The Impacts of Meteorology on the Seasonal and Interannual Variabilities of Ozone Transport From North America to East Asia

摘要

The transport of North American (NA) ozone to East Asia is investigated through the analysis of a 20 year simulation (1987-2006) using a global chemical transport model (GEOS-Chem) and forward trajectories during the 1990s at three NA sites. NA ozone mainly influences northern East Asia (> 30°N), where NA ozone in the free troposphere peaks in spring and fall (~12 ppbv). At the surface, NA ozone ranges from 2 to 7 ppbv and peaks in winter, ~50% of which is from the NA boundary layer. The seasonality of the imported NA ozone reflects the combined effects of meteorology and chemistry. In summer, NA ozone can be diverted from reaching East Asia by strong downdrafts behind the European trough. In winter, the prevailing monsoon climate in East Asia can boost downdrafts of NA ozone to the surface. In spring and fall, the westerlies are stronger and shift farther south than in summer, bring more NA ozone to the East Asian (EA) free troposphere than in summer. The imported NA ozone at the EA surface also varies with interannual meteorology. This interannual variation is found to closely correlate to the East Asian winter monsoon (EAWM). The stronger the EAWM in a winter is, the stronger are the downdrafts bringing more NA ozone to the EA surface in that winter and the subsequent spring. Because the anthropogenic NA emissions have decreased since 1999, the year an emission inventory was used in the simulation, the simulated NA influence may serve as an upper limit.