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Determine the Impacts of Asian Emissions on North America
GFDL 2002 second quarter milestone
PURPOSE:
Evidence suggests a substantial Asian impact on both North American air quality and regional radiative forcing, based on several factors: the prevailing winds aloft blowing from the west, recent observations of trace gases and dust over North America, and numerical simulations of transport and chemistry. Key questions raised regarding this impact are:
- What is the current magnitude of the impact of Asian emissions?
- What are the relative environmental roles of extreme episodes vs. background impacts?
- What will the impact, both episodic and background, be in the future?>
The GFDL atmospheric chemistry modeling group is developing global chemistry /transport models (GCTM) to answer, in a quantitative manner, the three questions posed above. This research is part of a major international effort to study the intercontinental transport and chemical transformation of pollution (http://www.al.noaa.gov/WWWHD/pubdocs/ITCT/).
Figure 1. Simulated percent impact of present Asian emissions on near-surface (940 mb) springtime Ozone (O3) and Carbon Monoxide (CO)
EFFORTS:
Recently a GFDL GCTM, employing winds generated by a general circulation model, has quantified the present seasonal impact of Asian emissions on global tropospheric levels of carbon monoxide (CO), nitrogen oxides (NOx) and ozone (O3). Figure 1 shows the percent contribution of a directly emitted Asian pollutant, CO, as well as the percent impact of Asian pollutants on tropospheric O3 levels. During the spring, seasonal average impacts are relatively modest over North America, ranging from 10 to 20% in the boundary layer for CO and O3. However, intercontinental transport occurs as highly episodic short-term events, as the six daily snapshots of the transport of Asian CO demonstrate in Figure 2. This is true for all the directly emitted pollutants (such as CO, NOx) and fine aerosol (PM2.5), as well as for the impact of these pollutants on tropospheric O3. For example, the average contribution of Asian CO is 30-40 ppbv (parts per billion by volume) in the boundary layer of western North America. However, transport episodes deliver more than 140 ppbv of Asian CO. The impact of such episodic transport events is more clearly shown in Figure 3, where present Asian emission levels result in a modest, though non-negligible, 5 - 10 ppbv background contribution to total O3 levels in the Southern California boundary layer. On the other hand, extreme transport events contribute up to 20 ppbv.
Figure 2. Simulated daily snapshots of Asian CO being transported over the North Pacific
To investigate this, we employ a pollution scenario for Asia that assumes continued development with no improvement in Asian emission technology and which resides in the upper levels of recent IPCC scenarios. Model results using this scenario predict that future total O3 levels on the west coast of the US will frequently exceed the recently revised EPA air quality standards of 80 ppbv, shown by the blue dashed line in Figure 3. The simulation suggests that a large fraction of this ozone increase is driven by Asian pollution.
Figure 3. Simulated daily time series of near surface (940mb)
Ozone (O3) over Southern
California for emission levels at present and around the year 2030
CUSTOMERS:
The GFDL modeling efforts, along with those of other research groups, have provided the theoretical basis for the conception and design of a number of recent US measurement campaigns that have either focused on the pollution and natural dust emitted from Asia (e.g., recent campaigns managed by NASA and NSF) or on its arrival over the western US (e.g., the Intercontinental Transport and Chemical Transformation [ITCT] Project managed by NOAA). The GFDL modeling efforts will play an important role in the post-mission analysis of these measurement programs. In the future, improved simulations will provide both a history of past and current impacts of Asian emissions on North American air quality and the ability to predict future impacts. Both of these capabilities are of interest to: those who focus on the protection of the health of the general public; the EPA, which designs national air quality regulations; and those in NOAA who are planning to develop air quality forecasting capabilities. In the longer term, it is likely that international air quality treaties will need to be negotiated. In such a situation, the modeling capabilities being developed here will play a crucual role.
SIGNIFICANCE:
While US agencies currently focus on US pollution and its impact on human health and agricultural productivity, it is becoming clear that the intercontinental transport of pollution, specifically ozone (O3) and its precursors (e.g. NOx, CO) and fine particles (PM2.5), plays a significant role in both US air quality and the regional radiative forcing of climate. It is critical that we develop the capability to evaluate both current impacts and possible future impacts due to increased emissions resulting from further economic development in Asia.
SUCCESS:
We have shown that Asian emissions already have, on average, a modest (~10%) impact on North American ozone levels, and that this impact more than doubles when maximum daily impacts are identified. Moreover, we predict that future projections of Asian growth will lead to pollution emission levels that challenge current US air quality standards, unless modern emission technology is extensively employed in Asia. Recent modeling efforts at GFDL and other research centers have clearly demonstrated the potential for realistic quantitative assessments of regional contributions to global air quality.
NEXT STEPS:
We will first focus on the analysis of the recent measurement campaigns, in particular this spring's ITCT campaign off the west coast of the US. We will also initiate 20-year integrations using NCEP re-analysis winds that examine the interannual variability and trends in the global distributions of ozone, dust, inorganic and organic aerosols, and black carbon and their resulting radiative forcings. We are particularly interested in identifying the size of the natural noise (interannual transport variability) in comparison to the human signal (increasing regional pollutant emissions). The ultimate goal is to develop the tools to accurately quantify contributions from other nations to both short-term weather-driven pollution events and long-term climate- and emission-driven air quality trends in the US.