Fine particulate (PM2.5) continues to be a major air pollution problem due to its adverse effects on human health and the environment. Recently, the rapid industrialization now taking place in Asia is producing a large and growing quantity of NOX, SO2, CO and other atmospheric emissions. This research is part of the work of project "Intercontinental Transport and Climatic Effects of Air Pollutants" (ICAP) sponsored by U.S. Environment Protection Agency (EPA). It presents modeling and interpretation of the impact of Asian anthropogenic emissions on air quality in the U.S. through long distance transport of Asian air pollutants. In order to better quantify the magnitude and impacts of Asia anthropogenic emission on U.S. air quality caused by intercontinental transport, a 3-D regional air quality model, the EPA Models-3 Community Multiscale Air Quality (CMAQ) system has been employed in this study. The model is applied to both baseline simulation of 2001 and the sensitivity simulation in which anthropogenic Asian emissions were turned off. The baseline simulation was evaluated with both surface data over U.S. and aircraft observation over Western Pacific regions. Compared to surface measurement over U.S., over 50% of the simulated PM2.5 concentrations fall within a factor of two of the observations in all seasons except winter. Model evaluation using vertical aircraft data indicates that the model is able to well reproduce the vertical distributions of SO42-, NO3-, NH4+ and CO. Concentrations of all species decrease rapidly with increasing altitude below 6 km, followed by a much slower decrease between 6 and 12 km. Time series analysis indicates that model can place the plume in the right location, but tends to under-predict the peak values of the plume intensity.This study shows that the impact of Asian man-made emissions is persistent throughout the entire year rather than episodic. There also exists a significant seasonal cycle with an April/May maximum (1~1.2 μg/m3 by average) and a July/August minimum (0.2 ~ 0.3 μg/m3 by average) for both eastern and western U.S. despite of the fact that Asia has the monthly average maximum for PM2.5 in January. During the simulated year, SO42- is the major component of Asian pollutants in each month for both eastern and western U.S. Cross section study along 140oE indicates although the concentration of PM2.5 species has a maximum value below 1 km, while distribution of eastward fluxes of Asian aerosol species shows the strongest flux at 2-4 km due to strong westward wind flow. By integrating the flux at different layers along ~140oE, it turns out that maximum flux of PM2.5 is at 35-45oN in the boundary (< 2,500m) and at 20?35oN in the free troposphere (2,500 -16,000 m). Further study on the transport pattern of SO42- and NO3- indicates that SO42- is transported mainly from Asia directly to the U.S., while NO3- may be transported through transport of its precursors (mainly PAN).We also compare the model results over the continental U.S. domain using three different continental influxes; one from a hemispheric-scale CMAQ simulation, and the other two from a global model GEOS-CHEM simulation and from static boundary conditions. The simulation with boundary conditions (BCs) provided by the CMAQ modeling has a better model performance in the western U.S. than the GEOS-CHEM and static BCs when compared against CASTNet observation. While further studies on other species are needed, our results suggest that CMAQ modeling can be a suitable tool to address intercontinental transport of air pollution, an issue of hemispheric scale in nature that has been previously addressed by field campaigns or global models.
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Intercontinental and Regional Modeling of Air Pollutants Transport over the Pacific Regions
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