Atmospheric boundary layer sources for upper tropospheric air over the Asian summer monsoon region
Previous satellite measurements and model simulations have shown that the Asian summer monsoon (ASM) anticyclone is co-located with higher concentrations of pollutants, which are emitted in the continental atmospheric boundary layer (ABL). Backward trajectory calculations show that the air at the 150-hPa level has the maximum frequency of ABL sources within 30 days over the most intensive convection regions and their downwind areas, which are not located within the ASM anticyclone, but rather at the southern flank or periphery of the ASM anticyclone. The upper tropospheric airs originated from the ABL sources include two parts: one from the ocean, which has the dominant impact to the south of 20°N, particularly over the South China Sea (SCS) and the west tropical Pacific Ocean; and another from the continent, which is dominant between 10°N and 30°N, particularly over the Bay of Bengal (BoB), continental India, the Arabian Sea, and the Arabian Peninsula. It is the latter part that forms the higher pollutant concentration within the ASM anticyclone as shown by satellite measurements. Air in the ABL sources (both polluted and unpolluted) converges to the intensive convection region in the lower troposphere, and then traverses the middle troposphere through a wide group of upward pipes, and finally to the upper troposphere. These pipes in the middle troposphere are defined by the ASM intensive convections and cover the south of continental India, the BoB, the Tibetan Plateau, the Indochina Peninsula, the SCS, and the Philippine Sea.