Asian influence on surface ozone in the United States: A comparison of chemistry, seasonality, and transport mechanisms

Ongoing growth in Asia has increased emissions of several ozone precursors which are increasingly impacting surface ozone levels in the United States. For this study we use the offline Community Atmospheric Model with Chemistry driven by National Center for Environmental Protection meteorology for 2001-2005, plus additional tagged tracers, to examine the chemistry, seasonality, and transport of Asian emissions as they are lofted from the Asian boundary layer into the free troposphere over the Pacific Ocean and into the United States. At the surface in the western United States, Asian ozone (O(3)A) mixing ratios are maximum in the spring at 3.36 +/- 1.3 ppbv and are minimum in the summer at 1.36 +/- 0.7 ppbv (mean +/- standard deviation over time). Transport of O(3)A and its precursors to the surface in the United States depends on the structure of the elevated O(3)A plume and on available meteorological transport mechanisms, such as dry air streams associated with midlatitude cyclones, which can transport air from plumes with elevated levels of Asian pollution in the free troposphere to the surface. We show that the structure of such plumes has a strong seasonal dependence, entering the United States in the spring, widely dispersed between roughly 0 to 6 km and 20 degrees N to 50 degrees N in the lower free troposphere and boundary layer, with O(3)A mixing ratios between 5 and 10 ppbv. In summer the plume is less dispersed and is located in the upper free troposphere, centered at 8 km with peak O(3)A of 11 ppbv.