Atmos. Chem. Phys. Discuss., 10, 109-152, 2010
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Quantifying pollution inflow and outflow over East Asia through coupling regional and global models
M. Lin1, T. Holloway1, G. R. Carmichael2, and A. M. Fiore3
1Center for Sustainability and the Global Environment (SAGE), Nelson Institute for Environmental Studies, University of Wisconsin-Madison, Madison, Wisconsin, USA
2Center for Global and Regional Environmental Research, University of Iowa, Iowa City, Iowa, USA
3NOAA Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey, USA

Abstract. Understanding the exchange processes between the atmospheric boundary layer and the free troposphere is crucial for estimating hemispheric transport of air pollution. Most studies of hemispheric air pollution transport have taken a large-scale perspective: using global chemical transport models and focusing on synoptic-scale export events. These global models have fairly coarse spatial and temporal resolutions, and thus have a limited ability to represent boundary layer processes and urban photochemistry. In support of United Nations Task Force on Hemispheric Transport of Air Pollution (TF HTAP;, this study employs two high-resolution atmospheric chemistry models (WRF-Chem and CMAQ; 36×36 km) coupled with a global model (MOZART; 1.9×1.9°) to examine the importance of fine-scale transport and chemistry processes in controlling pollution export and import over the Asian continent. We find that the vertical lifting and outflow of Asian pollution is enhanced in the regional models throughout the study period (March 2001) as contrast to the global model. Episodic outflow of CO, PAN, and O3 to the upper troposphere during cold frontal passages is twice as great in the WRF-Chem model as compared with the MOZART model. The TRACE-P aircraft measurements indicate that the pollution plumes in MOZART are too weak and too low in the altitude, which we attribute to the global model's inability to capture rapid deep convection that develops along the leading edge of the convergence band during frontal events. In contrast to pollution export from Asia, we find little difference in the regional vs. global model transport of European (EU) pollution into surface air over East Asia (EA). Instead, the local surface characteristics – sensitivity – strongly influence surface O3 responses. For instance, the O3 response to 20% decreases in EU emissions imported into our regional model domain is strongest (0.4–0.6 ppbv) over mountainous regions and weakest (0.1–0.3 ppbv) in megacities. The spatial averaged O3 response over EA estimated by our regional models is ~0.1 ppbv lower than global model estimates. Our results suggest that global models tend to underestimate the total budget of Asian pollutants exported to the free troposphere given their limited ability to properly capture vertical convection and lifting. Due to the compensating effects on surface O3 responses over downwind continents, future high-resolution hemispheric model analysis should provide additional insights into how the export and import processes interact, and will help to narrow the uncertainty of intercontinental source-receptor relationships.

Citation: Lin, M., Holloway, T., Carmichael, G. R., and Fiore, A. M.: Quantifying pollution inflow and outflow over East Asia through coupling regional and global models, Atmos. Chem. Phys. Discuss., 10, 109-152, doi:10.5194/acpd-10-109-2010, 2010.
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