Maring, A McComiskey, Ann M Middlebrook, K Noone, C D O'Dowd, J Ogren, Joyce Penner, P K Quinn, A R Ravishankara, D L Savoie, Krishna M Achuta Rao, Y Shinozuka, Y Tang, R J Weber, and Y Wu, 2006: Aerosol direct radiative effects over the northwest Atlantic, northwest Pacific, and North Indian Oceans: estimates based on in-situ chemical and optical measurements and chemical transport modeling. Atmospheric Chemistry and Physics, 6(6), 1657-1732. Abstract PDF
The largest uncertainty in the radiative forcing of climate change over the industrial era is that due to aerosols, a substantial fraction of which is the uncertainty associated with scattering and absorption of shortwave (solar) radiation by anthropogenic aerosols in cloud-free conditions (IPCC, 2001). Quantifying and reducing the uncertainty in aerosol influences on climate is critical to understanding climate change over the industrial period and to improving predictions of future climate change for assumed emission scenarios. Measurements of aerosol properties during major field campaigns in several regions of the globe during the past decade are contributing to an enhanced understanding of atmospheric aerosols and their effects on light scattering and climate. The present study, which focuses on three regions downwind of major urban/population centers (North Indian Ocean (NIO) during INDOEX, the Northwest Pacific Ocean (NWP) during ACE-Asia, and the Northwest Atlantic Ocean (NWA) during ICARTT), incorporates understanding gained from field observations of aerosol distributions and properties into calculations of perturbations in radiative fluxes due to these aerosols. This study evaluates the current state of observations and of two chemical transport models (STEM and MOZART). Measurements of burdens, extinction optical depth (AOD), and direct radiative effect of aerosols (DRE – change in radiative flux due to total aerosols) are used as measurement-model check points to assess uncertainties. In-situ measured and remotely sensed aerosol properties for each region (mixing state, mass scattering efficiency, single scattering albedo, and angular scattering properties and their dependences on relative humidity) are used as input parameters to two radiative transfer models (GFDL and University of Michigan) to constrain estimates of aerosol radiative effects, with uncertainties in each step propagated through the analysis. Constraining the radiative transfer calculations by observational inputs increases the clear-sky, 24-h averaged AOD (34±8%), top of atmosphere (TOA) DRE (32±12%), and TOA direct climate forcing of aerosols (DCF – change in radiative flux due to anthropogenic aerosols) (37±7%) relative to values obtained with "a priori" parameterizations of aerosol loadings and properties (GFDL RTM). The resulting constrained clear-sky TOA DCF is −3.3±0.47, −14±2.6, −6.4±2.1 Wm−2 for the NIO, NWP, and NWA, respectively. With the use of constrained quantities (extensive and intensive parameters) the calculated uncertainty in DCF was 25% less than the "structural uncertainties" used in the IPCC-2001 global estimates of direct aerosol climate forcing. Such comparisons with observations and resultant reductions in uncertainties are essential for improving and developing confidence in climate model calculations incorporating aerosol forcing.
Crevoisier, Cyril, M Gloor, E Gloaquen, Larry W Horowitz, Jorge L Sarmiento, C Sweeney, and P P Tans, 2006: A direct carbon budgeting approach to infer carbon sources and sinks. Design and synthetic application to complement the NACP observation network. Tellus B, 58B(5), DOI:10.1111/j.1600-0889.2006.00214.x. Abstract
In order to exploit the upcoming regular measurements of vertical carbon dioxide (CO2 profiles over North America implemented in the framework of the North American Carbon Program (NACP), we design a direct carbon budgeting approach to infer carbon sources and sinks over the continent using model simulations. Direct budgeting puts a control volume on top of North America, balances air mass in- and outflows into the volume and solves for the surface fluxes. The flows are derived from the observations through a geostatistical interpolation technique called Kriging combined with transport fields from weather analysis. The use of CO2 vertical profiles simulated by the atmospheric transport model MOZART-2 at the planned 19 stations of the NACP network has given an estimation of the error of 0.39 GtC yr-1 within the model world. Reducing this error may be achieved through a better estimation of mass fluxes associated with convective processes affecting North America. Complementary stations in the north-west and the north-east are also needed to resolve the variability of CO2 in these regions. For instance, the addition of a single station near 52°N; 110°W is shown to decrease the estimation error to 0.34 GtC yr-1.
The formulation and simulation characteristics of two new global coupled climate models developed at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL) are described. The models were designed to simulate atmospheric and oceanic climate and variability from the diurnal time scale through multicentury climate change, given our computational constraints. In particular, an important goal was to use the same model for both experimental seasonal to interannual forecasting and the study of multicentury global climate change, and this goal has been achieved.
Two versions of the coupled model are described, called CM2.0 and CM2.1. The versions differ primarily in the dynamical core used in the atmospheric component, along with the cloud tuning and some details of the land and ocean components. For both coupled models, the resolution of the land and atmospheric components is 2° latitude × 2.5° longitude; the atmospheric model has 24 vertical levels. The ocean resolution is 1° in latitude and longitude, with meridional resolution equatorward of 30° becoming progressively finer, such that the meridional resolution is 1/3° at the equator. There are 50 vertical levels in the ocean, with 22 evenly spaced levels within the top 220 m. The ocean component has poles over North America and Eurasia to avoid polar filtering. Neither coupled model employs flux adjustments.
The control simulations have stable, realistic climates when integrated over multiple centuries. Both models have simulations of ENSO that are substantially improved relative to previous GFDL coupled models. The CM2.0 model has been further evaluated as an ENSO forecast model and has good skill (CM2.1 has not been evaluated as an ENSO forecast model). Generally reduced temperature and salinity biases exist in CM2.1 relative to CM2.0. These reductions are associated with 1) improved simulations of surface wind stress in CM2.1 and associated changes in oceanic gyre circulations; 2) changes in cloud tuning and the land model, both of which act to increase the net surface shortwave radiation in CM2.1, thereby reducing an overall cold bias present in CM2.0; and 3) a reduction of ocean lateral viscosity in the extratropics in CM2.1, which reduces sea ice biases in the North Atlantic.
Both models have been used to conduct a suite of climate change simulations for the 2007 Intergovernmental Panel on Climate Change (IPCC) assessment report and are able to simulate the main features of the observed warming of the twentieth century. The climate sensitivities of the CM2.0 and CM2.1 models are 2.9 and 3.4 K, respectively. These sensitivities are defined by coupling the atmospheric components of CM2.0 and CM2.1 to a slab ocean model and allowing the model to come into equilibrium with a doubling of atmospheric CO2. The output from a suite of integrations conducted with these models is freely available online (see http://nomads.gfdl.noaa.gov/).
Manuscript received 8 December 2004, in final form 18 March 2005
Dentener, Frank, J Drevet, Jean-Francois Lamarque, I Bey, B Eickhout, Arlene M Fiore, Didier Hauglustaine, Larry W Horowitz, M Krol, U C Kulshrestha, M G Lawrence, Corinne Galy-Lacaux, S Rast, Drew Shindell, David S Stevenson, Twan van Noije, C Atherton, N Bell, D Bergman, T Butler, J Cofala, B Collins, R Doherty, K Ellingsen, J Galloway, M Gauss, V Montanaro, J F Müller, G Pitari, J Rodriguez, M Sanderson, F Solmon, S Strahan, Martin G Schultz, Kengo Sudo, Sophie Szopa, and O Wild, 2006: Nitrogen and sulfur deposition on regional and global scales: A multimodel evaluation. Global Biogeochemical Cycles, 20, GB4003, DOI:10.1029/2005GB002672. Abstract
We use 23 atmospheric chemistry transport models to calculate current and future (2030) deposition of reactive nitrogen (NOy, NHx) and sulfate (SOx) to land and ocean surfaces. The models are driven by three emission scenarios: (1) current air quality legislation (CLE); (2) an optimistic case of the maximum emissions reductions currently technologically feasible (MFR); and (3) the contrasting pessimistic IPCC SRES A2 scenario. An extensive evaluation of the present-day deposition using nearly all information on wet deposition available worldwide shows a good agreement with observations in Europe and North America, where 60–70% of the model-calculated wet deposition rates agree to within ±50% with quality-controlled measurements. Models systematically overestimate NHx deposition in South Asia, and underestimate NOy deposition in East Asia. We show that there are substantial differences among models for the removal mechanisms of NOy, NHx, and SOx, leading to ±1 σ variance in total deposition fluxes of about 30% in the anthropogenic emissions regions, and up to a factor of 2 outside. In all cases the mean model constructed from the ensemble calculations is among the best when comparing to measurements. Currently, 36–51% of all NOy, NHx, and SOx is deposited over the ocean, and 50–80% of the fraction of deposition on land falls on natural (nonagricultural) vegetation. Currently, 11% of the world's natural vegetation receives nitrogen deposition in excess of the “critical load” threshold of 1000 mg(N) m−2 yr−1. The regions most affected are the United States (20% of vegetation), western Europe (30%), eastern Europe (80%), South Asia (60%), East Asia (40%), southeast Asia (30%), and Japan (50%). Future deposition fluxes are mainly driven by changes in emissions, and less importantly by changes in atmospheric chemistry and climate. The global fraction of vegetation exposed to nitrogen loads in excess of 1000 mg(N) m−2 yr−1 increases globally to 17% for CLE and 25% for A2. In MFR, the reductions in NOy are offset by further increases for NHx deposition. The regions most affected by exceedingly high nitrogen loads for CLE and A2 are Europe and Asia, but also parts of Africa.
Dentener, Frank, David S Stevenson, K Ellingsen, Twan van Noije, Martin G Schultz, Larry W Horowitz, and Arlene M Fiore, et al., 2006: The Global Atmospheric Environment for the Next Generation. Environmental Science & Technology, 40(11), DOI:10.1021/es0523845. Abstract
Air quality, ecosystem exposure to nitrogen deposition, and climate change are intimately coupled problems: we assess changes in the global atmospheric environment between 2000 and 2030 using 26 state-of-the-art global atmospheric chemistry models and three different emissions scenarios. The first (CLE) scenario reflects implementation of current air quality legislation around the world, while the second (MFR) represents a more optimistic case in which all currently feasible technologies are applied to achieve maximum emission reductions. We contrast these scenarios with the more pessimistic IPCC SRES A2 scenario. Ensemble simulations for the year 2000 are consistent among models and show a reasonable agreement with surface ozone, wet deposition, and NO2 satellite observations. Large parts of the world are currently exposed to high ozone concentrations and high deposition of nitrogen to ecosystems. By 2030, global surface ozone is calculated to increase globally by 1.5 ± 1.2 ppb (CLE) and 4.3 ± 2.2 ppb (A2), using the ensemble mean model results and associated ±1 standard deviations. Only the progressive MFR scenario will reduce ozone, by -2.3 ± 1.1 ppb. Climate change is expected to modify surface ozone by -0.8 ± 0.6 ppb, with larger decreases over sea than over land. Radiative forcing by ozone increases by 63 ± 15 and 155 ± 37 mW m-2 for CLE and A2, respectively, and decreases by -45 ± 15 mW m-2 for MFR. We compute that at present 10.1% of the global natural terrestrial ecosystems are exposed to nitrogen deposition above a critical load of
1 g N m-2 yr-1. These percentages increase by 2030 to 15.8% (CLE), 10.5% (MFR), and 25% (A2). This study shows the importance of enforcing current worldwide air quality legislation and the major benefits of going further. Nonattainment of these air quality policy objectives, such as expressed by the SRES-A2 scenario, would further degrade the global atmospheric environment.
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Over the past century, atmospheric methane (CH4) rose dramatically before leveling off in the late 1990s. The processes controlling this trend are poorly understood, limiting confidence in projections of future CH4. The MOZART-2 global tropospheric chemistry model qualitatively captures the observed CH4 trend (increasing in the early 1990s and then leveling off) with constant emissions. From 1991–1995 to 2000–2004, the CH4 lifetime versus tropospheric OH decreases by 1.6%, reflecting increases in OH and temperature. The rise in OH stems from an increase in lightning NOx as parameterized in the model. A simulation including annually varying anthropogenic and wetland CH4 emissions, as well as the changes in meteorology, best reproduces the observed CH4 distribution, trend, and seasonal cycles. Projections of future CH4 abundances should consider climate-driven changes in CH4 sources and sinks.
Ginoux, Paul, Larry W Horowitz, V Ramaswamy, I V Geogdzhayev, B Holben, Georgiy Stenchikov, and X Tie, 2006: Evaluation of aerosol distribution and optical depth in the Geophysical Fluid Dynamics Laboratory coupled model CM2.1 for present climate. Journal of Geophysical Research, 111, D22210, DOI:10.1029/2005JD006707. Abstract
This study evaluates the strengths and weaknesses of aerosol distributions and optical depths that are used to force the GFDL coupled climate model CM2.1. The concentrations of sulfate, organic carbon, black carbon, and dust are simulated using the MOZART model (Horowitz, 2006), while sea-salt concentrations are obtained from a previous study by Haywood et al. (1999). These aerosol distributions and precalculated relative-humidity-dependent specific extinction are utilized in the CM2.1 radiative scheme to calculate the aerosol optical depth. Our evaluation of the mean values (1996–2000) of simulated aerosols is based on comparisons with long-term mean climatological data from ground-based and remote sensing observations as well as previous modeling studies. Overall, the predicted concentrations of aerosol are within a factor 2 of the observed values and have a tendency to be overestimated. Comparison with satellite data shows an agreement within 10% of global mean optical depth. This agreement masks regional differences of opposite signs in the optical depth. Essentially, the excessive optical depth from sulfate aerosols compensates for the underestimated contribution from organic and sea-salt aerosols. The largest discrepancies are over the northeastern United States (predicted optical depths are too high) and over biomass burning regions and southern oceans (predicted optical depths are too low). This analysis indicates that the aerosol properties are very sensitive to humidity, and major improvements could be achieved by properly taking into account their hygroscopic growth together with corresponding modifications of their optical properties.
Horowitz, Larry W., 2006: Past, present, and future concentrations of tropospheric ozone and aerosols: Methodology, ozone evaluation, and sensitivity to aerosol wet removal. Journal of Geophysical Research, 111, D22211, DOI:10.1029/2005JD006937. Abstract PDF
Tropospheric ozone and aerosols are radiatively important trace species, whose concentrations have increased dramatically since preindustrial times and are projected to continue to change in the future. The evolution of ozone and aerosol concentrations from 1860 to 2100 is simulated on the basis of estimated historical emissions and four different future emission scenarios (Intergovernmental Panel on Climate Change Special Report on Emissions Scenarios A2, A1B, B1, and A1FI). The simulations suggest that the tropospheric burden of ozone has increased by 50% and sulfate and carbonaceous aerosol burdens have increased by factors of 3 and 6, respectively, since preindustrial times. Projected ozone changes over the next century range from -6% to +43%, depending on the emissions scenario. Sulfate concentrations are projected to increase for the next several decades but then to decrease by 2100 to 4–45% below their 2000 values. Simulated ozone concentrations agree well with present-day observations and recent trends. Preindustrial surface concentrations of ozone are shown to be sensitive to the assumed anthropogenic and biomass burning emissions, but in all cases they overestimate the few available measurements from that era. Simulated tropospheric burdens of aerosols are sensitive by up to a factor of 2 to assumptions about the rate of aerosol wet deposition in the model. The concentrations of ozone and aerosols produced by this study are provided as climate-forcing agents in the Geophysical Fluid Dynamics Laboratory coupled climate model to estimate their effects on climate. The aerosol distributions from this study and the resulting optical depths are evaluated in a companion paper by P. Ginoux et al. (2006).
Kinne, S, M Schulz, C Textor, S Guibert, Yves Balkanski, Susanne E Bauer, Paul Ginoux, Michael Herzog, and Larry W Horowitz, et al., 2006: An AeroCom initial assessment – optical properties in aerosol component modules of global models. Atmospheric Chemistry and Physics, 6, 1815-1834. Abstract PDF
The AeroCom exercise diagnoses multi-component aerosol modules in global modeling. In an initial assessment simulated global distributions for mass and mid-visible aerosol optical thickness (aot) were compared among 20 different modules. Model diversity was also explored in the context of previous comparisons. For the component combined aot general agreement has improved for the annual global mean. At 0.11 to 0.14, simulated aot values are at the lower end of global averages suggested by remote sensing from ground (AERONET ca. 0.135) and space (satellite composite ca. 0.15). More detailed comparisons, however, reveal that larger differences in regional distribution and significant differences in compositional mixture remain. Of particular concern are large model diversities for contributions by dust and carbonaceous aerosol, because they lead to significant uncertainty in aerosol absorption (aab). Since aot and aab, both, influence the aerosol impact on the radiative energy-balance, the aerosol (direct) forcing uncertainty in modeling is larger than differences in aot might suggest. New diagnostic approaches are proposed to trace model differences in terms of aerosol processing and transport: These include the prescription of common input (e.g. amount, size and injection of aerosol component emissions) and the use of observational capabilities from ground (e.g. measurements networks) or space (e.g. correlations between aerosol and clouds).
Shindell, Drew, G Faluvegi, David S Stevenson, M Krol, Louisa K Emmons, Jean-Francois Lamarque, Gabrielle Pétron, Frank Dentener, Martin G Schultz, K Ellingsen, O Wild, Arlene M Fiore, and Larry W Horowitz, et al., 2006: Multimodel simulations of carbon monoxide: Comparison with observations and projected near-future changes. Journal of Geophysical Research, 111, D19306, DOI:10.1029/2006JD007100. Abstract
We analyze present-day and future carbon monoxide (CO) simulations in 26 state-of-the-art atmospheric chemistry models run to study future air quality and climate change. In comparison with near-global satellite observations from the MOPITT instrument and local surface measurements, the models show large underestimates of Northern Hemisphere (NH) extratropical CO, while typically performing reasonably well elsewhere. The results suggest that year-round emissions, probably from fossil fuel burning in east Asia and seasonal biomass burning emissions in south-central Africa, are greatly underestimated in current inventories such as IIASA and EDGAR3.2. Variability among models is large, likely resulting primarily from intermodel differences in representations and emissions of nonmethane volatile organic compounds (NMVOCs) and in hydrologic cycles, which affect OH and soluble hydrocarbon intermediates. Global mean projections of the 2030 CO response to emissions changes are quite robust. Global mean midtropospheric (500 hPa) CO increases by 12.6 ± 3.5 ppbv (16%) for the high-emissions (A2) scenario, by 1.7 ± 1.8 ppbv (2%) for the midrange (CLE) scenario, and decreases by 8.1 ± 2.3 ppbv (11%) for the low-emissions (MFR) scenario. Projected 2030 climate changes decrease global 500 hPa CO by 1.4 ± 1.4 ppbv. Local changes can be much larger. In response to climate change, substantial effects are seen in the tropics, but intermodel variability is quite large. The regional CO responses to emissions changes are robust across models, however. These range from decreases of 10–20 ppbv over much of the industrialized NH for the CLE scenario to CO increases worldwide and year-round under A2, with the largest changes over central Africa (20–30 ppbv), southern Brazil (20–35 ppbv) and south and east Asia (30–70 ppbv). The trajectory of future emissions thus has the potential to profoundly affect air quality over most of the world's populated areas.
Stevenson, David S., Frank Dentener, M Schulz, K Ellingsen, Twan van Noije, O Wild, Arlene M Fiore, and Larry W Horowitz, et al., 2006: Multimodel ensemble simulations of present-day and near-future tropospheric ozone. Journal of Geophysical Research, 111, D08301, DOI:10.1029/2005JD006338. Abstract
Global tropospheric ozone distributions, budgets, and radiative forcings from an ensemble of 26 state-of-the-art atmospheric chemistry models have been intercompared and synthesized as part of a wider study into both the air quality and climate roles of ozone. Results from three 2030 emissions scenarios, broadly representing “optimistic,” “likely,” and “pessimistic” options, are compared to a base year 2000 simulation. This base case realistically represents the current global distribution of tropospheric ozone. A further set of simulations considers the influence of climate change over the same time period by forcing the central emissions scenario with a surface warming of around 0.7K. The use of a large multimodel ensemble allows us to identify key areas of uncertainty and improves the robustness of the results. Ensemble mean changes in tropospheric ozone burden between 2000 and 2030 for the 3 scenarios range from a 5% decrease, through a 6% increase, to a 15% increase. The intermodel uncertainty (±1 standard deviation) associated with these values is about ±25%. Model outliers have no significant influence on the ensemble mean results. Combining ozone and methane changes, the three scenarios produce radiative forcings of −50, 180, and 300 mW m−2, compared to a CO2 forcing over the same time period of 800–1100 mW m−2. These values indicate the importance of air pollution emissions in short- to medium-term climate forcing and the potential for stringent/lax control measures to improve/worsen future climate forcing. The model sensitivity of ozone to imposed climate change varies between models but modulates zonal mean mixing ratios by ±5 ppbv via a variety of feedback mechanisms, in particular those involving water vapor and stratosphere-troposphere exchange. This level of climate change also reduces the methane lifetime by around 4%. The ensemble mean year 2000 tropospheric ozone budget indicates chemical production, chemical destruction, dry deposition and stratospheric input fluxes of 5100, 4650, 1000, and 550 Tg(O3) yr−1, respectively. These values are significantly different to the mean budget documented by the Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report (TAR). The mean ozone burden (340 Tg(O3)) is 10% larger than the IPCC TAR estimate, while the mean ozone lifetime (22 days) is 10% shorter. Results from individual models show a correlation between ozone burden and lifetime, and each model's ozone burden and lifetime respond in similar ways across the emissions scenarios. The response to climate change is much less consistent. Models show more variability in the tropics compared to midlatitudes. Some of the most uncertain areas of the models include treatments of deep tropical convection, including lightning NO x production; isoprene emissions from vegetation and isoprene's degradation chemistry; stratosphere-troposphere exchange; biomass burning; and water vapor concentrations.
Textor, C, M Schulz, S Guibert, S Kinne, Yves Balkanski, Susanne E Bauer, T F Berglen, Paul Ginoux, and Larry W Horowitz, et al., 2006: Analysis and quantification of the diversities of aerosol life cycles within AeroCom. Atmospheric Chemistry and Physics, 6(7), 1777-1813. Abstract PDF
Simulation results of global aerosol models have been assembled in the framework of the AeroCom intercomparison exercise. In this paper, we analyze the life cycles of dust, sea salt, sulfate, black carbon and particulate organic matter as simulated by sixteen global aerosol models. The differences among the results (model diversities) for sources and sinks, burdens, particle sizes, water uptakes, and spatial dispersals have been established. These diversities have large consequences for the calculated radiative forcing and the aerosol concentrations at the surface. Processes and parameters are identified which deserve further research.
The AeroCom all-models-average emissions are dominated by the mass of sea salt (SS), followed by dust (DU), sulfate (SO4), particulate organic matter (POM), and finally black carbon (BC). Interactive parameterizations of the emissions and contrasting particles sizes of SS and DU lead generally to higher diversities of these species, and for total aerosol. The lower diversity of the emissions of the fine aerosols, BC, POM, and SO4, is due to the use of similar emission inventories, and does therefore not necessarily indicate a better understanding of their sources. The diversity of SO4-sources is mainly caused by the disagreement on depositional loss of precursor gases and on chemical production. The diversities of the emissions are passed on to the burdens, but the latter are also strongly affected by the model-specific treatments of transport and aerosol processes. The burdens of dry masses decrease from largest to smallest: DU, SS, SO4, POM, and BC.
The all-models-average residence time is shortest for SS with about half a day, followed by SO4 and DU with four days, and POM and BC with six and seven days, respectively. The wet deposition rate is controlled by the solubility and increases from DU, BC, POM to SO4 and SS. It is the dominant sink for SO4, BC, and POM, and contributes about one third to the total removal of SS and DU species. For SS and DU we find high diversities for the removal rate coefficients and deposition pathways. Models do neither agree on the split between wet and dry deposition, nor on that between sedimentation and other dry deposition processes. We diagnose an extremely high diversity for the uptake of ambient water vapor that influences the particle size and thus the sink rate coefficients. Furthermore, we find little agreement among the model results for the partitioning of wet removal into scavenging by convective and stratiform rain.
Large differences exist for aerosol dispersal both in the vertical and in the horizontal direction. In some models, a minimum of total aerosol concentration is simulated at the surface. Aerosol dispersal is most pronounced for SO4 and BC and lowest for SS. Diversities are higher for meridional than for vertical dispersal, they are similar for the individual species and highest for SS and DU. For these two components we do not find a correlation between vertical and meridional aerosol dispersal. In addition the degree of dispersals of SS and DU is not related to their residence times. SO4, BC, and POM, however, show increased meridional dispersal in models with larger vertical dispersal, and dispersal is larger for longer simulated residence times.
van Noije, Twan, H J Eskes, Frank Dentener, David S Stevenson, K Ellingsen, Martin G Schultz, O Wild, M Amann, C Atherton, D J Bergmann, I Bey, K F Boersma, T Butler, J Cofala, J Drevet, Arlene M Fiore, M Gauss, Didier Hauglustaine, Larry W Horowitz, I Isaksen, M Krol, Jean-Francois Lamarque, M G Lawrence, Randall V Martin, V Montanaro, J F Müller, G Pitari, Michael J Prather, J A Pyle, A Richter, J Rodriguez, N H Savage, S Strahan, Kengo Sudo, Sophie Szopa, and M van Roozendael, 2006: Multi-model ensemble simulations of tropospheric NO2 compared with GOME retrievals for the year 2000. Atmospheric Chemistry and Physics, 6, 2943-2979. Abstract PDF
We present a systematic comparison of tropospheric NO2 from 17 global atmospheric chemistry models with three state-of-the-art retrievals from the Global Ozone Monitoring Experiment (GOME) for the year 2000. The models used constant anthropogenic emissions from IIASA/EDGAR3.2 and monthly emissions from biomass burning based on the 1997–2002 average carbon emissions from the Global Fire Emissions Database (GFED). Model output is analyzed at 10:30 local time, close to the overpass time of the ERS-2 satellite, and collocated with the measurements to account for sampling biases due to incomplete spatiotemporal coverage of the instrument. We assessed the importance of different contributions to the sampling bias: correlations on seasonal time scale give rise to a positive bias of 30–50% in the retrieved annual means over regions dominated by emissions from biomass burning. Over the industrial regions of the eastern United States, Europe and eastern China the retrieved annual means have a negative bias with significant contributions (between –25% and +10% of the NO2 column) resulting from correlations on time scales from a day to a month. We present global maps of modeled and retrieved annual mean NO2 column densities, together with the corresponding ensemble means and standard deviations for models and retrievals. The spatial correlation between the individual models and retrievals are high, typically in the range 0.81–0.93 after smoothing the data to a common resolution. On average the models underestimate the retrievals in industrial regions, especially over eastern China and over the Highveld region of South Africa, and overestimate the retrievals in regions dominated by biomass burning during the dry season. The discrepancy over South America south of the Amazon disappears when we use the GFED emissions specific to the year 2000. The seasonal cycle is analyzed in detail for eight different continental regions. Over regions dominated by biomass burning, the timing of the seasonal cycle is generally well reproduced by the models. However, over Central Africa south of the Equator the models peak one to two months earlier than the retrievals. We further evaluate a recent proposal to reduce the NOx emission factors for savanna fires by 40% and find that this leads to an improvement of the amplitude of the seasonal cycle over the biomass burning regions of Northern and Central Africa. In these regions the models tend to underestimate the retrievals during the wet season, suggesting that the soil emissions are higher than assumed in the models. In general, the discrepancies between models and retrievals cannot be explained by a priori profile assumptions made in the retrievals, neither by diurnal variations in anthropogenic emissions, which lead to a marginal reduction of the NO2 abundance at 10:30 local time (by 2.5–4.1% over Europe). Overall, there are significant differences among the various models and, in particular, among the three retrievals. The discrepancies among the retrievals (10–50% in the annual mean over polluted regions) indicate that the previously estimated retrieval uncertainties have a large systematic component. Our findings imply that top-down estimations of NOx emissions from satellite retrievals of tropospheric NO2 are strongly dependent on the choice of model and retrieval.
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We present a systematic comparison of tropospheric NO2 from 17 global atmospheric chemistry models with three state-of-the-art retrievals from the Global Ozone Monitoring Experiment (GOME) for the year 2000. The models used constant anthropogenic emissions from IIASA/EDGAR3.2 and monthly emissions from biomass burning based on the 1997–2002 average carbon emissions from the Global Fire Emissions Database (GFED). Model output is analyzed at 10:30 local time, close to the overpass time of the ERS-2 satellite, and collocated with the measurements to account for sampling biases due to incomplete spatiotemporal coverage of the instrument. We assessed the importance of different contributions to the sampling bias: correlations on seasonal time scale give rise to a positive bias of 30–50% in the retrieved annual means over regions dominated by emissions from biomass burning. Over the industrial regions of the eastern United States, Europe and eastern China the retrieved annual means have a negative bias with significant contributions (between –25% and +10% of the NO2 column) resulting from correlations on time scales from a day to a month. We present global maps of modeled and retrieved annual mean NO2 column densities, together with the corresponding ensemble means and standard deviations for models and retrievals. The spatial correlation between the individual models and retrievals are high, typically in the range 0.81–0.93 after smoothing the data to a common resolution. On average the models underestimate the retrievals in industrial regions, especially over eastern China and over the Highveld region of South Africa, and overestimate the retrievals in regions dominated by biomass burning during the dry season. The discrepancy over South America south of the Amazon disappears when we use the GFED emissions specific to the year 2000. The seasonal cycle is analyzed in detail for eight different continental regions. Over regions dominated by biomass burning, the timing of the seasonal cycle is generally well reproduced by the models. However, over Central Africa south of the Equator the models peak one to two months earlier than the retrievals. We further evaluate a recent proposal to reduce the NOx emission factors for savanna fires by 40% and find that this leads to an improvement of the amplitude of the seasonal cycle over the biomass burning regions of Northern and Central Africa. In these regions the models tend to underestimate the retrievals during the wet season, suggesting that the soil emissions are higher than assumed in the models. In general, the discrepancies between models and retrievals cannot be explained by a priori profile assumptions made in the retrievals, neither by diurnal variations in anthropogenic emissions, which lead to a marginal reduction of the NO2 abundance at 10:30 local time (by 2.5–4.1% over Europe). Overall, there are significant differences among the various models and, in particular, among the three retrievals. The discrepancies among the retrievals (10–50% in the annual mean over polluted regions) indicate that the previously estimated retrieval uncertainties have a large systematic component. Our findings imply that top-down estimations of NOx emissions from satellite retrievals of tropospheric NO2 are strongly dependent on the choice of model and retrieval.
West, J J., Arlene M Fiore, Larry W Horowitz, and D L Mauzerall, 2006: Global health benefits of mitigating ozone pollution with methane emission controls. Proceedings of the National Academy of Sciences, 103(11), DOI:10.1073/pnas.0600201103. Abstract
Methane (CH4) contributes to the growing global background concentration of tropospheric ozone (O3), an air pollutant associated with premature mortality. Methane and ozone are also important greenhouse gases. Reducing methane emissions therefore decreases surface ozone everywhere while slowing climate warming, but although methane mitigation has been considered to address climate change, it has not for air quality. Here we show that global decreases in surface ozone concentrations, due to methane mitigation, result in substantial and widespread decreases in premature human mortality. Reducing global anthropogenic methane emissions by 20% beginning in 2010 would decrease the average daily maximum 8-h surface ozone by 1 part per billion by volume globally. By using epidemiologic ozone-mortality relationships, this ozone reduction is estimated to prevent 30,000 premature all-cause mortalities globally in 2030, and 370,000 between 2010 and 2030. If only cardiovascular and respiratory mortalities are considered, 17,000 global mortalities can be avoided in 2030. The marginal cost-effectiveness of this 20% methane reduction is estimated to be $420,000 per avoided mortality. If avoided mortalities are valued at $1 million each, the benefit is $240 per tonne of CH4 ($12 per tonne of CO2 equivalent), which exceeds the marginal cost of the methane reduction. These estimated air pollution ancillary benefits of climate-motivated methane emission reductions are comparable with those estimated previously for CO2. Methane mitigation offers a unique opportunity to improve air quality globally and can be a cost-effective component of international ozone management, bringing multiple benefits for air quality, public health, agriculture, climate, and energy.
human health | mortality | tropospheric ozone | air quality
Fiore, Arlene M., Larry W Horowitz, D W Purves, Hiram Levy II, M J Evans, Yan Wang, Q Li, and R M Yantosca, June 2005: Evaluating the contribution of changes in isoprene emissions to surface ozone trends over the eastern United States. Journal of Geophysical Research, 110, D12303, DOI:10.1029/2004JD005485. Abstract
Reducing surface ozone (O3) to concentrations in compliance with the national air quality standard has proven to be challenging, despite tighter controls on O3 precursor emissions over the past few decades. New evidence indicates that isoprene emissions changed considerably from the mid-1980s to the mid-1990s owing to land-use changes in the eastern United States (Purves et al., 2004). Over this period, U.S. anthropogenic VOC (AVOC) emissions decreased substantially. Here we apply two chemical transport models (GEOS-CHEM and MOZART-2) to test the hypothesis, put forth by Purves et al. (2004), that the absence of decreasing O3 trends over much of the eastern United States may reflect a balance between increases in isoprene emissions and decreases in AVOC emissions. We find little evidence for this hypothesis; over most of the domain, mean July afternoon (1300–1700 local time) surface O3 is more responsive (ranging from -9 to +7 ppbv) to the reported changes in anthropogenic NOx emissions than to the concurrent isoprene (-2 to +2 ppbv) or AVOC (-2 to 0 ppbv) emission changes. The estimated magnitude of the O3 response to anthropogenic NOx emission changes, however, depends on the base isoprene emission inventory used in the model. The combined effect of the reported changes in eastern U.S. anthropogenic plus biogenic emissions is insufficient to explain observed changes in mean July afternoon surface O3 concentrations, suggesting a possible role for decadal changes in meteorology, hemispheric background O3, or subgrid-scale chemistry. We demonstrate that two major uncertainties, the base isoprene emission inventory and the fate of isoprene nitrates (which influence surface O3 in the model by -15 to +4 and +4 to +12 ppbv, respectively), preclude a well-constrained quantification of the present-day contribution of biogenic or anthropogenic emissions to surface O3 concentrations, particularly in the high-isoprene-emitting southeastern United States. Better constraints on isoprene emissions and chemistry are needed to quantitatively address the role of isoprene in eastern U.S. air quality.
Lamarque, Jean-Francois, J T Kiehl, G P Brasseur, T Butler, Philip Cameron-Smith, William D Collins, William J Collins, Claire Granier, Didier Hauglustaine, Peter G Hess, E A Holland, Larry W Horowitz, M G Lawrence, D McKenna, P Merilees, Michael J Prather, Krishna M Achuta Rao, D Rotman, Drew Shindell, and P Thornton, 2005: Assessing future nitrogen deposition and carbon cycle feedback using a multimodel approach: Analysis of nitrogen deposition. Journal of Geophysical Research, 110, D19303, DOI:10.1029/2005JD005825. Abstract
In this study, we present the results of nitrogen deposition on land from a set of 29 simulations from six different tropospheric chemistry models pertaining to present-day and 2100 conditions. Nitrogen deposition refers here to the deposition (wet and dry) of all nitrogen-containing gas phase chemical species resulting from NOx (NO + NO2) emissions. We show that under the assumed IPCC SRES A2 scenario the global annual average nitrogen deposition over land is expected to increase by a factor of ~2.5, mostly because of the increase in nitrogen emissions. This will significantly expand the areas with annual average deposition exceeding 1 gN/m2/year. Using the results from all models, we have documented the strong linear relationship between models on the fraction of the nitrogen emissions that is deposited, regardless of the emissions (present day or 2100). On average, approximately 70% of the emitted nitrogen is deposited over the landmasses. For present-day conditions the results from this study suggest that the deposition over land ranges between 25 and 40 Tg(N)/year. By 2100, under the A2 scenario, the deposition over the continents is expected to range between 60 and 100 Tg(N)/year. Over forests the deposition is expected to increase from 10 Tg(N)/year to 20 Tg(N)/year. In 2100 the nitrogen deposition changes from changes in the climate account for much less than the changes from increased nitrogen emissions.
Liu, J, D L Mauzerall, and Larry W Horowitz, 2005: Analysis of seasonal and interannual variability in transpacific transport. Journal of Geophysical Research, 110, D04302, DOI:10.1029/2004JD005207. Abstract
The purpose of our analysis is both to evaluate the meteorological component of the seasonal and interannual variability of transpacific transport and to identify meteorological features that can be used to estimate transpacific transport. To accomplish this goal, we simulate the transport of nine continental tracers with uniform emissions and two-week lifetimes using the global Model of Ozone and Related Tracers Version 2 (MOZART-2) driven with NCEP reanalysis meteorology from 1991-2001. In addition, we define a transpacific transport potential, a measure of the quantity of a tracer transported from a particular region normalized by its total emissions from that region, across a meridional plane in the eastern Pacific at 130°W. We find that at midlatitudes, the east Asian and Indian tracers have the largest transport potentials, particularly in spring. The interannual variability of the transpacific transport potentials of most tracers is relatively high in winter and fall (particularly in February and September) but is low from April to August. At high latitudes the former Soviet Union, east Asian, and European tracers have the largest transpacific transport potentials, especially in late summer and fall, when the lowest interannual variability is observed. We find that El Niño winters are associated with stronger eastward transport of east Asian emissions in the subtropical eastern Pacific. Transport of the east Asian tracer in the central North Pacific is well correlated with the North Pacific Index. However, we find that the interannual variability of transport across the west coast of North America is mostly driven by local meteorology. We therefore created a new index based on meteorology over the eastern Pacific, which we call the Eastern Pacific Index (EPI). The EPI captures most of the interannual variability of transpacific transport at both middle- and high-latitude regions across the west coast of North America.
This study simulates the direct radiative forcing of organic aerosol using the GFDL AM2 GCM. The aerosol climatology is provided by the MOZART chemical transport model (CTM). The approach to calculating aerosol optical properties explicitly considers relative humidity–dependent hygroscopic growth by employing a functional group–based thermodynamic model, and makes use of the size distribution derived from AERONET measurements. The preindustrial (PI) and present-day (PD) global burdens of organic carbon are 0.17 and 1.36 Tg OC, respectively. The annual global mean total-sky and clear-sky top-of-the atmosphere (TOA) forcings (PI to PD) are estimated as −0.34 and −0.71 W m−2, respectively. Geographically the radiative cooling largely lies over the source regions, namely part of South America, Central Africa, Europe and South and East Asia. The annual global mean total-sky and clear-sky surface forcings are −0.63 and −0.98 W m−2, respectively. A series of sensitivity analyses shows that the treatments of hygroscopic growth and optical properties of organic aerosol are intertwined in the determination of the global organic aerosol forcing. For example, complete deprivation of water uptake by hydrophilic organic particles reduces the standard (total-sky) and clear-sky TOA forcing estimates by 18% and 20%, respectively, while the uptake by a highly soluble organic compound (malonic acid) enhances them by 18% and 32%, respectively. Treating particles as non-absorbing enhances aerosol reflection and increases the total-sky and clear-sky TOA forcing by 47% and 18%, respectively, while neglecting the scattering brought about by the water associated with particles reduces them by 24% and 7%, respectively.
Ming, Yi, V Ramaswamy, Paul Ginoux, Larry W Horowitz, and L M Russell, 2005: Geophysical Fluid Dynamics Laboratory general circulation model investigation of the indirect radiative effects of anthropogenic sulfate aerosol. Journal of Geophysical Research, 110, D22206, DOI:10.1029/2005JD006161. Abstract
The Geophysical Fluid Dynamics Laboratory (GFDL) atmosphere general circulation model, with its new cloud scheme, is employed to study the indirect radiative effect of anthropogenic sulfate aerosol during the industrial period. The preindustrial and present-day monthly mean aerosol climatologies are generated from running the Model for Ozone And Related chemical Tracers (MOZART) chemistry-transport model. The respective global annual mean sulfate burdens are 0.22 and 0.81 Tg S. Cloud droplet number concentrations are related to sulfate mass concentrations using an empirical relationship (Boucher and Lohmann, 1995). A distinction is made between "forcing" and flux change at the top of the atmosphere in this study. The simulations, performed with prescribed sea surface temperature, show that the first indirect "forcing" ("Twomey" effect) amounts to an annual mean of -1.5 W m-2, concentrated largely over the oceans in the Northern Hemisphere (NH). The annual mean flux change owing to the response of the model to the first indirect effect is -1.4 W m-2, similar to the annual mean forcing. However, the model's response causes a rearrangement of cloud distribution as well as changes in longwave flux (smaller than solar flux changes). There is thus a differing geographical nature of the radiation field than for the forcing even though the global means are similar. The second indirect effect, which is necessarily an estimate made in terms of the model's response, amounts to -0.9 W m-2, but the statistical significance of the simulated geographical distribution of this effect is relatively low owing to the model's natural variability. Both the first and second effects are approximately linearly additive, giving rise to a combined annual mean flux change of -2.3 W m-2, with the NH responsible for 77% of the total flux change. Statistically significant model responses are obtained for the zonal mean total indirect effect in the entire NH and in the Southern Hemisphere low latitudes and midlatitudes (north of 45°S). The area of significance extends more than for the first and second effects considered separately. A comparison with a number of previous studies based on the same sulfate-droplet relationship shows that, after distinguishing between forcing and flux change, the global mean change in watts per square meter for the total effect computed in this study is comparable to existing studies in spite of the differences in cloud schemes.
The global distribution of tropospheric ozone (O3) depends on the emission of precursors, chemistry, and transport. For small perturbations to emissions, the global radiative forcing resulting from changes in O3 can be expressed as a sum of forcings from emission changes in different regions. Tropospheric O3 is considered in present climate policies only through the inclusion of indirect effect of CH4 on radiative forcing through its impact on O3 concentrations. The short-lived O3 precursors (NOx , CO, and NMHCs) are not directly included in the Kyoto Protocol or any similar climate mitigation agreement. In this study, we quantify the global radiative forcing resulting from a marginal reduction (10%) in anthropogenic emissions of NOx alone from nine geographic regions and a combined marginal reduction in NOx , CO, and NMHCs emissions from three regions. We simulate, using the global chemistry transport model MOZART-2, the change in the distribution of global O3 resulting from these emission reductions. In addition to the short-term reduction in O3, these emission reductions also increase CH4concentrations (by decreasing OH); this increase in CH4 in turn counteracts part of the initial reduction in O3 concentrations. We calculate the global radiative forcing resulting from the regional emission reductions, accounting for changes in both O3 and CH4. Our results show that changes in O3 production and resulting distribution depend strongly on the geographical location of the reduction in precursor emissions. We find that the global O3 distribution and radiative forcing are most sensitive to changes in precursor emissions from tropical regions and least sensitive to changes from midlatitude and high-latitude regions. Changes in CH4 and O3 concentrations resulting from NOx emission reductions alone produce offsetting changes in radiative forcing, leaving a small positive residual forcing (warming) for all regions. In contrast, for combined reductions of anthropogenic emissions of NOx , CO, and NMHCs, changes in O3 and CH4 concentrations result in a net negative radiative forcing (cooling). Thus we conclude that simultaneous reductions of CO, NMHCs, and NOx lead to a net reduction in radiative forcing due to resulting changes in tropospheric O3 and CH4 while reductions in NOx emissions alone do not.
for climate research developed at the Geophysical Fluid Dynamics Laboratory (GFDL) are presented. The atmosphere model, known as AM2, includes a new gridpoint dynamical core, a prognostic cloud scheme, and a multispecies aerosol climatology, as well as components from previous models used at GFDL. The land model, known as LM2, includes soil sensible and latent heat storage, groundwater storage, and stomatal resistance. The performance of the coupled model AM2–LM2 is evaluated with a series of prescribed sea surface temperature (SST) simulations. Particular focus is given to the model's climatology and the characteristics of interannual variability related to E1 Niño– Southern Oscillation (ENSO).
One AM2–LM2 integration was performed according to the prescriptions of the second Atmospheric Model Intercomparison Project (AMIP II) and data were submitted to the Program for Climate Model Diagnosis and Intercomparison (PCMDI). Particular strengths of AM2–LM2, as judged by comparison to other models participating in AMIP II, include its circulation and distributions of precipitation. Prominent problems of AM2– LM2 include a cold bias to surface and tropospheric temperatures, weak tropical cyclone activity, and weak tropical intraseasonal activity associated with the Madden–Julian oscillation.
An ensemble of 10 AM2–LM2 integrations with observed SSTs for the second half of the twentieth century permits a statistically reliable assessment of the model's response to ENSO. In general, AM2–LM2 produces a realistic simulation of the anomalies in tropical precipitation and extratropical circulation that are associated with ENSO.
Mineral dust aerosols originating from arid regions are simulated in an atmospheric global chemical transport model. Based on model results and observations of dust concentration, we hypothesize that air pollution increases the scavenging of dust by producing high levels of readily soluble materials on the dust surface, which makes dust aerosols effective cloud condensation nuclei (CCN). This implies that air pollution could have caused an increase of dust deposition to the coastal oceans of East Asia and a decrease by as much as 50% in the eastern North Pacific.
Goldstein, A H., D B Millet, M McKay, L Jaeglé, Larry W Horowitz, Owen R Cooper, R Hudman, D J Jacob, S J Oltmans, and A Clarke, December 2004: Impact of Asian emissions on observations at Trinidad Head, California, during ITCT 2K2. Journal of Geophysical Research, 109, D23S17, DOI:10.1029/2003JD004406. Abstract
Field measurements of a wide suite of trace gases and aerosols were carried out during April and May 2002, along with extensive chemical transport modeling, as part of the NOAA Intercontinental Transport and Chemical Transformation study. Here, we use a combination of in-situ ground-based measurements from Trinidad Head, CA, chemical transport modeling, and backward trajectory analysis to examine the impact of long-range transport from Asia on the composition of air masses arriving at the California coast at the surface. The impact of Asian emissions is explored in terms of both episodic enhancements and contribution to background concentrations. We find that variability in CO concentrations at the ground site was largely driven by North American emissions, and that individual Asian plumes did not cause any observable pollution enhancement episodes at Trinidad Head. Despite this, model simulations suggest that Asian emissions were responsible for 33% of the CO observed at Trinidad Head, providing a larger mean contribution than direct emissions from any other region of the globe. Surface ozone levels were found to depend primarily on local atmospheric mixing, with surface deposition leading to low concentrations under stagnant conditions. Model simulations suggested that on average 4 ± 1 ppb of ozone (10% of observed) at Trinidad Head was transported from Asia.
Tang, Y, Gregory R Carmichael, and Larry W Horowitz, et al., 2004: Multiscale simulations of tropospheric chemistry in the eastern Pacific and on the U.S. West Coast during spring 2002. Journal of Geophysical Research, 109, D23S11, DOI:10.1029/2004JD004513. Abstract
Regional modeling analysis for the Intercontinental Transport and Chemical Transformation 2002 (ITCT 2K2) experiment over the eastern Pacific and U.S. West Coast is performed using a multiscale modeling system, including the regional tracer model Chemical Weather Forecasting System (CFORS), the Sulfur Transport and Emissions Model 2003 (STEM-2K3) regional chemical transport model, and an off-line coupling with the Model of Ozone and Related Chemical Tracers (MOZART) global chemical transport model. CO regional tracers calculated online in the CFORS model are used to identify aircraft measurement periods with Asian influences. Asian-influenced air masses measured by the National Oceanic and Atmospheric Administration (NOAA) WP-3 aircraft in this experiment are found to have lower ?Acetone/?CO, ?Methanol/?CO, and ?Propane/?Ethyne ratios than air masses influenced by U.S. emissions, reflecting differences in regional emission signals. The Asian air masses in the eastern Pacific are found to usually be well aged (>5 days), to be highly diffused, and to have low NOy levels. Chemical budget analysis is performed for two flights, and the O3 net chemical budgets are found to be negative (net destructive) in the places dominated by Asian influences or clear sites and positive in polluted American air masses. During the trans-Pacific transport, part of gaseous HNO3 was converted to nitrate particle, and this conversion was attributed to NOy decline. Without the aerosol consideration, the model tends to overestimate HNO3 background concentration along the coast region. At the measurement site of Trinidad Head, northern California, high-concentration pollutants are usually associated with calm wind scenarios, implying that the accumulation of local pollutants leads to the high concentration. Seasonal variations are also discussed from April to May for this site. A high-resolution nesting simulation with 12-km horizontal resolution is used to study the WP-3 flight over Los Angeles and surrounding areas. This nested simulation significantly improved the predictions for emitted and secondary generated species. The difference of photochemical behavior between the coarse (60-km) and nesting simulations is discussed and compared with the observation.
Emmons, Louisa K., Peter G Hess, A Klonecki, X Tie, Larry W Horowitz, Jean-Francois Lamarque, Douglas Kinnison, G P Brasseur, R Atlas, E Browell, C Cantrell, F Eisele, R L Mauldin, J Merrill, B Ridley, and Rick Shetter, 2003: Budget of tropospheric ozone during TOPSE from two chemical transport models. Journal of Geophysical Research, 108(D8), 8372, DOI:10.1029/2002JD002665. Abstract PDF
The tropospheric ozone budget during the Tropospheric Ozone Production about the Spring Equinox (TOPSE) campaign has been studied using two chemical transport models (CTMs): HANK and the Model of Ozone and Related chemical Tracers, version 2 (MOZART-2). The two models have similar chemical schemes but use different meteorological fields, with HANK using MM5 (Pennsylvania State University, National Center for Atmospheric Research Mesoscale Modeling System) and MOZART-2 driven by European Centre for Medium-Range Weather Forecasts (ECMWF) fields. Both models simulate ozone in good agreement with the observations but underestimate NOx. The models indicate that in the troposphere, averaged over the northern middle and high latitudes, chemical production of ozone drives the increase of ozone seen in the spring. Both ozone gross chemical production and loss increase greatly over the spring months. The in situ production is much larger than the net stratospheric input, and the deposition and horizontal fluxes are relatively small in comparison to chemical destruction. The net production depends sensitively on the concentrations of H2O, HO2 and NO, which differ slightly in the two models. Both models underestimate the chemical production calculated in a steady state model using TOPSE measurements, but the chemical loss rates agree well. Measures of the stratospheric influence on tropospheric ozone in relation to in situ ozone production are discussed. Two different estimates of the stratospheric fraction of O3 in the Northern Hemisphere troposphere indicate it decreases from 30-50% in February to 15-30% in June. A sensitivity study of the effect of a perturbation in the vertical flux on tropospheric ozone indicates the contribution from the stratosphere is approximately 15%.
Horowitz, Larry W., S Walters, D L Mauzerall, Louisa K Emmons, Philip J Rasch, Claire Granier, X Tie, Jean-Francois Lamarque, M Schulz, G S Tyndall, Isidoro Orlanski, and G P Brasseur, December 2003: A global simulation of tropospheric ozone and related tracers: description and evaluation of MOZART, version 2. Journal of Geophysical Research, 108(D24), 4784, DOI:10.1029/2002JD002853. Abstract
We have developed a global three-dimensional chemical transport model called Model of Ozone and Related Chemical Tracers (MOZART), version 2. This model, which will be made available to the community, is built on the framework of the National Center for Atmospheric Research (NCAR) Model of Atmospheric Transport and Chemistry (MATCH) and can easily be driven with various meteorological inputs and model resolutions. In this work, we describe the standard configuration of the model, in which the model is driven by meteorological inputs every 3 hours from the middle atmosphere version of the NCAR Community Climate Model (MACCM3) and uses a 20-min time step and a horizontal resolution of 2.8° latitude × 2.8° longitude with 34 vertical levels extending up to approximately 40 km. The model includes a detailed chemistry scheme for tropospheric ozone, nitrogen oxides, and hydrocarbon chemistry, with 63 chemical species. Tracer advection is performed using a flux-form semi-Lagrangian scheme with a pressure fixer. Subgrid-scale convective and boundary layer parameterizations are included in the model. Surface emissions include sources from fossil fuel combustion, biofuel and biomass burning, biogenic and soil emissions, and oceanic emissions. Parameterizations of dry and wet deposition are included. Stratospheric concentrations of several long-lived species (including ozone) are constrained by relaxation toward climatological values. The distribution of tropospheric ozone is well simulated in the model, including seasonality and horizontal and vertical gradients. However, the model tends to overestimate ozone near the tropopause at high northern latitudes. Concentrations of nitrogen oxides (NOx) and nitric acid (HNO3) agree well with observed values, but peroxyacetylnitrate (PAN) is overestimated by the model in the upper troposphere at several locations. Carbon monoxide (CO) is simulated well at most locations, but the seasonal cycle is underestimated at some sites in the Northern Hemisphere. We find that in situ photochemical production and loss dominate the tropospheric ozone budget, over input from the stratosphere and dry deposition. Approximately 75% of the tropospheric production and loss of ozone occurs within the tropics, with large net production in the tropical upper troposphere. Tropospheric production and loss of ozone are three to four times greater in the northern extratropics than the southern extratropics. The global sources of CO consist of photochemical production (55%) and direct emissions (45%). The tropics dominate the chemistry of CO, accounting for about 75% of the tropospheric production and loss. The global budgets of tropospheric ozone and CO are generally consistent with the range found in recent studies. The lifetime of methane (9.5 years) and methylchloroform (5.7 years) versus oxidation by tropospheric hydroxyl radical (OH), two useful measures of the global abundance of OH, agree well with recent estimates. Concentrations of nonmethane hydrocarbons and oxygenated intermediates (carbonyls and peroxides) generally agree well with observations.
zone is an air quality problem today for much of the
world’s population. Regions can exceed the ozone air
quality standards (AQS) through a combination of local
emissions, meteorology favoring pollution episodes, and the
clean-air baseline levels of ozone upon which pollution
builds. The IPCC 2001 assessment studied a range of global
emission scenarios and found that all but one projects
increases in global tropospheric ozone during the 21st
century. By 2030, near-surface increases over much of the
northern hemisphere are estimated to be about 5 ppb (+2 to
+7 ppb over the range of scenarios). By 2100 the two more
extreme scenarios project baseline ozone increases of >20
ppb, while the other four scenarios give changes of
4to
+10 ppb. Even modest increases in the background
abundance of tropospheric ozone might defeat current
AQS strategies. The larger increases, however, would
gravely threaten both urban and rural air quality over
most of the northern hemisphere.
Tie, X, Louisa K Emmons, Larry W Horowitz, G P Brasseur, B Ridley, E L Atlas, C Stround, Peter G Hess, A Klonecki, S Madronich, R W Talbot, and Jack E Dibb, February 2003: Effect of sulfate aerosol on tropospheric NOx and ozone budgets: Model simulations and TOPSE evidence. Journal of Geophysical Research, 108(D4), 8364, DOI:10.1029/2001JD001508. Abstract
The distributions of NOx and O3 are analyzed during TOPSE (Tropospheric Ozone Production about the Spring Equinox). In this study these data are compared with the calculations of a global chemical/transport model (Model for OZone And Related chemical Tracers (MOZART)). Specifically, the effect that hydrolysis of N2O5 on sulfate aerosols has on tropospheric NOx and O3 budgets is studied. The results show that without this heterogeneous reaction, the model significantly overestimates NOx concentrations at high latitudes of the Northern Hemisphere (NH) in winter and spring in comparison to the observations during TOPSE; with this reaction, modeled NOx concentrations are close to the measured values. This comparison provides evidence that the hydrolysis of N2O5 on sulfate aerosol plays an important role in controlling the tropospheric NOx and O3 budgets. The calculated reduction of NOx attributed to this reaction is 80 to 90% in winter at high latitudes over North America. Because of the reduction of NOx, O3 concentrations are also decreased. The maximum O3 reduction occurs in spring although the maximum NOx reduction occurs in winter when photochemical O3 production is relatively low. The uncertainties related to uptake coefficient and aerosol loading in the model is analyzed. The analysis indicates that the changes in NOxdue to these uncertainties are much smaller than the impact of hydrolysis of N2O5 on sulfate aerosol. The effect that hydrolysis of N2O5 on global NOx and O3 budgets are also assessed by the model. The results suggest that in the Northern Hemisphere, the average NOx budget decreases 50% due to this reaction in winter and 5% in summer. The average O3 budget is reduced by 8% in winter and 6% in summer. In the Southern Hemisphere (SH), the sulfate aerosol loading is significantly smaller than in the Northern Hemisphere. As a result, sulfate aerosol has little impact on NOx and O3 budgets of the Southern Hemisphere.
Prather, Michael J., D Ehhalt, and Larry W Horowitz, et al., 2001: Atmospheric Chemistry and Greenhouse Gases In Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge, UK, Cambridge University Press, 239-287.
Liang, J, Larry W Horowitz, D J Jacob, Yan Wang, and Arlene M Fiore, et al., 1998: Seasonal budgets of reactive nitrogen species and ozone over the United States, and export fluxes to the global atmosphere. Journal of Geophysical Research, 103(D11), 13,435-13,450. Abstract PDF
A three-dimensional, continental-scale photochemical model is used to investigate seasonal budgets of O3 and NO y species (including NO x and its oxidation products) in the boundary layer over the United States and to estimate the export of these species from the U.S. boundary layer to the global atmosphere. Model results are evaluated with year-round observations for O3, CO, and NO y species at nonurban sites. A seasonal transition from NO x to hydrocarbon-limited conditions for O3 production over the eastern United States is found to take place in the fall, with the reverse transition taking place in the spring. The mean NO x /NO y molar ratio in the U.S. boundary layer in the model ranges from 0.2 in summer to 0.6 in winter, in accord with observations, and reflecting largely the seasonal variation in the chemical lifetime of NO x . Formation of hydroxy organic nitrates during oxidation of isoprene, followed by decomposition of these nitrates to HNO3, is estimated to account for 30% of the chemical sink of NO x in the U.S. boundary layer in summer. Model results indicate that peroxyacylnitrates (PANs) are most abundant in the U.S. boundary layer in spring (25% of total NO y .), reflecting a combination of active photochemistry and low temperatures. About 20% of the NO x emitted from fossil fuel combustion in the United States in the model is exported out of the U.S. boundary layer as NO x or PANs (15% in summer, 25% in winter). This export responds less than proportionally to changes in NO x emissions in summer, but more than proportionally in winter. The annual mean export of NO x and PANs from the U.S. boundary layer is estimated to be 1.4 Tg N yr−1, representing an important source of NO x on the scale of the northern hemisphere troposphere. The eventual O3 production in the global troposphere due to the exported NO x and PANs is estimated to be twice as large, on an annual basis, as the direct export of O3 pollution from the U.S. boundary layer. Fossil fuel combustion in the United States is estimated to account for about 10% of the total source of O3 in the northern hemisphere troposphere on an annual basis
Olson, J, and Larry W Horowitz, et al., 1997: Results from the Intergovernmental Panel on Climate Change Photochemical Model Intercomparison. Journal of Geophysical Research, 102(D5), 5979-5991. Abstract PDF
Results from the Intergovernmental Panel on Climate Change (IPCC) tropospheric photochemical model intercomparison (PhotoComp) are presented with a brief discussion of the factors that may contribute to differences in the modeled behaviors of HOx cycling and the accompanying O3 tendencies. PhotoComp was a tightly controlled model experiment in which the IPCC 1994 assessment sought to determine the consistency among models that are used to predict changes in tropospheric ozone, an important greenhouse gas. Calculated tropospheric photodissociation rates displayed significant differences, with a root-mean-square (rms) error of the reported model results ranging from about ±6-9% of the mean (for O3 and NO2) to up to ±15% (H2O2 and CH2O). Models using multistream methods in radiative transfer calculations showed distinctly higher rates for photodissociation of NO2 and CH2O compared to models using two-stream methods, and this difference accounted for up to one third of the rms error for these two rates. In general, some small but systematic differences between models were noted for the predicted chemical tendencies in cases that did not include reactions of nonmethane hydrocarbons (NMHC). These differences in modeled O3 tendencies in some cases could be identified, for example, as being due to differences in photodissociation rates, but in others they could not and must be ascribed to unidentified errors. O3tendencies showed rms errors of about ±10% in the moist, surface level cases with NOx concentrations equal to a few tens of parts per trillion by volume. Most of these model to model differences can be traced to differences in the destruction of O3 due to reaction with HO2. Differences in HO2, in turn, are likely due to (1) inconsistent reaction rates used by the models for the conversion of HO2 to H2O2 and (2) differences in the model-calculated photolysis of H2O2 and CH2O. In the middle tropospheric "polluted" scenario with NOx concentrations larger than a few parts per billion by volume, O3 tendencies showed rms errors of ±10-30%. These model to model differences most likely stem from differences in the calculated rates of O3 photolysis to O(1D), which provides about 80% of the HOx source under these conditions. The introduction of hydrocarbons dramatically increased both the rate of NOx loss and its model differences, which, in turn, are reflected in an increased spread of predicted O3. Including NMHC in the simulation approximately doubled the rms error for O3 concentration.
