A 9-year climatology of airstreams in East Asia and implications for the transport of pollutants and downstream impacts

Ambient measurements of PM₁₀ levels in Taiwan over East Asia have shown unusual decreases for the 1994-2002 period. To assess these downward trends of PM₁₀, we develop methods for calculating climatologies of PM₁₀ and airstreams, and a new method for quantifying airstreams and their associated impacts on PM₁₀ and PM₁₀-influencing factors. These methods were verified against anomalously low PM₁₀ levels during the 1998 ENSO, and anomalously high PM₁₀ levels during spring 2000. Calculations show the following: 20-23% increased occurrence of airstreams from the north direction in spring 2000 coincide with 14-16% increases in ambient PM₁₀ levels downstream over Taiwan; 60% increased occurrence of airstreams over western Pacific coincide with 10-15% drops in ambient PM₁₀ levels downstream during the 1998 ENSO. Comparison of airstreams between 1994-1997 and 1999-2002 show a 15-29% reduction over the eastern direction and a 7-17% increase over the western direction. The most significant findings associated with these changes are the reduction in close-to-surface inversions, the increase in rainfall, the increase in continental outflows, and the counteracting effects between these processes. The reduction in close-to-surface inversions dilutes ambient particles from the surface toward the free troposphere, thereby reducing ambient PM₁₀ levels; increases in rainfall provide efficient wet scavenging mechanisms for removing particles from the ambient air. These two processes counteract and dominate over the increase in airstreams over the northwestern direction, where continental pollutants are exported downstream, and results in the downward trends in PM₁₀.