Arlene Fiore's Research Page

Policy-Relevant Background Ozone in Surface Air over the United States

Mitigating Ozone Pollution and Climate Change through Methane Control

The Contribution of Isoprene to Eastern U.S. Surface Ozone

Intercontinental Transport of Pollution

Applicability of coarse-resolution global models to simulations of ozone pollution episodes in the United States

Trends in Measured Ozone Concentrations over the United States

POLICY-RELEVANT BACKGROUND OZONE IN SURFACE AIR OVER THE UNITED STATES

What is background ozone in surface air over the United States? And why is it "policy-relevant"?

The EPA is currently reviewing the scientific criteria upon which the national ozone air quality standard is based, and EPA considers background ozone during this process. The standard must be attainable via domestic anthropogenic emission controls. "Policy-relevant background" is defined to be the ozone concentrations that would be present if North American anthropogenic emissions of ozone precursors were turned off. Policy-relevant background thus consists of any naturally produced ozone, plus any ozone produced from anthropogenic precursor emissions outside of North America, that is present in surface air over the United States. A quantitative estimate of policy-relevant background is needed by EPA for two reasons: (1) to ensure that the standard is not set too close to background levels, and (2) to calculate the health risk associated with exposure to the increment of ozone above the background (i.e., exposure to ozone produced from North American ozone precursor sources). Since observations only exist for total ozone, we need to use models to estimate the contribution to observed surface ozone that is produced from North American anthropogenic precursor emissions. Our approach using the GEOS-CHEM model of tropospheric chemistry to estimate policy-relevant background ozone in surface air over the United States are described in the following publications:

A. Fiore, D.J. Jacob, H. Liu, R.M. Yantosca, T.D. Fairlie, Q. Li Variability in surface ozone background over the United States: Implications for air quality policy, J. Geophys. Res. 108, 4787, doi:10.1029/2003JD003855, 2003. [pdf]
Fiore, A.M., D.J. Jacob, I. Bey, R.M. Yantosca, B.D. Field, A.C. Fusco, and J.G. Wilkinson, Background ozone over the United States in summer: Origin,trend, and contribution to pollution episodes, J. Geophys. Res., 107 (D15), doi:10.1029/2001JD000982, 2002. [pdf]

The first draft of the EPA Criteria Document, reviewed by the Clean Air Science Advisory Committee (CASAC) in May 2005 can be accessed here.

Below are highlights from our work on the origin of background ozone, and the variability of policy-relevant background ozone in surface air over the United States.

VARIABILITY IN SURFACE OZONE BACKGROUND OVER THE UNITED STATES:
IMPLICATIONS FOR AIR QUALITY POLICY

When setting the U.S. national ozone air quality standard, the Environmental Protection Agency (EPA) accounts for a background ozone level above which risk to human health is assessed. The EPA is currently reviewing the scientific basis for its ozone standard, and in this review process has defined background ozone to be those concentrations in surface air that would exist if North American anthropogenic emissions were turned off. In the past, the EPA used observational statistics to define a 25-45 ppbv range of surface ozone background during the U.S. ozone pollution season. A 40 ppbv level was then adopted for use in risk assessments. The figure above shows our best estimate for the U.S. surface ozone background (green diamonds) that would exist if North American anthropogenic emissions were turned off, simulated with the GEOS-CHEM model.

We classified ozone data from the Clean Air Status and Trends Network ((CASTNET) into low-lying sites (generally below 1.5 km) and elevated sites (above 1.5 km). All elevated sites are in the west. We then aggregated our results to construct the cumulative probability distributions shown in the figure, for the 58 surface sites and the 12 elevated sites, for the three seasons, for the observations (black asterisks) and the model (red triangles). The corresponding distribution of background ozone concentrations is shown as green diamonds. The figure indicates that an appropriate background for use in risk assessment should vary as a function of season, altitude, and total ozone level. In particular, the depletion of the background during high-ozone events should be taken into account; the current 40 ppbv value used by the EPA will underestimate the risk posed by ozone concentrations above the background under these circumstances. The highest observed ozone concentrations at all altitudes in all seasons are associated with pollution from North American anthropogenic emissions, as seen by the difference between the green diamonds and red triangles. The background ozone concentrations shown here include the contribution from hemispheric pollution and would be even lower if international emission controls could be considered as part of a broader strategy to improve U.S. air quality. For a full account of this work, please see Fiore et al., 2003 (pdf).

BACKGROUND OZONE IN SURFACE AIR OVER THE UNITED STATES:
ORIGIN AND CONTRIBUTION TO POLLUTION EPISODES

The top panel shows the probability distribution of afternoon (1-5 p.m.) background ozone concentrations in surface air over the U.S. during the summer of 1995 in the GEOS-CHEM model model for two populations: (1) the ensemble of data from all U.S. grid squares on all summer days (black line) and (2) the data subset from highly polluted days, when total afternoon surface ozone exceeds 80 ppbv (green line). The background is defined here as ozone produced outside the North American boundary layer (surface to 700 hPa) using a tagged tracer method [Wang et al., 1998]. The figure shows that the background contribution is generally much smaller on polluted days, which are often associated with stagnation and subsidence inversions that suppress mixing from the free troposphere. This result indicates that inferring a background contribution from observations under clean conditions severely overestimates the actual background under the heavily polluted conditions associated with exceedances of the national air quality standard for ozone. A small population of points with high background concentrations and ozone above 80 ppbv is evident from the figure. Our analysis indicates that these values occur following subgrid-scale convective activity that transports ozone from the free troposphere to the surface.
The bottom panel of the figure below shows the enhancement to background ozone due to anthropogenic emissions in Asia and Europe, plotted as a function of total ozone concentrations in the model. Points represent daily afternoon (1-5 p.m.) values for all U.S. grid squares on all summer days. The enhancement from Asian and European emissions is defined here as the difference betwe en a GEOS-CHEM simulation with anthropogenic emissions within North America turned off , and one with all anthropogenic emissions turned off. The average enhancement in surface air over the U.S. is 4-7 ppbv, but as shown in the figure, this enhancement is particularly large (up to 14 ppbv) under moderately high ozone concentrations (50-70 ppbv), reflecting a combination of convective transport from the free troposphere and rapid photochemical production within the U.S. boundary layer. Please see Fiore et al. [2002] for more details.

MITIGATING OZONE POLLUTION AND CLIMATE CHANGE THROUGH METHANE CONTROL
Earlier work (see figure below) has shown that methane emission controls are a powerful level to reduce both global warming and air pollution via decreases in background tropospheric ozone. A preliminary cost-benefit analysis, led by Jason West shows that methane controls are indeed viable (West and Fiore, 2005).

The figure above shows that decreasing global anthropogenic methane emissions by 50% (in the GEOS-CHEM model) substantially lowers radiative forcing (a metric for assessing climate impact) and halves the incidence of days where surface ozone exceeds 80 ppbv in the United States. Methane is not traditionally considered to be an important ozone precursor in contributing to U.S. pollution episodes because of its long lifetime (low reactivity). In the free troposphere, however, methane oxidation contributes to background ozone, which can then be transported to the surface and contribute to a baseline above which regional ozone pollution episodes (fueled by nitrogen oxide and non-methane volatile organic compound emissions) can build.

We are currently using the MOZART-2 model to further assess the feasibility of controlling methane as a strategy for global air pollution management.

THE CONTRIBUTION OF ISOPRENE TO EASTERN U.S. SURFACE OZONE
Uncertainties in isoprene emissions and chemistry confound efforts to quantitatively address the role of isoprene in eastern U.S. air quality. We have recently shown that differences in current isoprene emission inventories and in the representation of organic isoprene nitrates (their yield and fate) in chemical mechanisms substantially influence estimates of surface ozone over the eastern United States, locally by up to 15 and 12 ppbv, respectively. The largest uncertainties occur in the high isoprene-emitting southeastern United States, as illustrated by the two figures below and discussed in [Fiore et al., 2005 (pdf) ]

The figure above shows that when we treat isoprene nitrates as a sink for nitrogen oxides in the MOZART-2 model, surface ozone decreases by 4-12 ppbv over the eastern United States (assuming a 12% yield). The yield and fate of isoprene nitrates are both highly uncertain.

The upper two panels in the figure above show the standard isoprene and nitrogen oxide emissions for July 2001 in the GEOS-CHEM model for a 1x1 degree (latitude by longitude) simulation over North America. We implemented an isoprene inventory (bottom left panel) developed by Drew Purves . This inventory is similar to the BEIS2 inventory used in many U.S. regional models. The difference in the two isoprene emission inventories yields local differences of up to 15 ppbv (the color scale saturates in the blue).

During the summer 2004 ICARTT aircraft campaign over the eastern United States, isoprene and a suite of its oxidation products were measured, including formaldehyde, individual and total alkyl nitrates and peroxynitrates. We are currently using the MOZART model to conduct sensitivity simulations in which we vary isoprene emissions (GEIA, MEGAN inventories) and treatments of isoprene nitrates (the yield and the fraction recycling to NOx). We are analyzing these simulations, in conjunction with the ICARTT measurements, to gain insights into isoprene-NOx-ozone chemistry over the eastern United States, and to attempt to constrain the isoprene emissions and the yield and fate of isoprene nitrates.

INTERCONTINENTAL TRANSPORT OF POLLUTION
In recent years, there has been a growing body of literature documenting evidence for air pollution transport from Asia to North America, North America to Europe, and Europe to Asia. The following publications attempt to summarize the evidence for intercontinental transport of ozone and aerosols and discuss opportunities for coordinated mitigation of air pollution and global warming.

T. Holloway, A. Fiore, and M. Galanter Hastings, Intercontinental Transport of Air Pollution: Will emerging science lead to a new hemispheric treaty?, Environ. Sci. & Technol. , 37 (20), 4535-4542, 2003. [Abstract (html)]
A. Fiore, T. Holloway, M.G. Hastings, A Global Perspective on Air Quality: Intercontinental Transport and Linkages with Climate , EM, December, 2003. [Abstract (html)]

Additional discussion of intercontinental transport can be found in our work on background ozone and on methane, and below. Future increases in the global ozone background could
thwart U.S. efforts to abate ozone pollution

The figure above shows the number of GEOS-CHEM model grid-square days per month in the United States with afternoon (1-5 p.m. local time) ozone concentrations in surface air in excess of a 70 ppbv threshold for a 1995 base case simulation (black bars) and a simulation of the 2030 atmosphere based upon emissions projections from the IPCC [2001] A1 scenario (red bars). In the 2030 A1 simulation, global emissions of ozone precursors increase, but the distribution shifts from the developed world to the developing world. In the United States, emissions of ozone precursors decline by 20-40% relative to 1995. A 70 ppbv threshold is used here as a metric to gauge changes in the frequency of ozone pollution events in our simulations. The 2030 A1 simulation reveals a longer ozone season, with increased exceedances of the 70 ppbv threshold as compared to 1995. Results are similar for 60 and 80 ppbv thresholds. These changes are attributed to the rise in the global ozone background that occurs in the 2030 A1 simulation, in part due to substantial increases in methane emissions. More details about this work, including the role of methane in linking ozone pollution and climate change mitigation objectives, are available in Fiore et al. [2002].

APPLICABILITY OF COARSE-RESOLUTION GLOBAL MODELS TO SIMULATIONS OF OZONE POLLUTION EPISODES IN THE UNITED STATES
Using principal component analysis, we evaluated and compared the global GEOS-CHEM model at both 4x5 and 2x2.5 degrees (latitude x longitude) horizontal resolution with observations from the EPA/AIRS network of observations sites and with the regional Multiscale Air Quality SImulation Platform (MAQSIP) regional model, which was the prototype for models currently being used in regulatory applications.

The full text of this paper is available here as a pdf, and highlights are discussed below.

APPLICATION OF EMPIRICAL ORTHOGONAL FUNCTIONS TO
EVALUATE OZONE SIMULATIONS

The figure above shows the first three Varimax-rotated empirical orthogonal functions (EOFs; left and middle columns) as derived from daily afternoon average (13-17 local time) surface ozone concentrations for the summer of 1995 in the observations averaged on the regional MAQSIP model grid (left) and in the MAQSIP model (middle). The percentage of total variance explained by each individual EOF is shown. The right column shows the principal components (daily variation in the strength of the EOFs; solid lines) derived from observations and the corresponding time series (dotted lines) obtained from projecting the simulated daily summer afternoon ozone concentrations onto the observationally-derived EOFs (left column). The east-west EOF identifies a longitudinal gradient as a major factor of variability for ozone in the eastern United States (upper panels). When the principal component time series (solid line; upper right panel) peaks, this east- west EOF is strongly expressed and ozone concentrations are elevated along the east coast (where the EOF is shown in red) and low in the central part of the country (where the EOF is shown in blue). The pattern is reversed when the time series is minimum. The second EOF highlights the industrial Midwest and the Northeast (middle panels) as a region where ozone concentrations vary coherently; the third EOF isolates the Southeast (bottom panels). The MAQSIP model reproduces all three EOFs (center column) and the associate time series (dotted lines in right column), indicating that it captures the synoptic-scale meteorological processes that modulate the observed day-to-day spatial and temporal variability in surface ozone concentrations.

TRENDS IN MEASURED OZONE CONCENTRATIONS IN SURFACE AIR OVER THE UNITED STATES
As an undergraduate researcher in Daniel Jacob's modeling group I analyzed trends in summer afternoon ozone concentrations from 1980-1995 measured at EPA/AIRS stations. Cynthia Lin has extended this work to include an analysis of trends in 8-hour and 1-hour average ozone concentrations, as well as trends in background levels of ozone over the United States. Links to these papers are provided below:
Lin, C.-Y. C., D.J. Jacob, and A.M. Fiore, Trends in exceedances of the ozone air quality standard in the continental United States, 1980-1998, Atmos. Environ., 35, , 3217-3228, 2001. [full text (html)]
Lin, C.-Y. C., D.J. Jacob, J.W. Munger, and A.M. Fiore, Increasing background ozone in surface air over the United States, Geophys. Res. Lett, 27, 3465-3468, 2000. [Full text (html)]
Fiore, A.M., D.J. Jacob, J.A. Logan, and J.H. Yin, Long-term trends in ground level ozone over the contiguous United States, 1980-1995, J. Geophys. Res., 103, 1471-1480, 1998. [Abstract (html)]