IMPACT OF AEROSOL DIRECT AND INDIRECT EFFECTS ON EAST ASIAN AIR QUALITY DURING EAST-AIRE CAMPAIGN PERIOD
Allen, Dale J
Dickerson, Russell R
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WRF-Chem simulations were performed for the March 2005 East Asian Studies of Tropospheric Aerosols: an International Regional Experiment (EAST-AIRE) Intensive Observation Campaign (IOC) to investigate the effects of aerosols on surface radiation and air quality. Domain-wide, WRF-Chem showed a decrease of 20 W/m2 in surface shortwave (SW) radiation due to the aerosol direct effect (ADE), consistent with observational studies. The ADE reduced mixing and caused 24-hr surface PM2.5 concentrations to increase in eastern China (4.4%), southern China (10%), western China (2.3%), and the Sichuan Basin (9.6%), due to a thinner planetary boundary layer (PBL) and increased stability. Conversely, surface 1-hour maximum ozone was reduced by 2.3% domain-wide and up to 12% in eastern China because less radiation reached the surface. Studies of the impact of reducing SO2 and black carbon (BC) emissions by 80% on aerosol amounts were performed via two sensitivity simulations. Reducing SO2 decreased surface PM2.5 concentrations in the Sichuan Basin and southern China by 5.4% and decreased ozone by up to 6 ppbv in the Sichuan Basin and Southern China. Reducing BC emissions decreased PM2.5 by 3% in eastern China and the Sichuan Basin but increased surface ozone by up to 3.6 ppbv in eastern China and the Sichuan Basin. This result indicates that the benefits of reducing PM2.5 associated with reducing absorbing aerosols may be partially offset by increases in ozone at least for a scenario when NOx and VOC emissions are unchanged. The relative importance of direct and indirect effects in altering the atmospheric composition was then studied with two case studies of periods that featured strong synoptic systems. The case studies demonstrated that changes in primary aerosol (i.e., dust, OC) and chemically stable trace gas concentrations (i.e., CO) were mainly driven by changes in meteorological conditions due to the direct and indirect effects with the direct effect showing a stronger impact over highly polluted and dry regions and the indirect effect dominating over humid areas. Secondary aerosols (i.e., sulfate, nitrate) were affected by both changes in meteorological and chemical processes The variation of ozone due to the indirect effect was found to be associated with changes in the NO2 photolysis rate due to changes in actinic flux driven by changes in AOD and/or COD.