Impacts of Asian summer monsoon on seasonal and interannual variations of aerosols over eastern China

[1] China is located in a large monsoon domain; variations in meteorological fields associated with the Asian summer monsoon can influence transport, deposition, and chemical reactions of aerosols over eastern China. We apply a global three-dimensional Goddard Earth Observing System (GEOS) chemical transport model (GEOS-Chem) driven by NASA/GEOS-4 assimilated meteorological data to quantify the impacts of the East Asian summer monsoon on seasonal and interannual variations of aerosols over eastern China. During the summer monsoon season, four channels of strong cross-equatorial flows located within 40°E–135°E are found to bring clean air to China from the Southern Hemisphere. These channels have the effect of diluting aerosol concentrations in eastern China. In the meantime, rain belts associated with the summer monsoon move from southeastern to northern China during June–August, leading to a large wet deposition of aerosols. As a result, aerosol concentrations over eastern China are the lowest in summer. Sensitivity studies with no seasonal variations in emissions indicate that the Asian summer monsoon can reduce surface layer PM2.5 (particles with a diameter of 2.5 μm or less) aerosol concentration averaged over eastern China (110°E–120°E, 20°N–45°N) by about 50–70%, as the concentration in July is compared to that in January. We also compare simulated PM2.5 concentrations in the weak monsoon year of 1998 with those in the strong monsoon year of 2002, assuming same emissions in simulations for these 2 years. Accounting for sulfate, nitrate, ammonium, black carbon, organic carbon, as well as submicron mineral dust and sea salt, surface layer PM2.5 concentration averaged over June–August and over eastern China is 7.06 μg m−3 (or 44.3%) higher in the weak monsoon year 1998 than in the strong monsoon year 2002, and the column burden of PM2.5 is 25.1 mg m−2 (or 73.1%) higher in 1998 than in 2002. As a result, over eastern China, the difference in summer aerosol optical depth between 1998 and 2002 is estimated to be about 0.7. These results have important implications for understanding air quality and climatic effects of aerosols in eastern China.

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