Modeling study on seasonal variation in aerosol extinction properties over China

To investigate the seasonal variation of aerosol optical depth (AOD), extinction coefficient (EXT), single scattering albedo (SSA) and the decomposed impacts from sulfate (SO₄^2-) and black carbon (BC) over China, numerical experiments are conducted from November 2007 to December 2008 by using WRF-Chem. Comparison of model results with measurements shows that model can reproduce the spatial distribution and seasonal variation of AOD and SSA. Over south China, AOD is largest in spring (0.6-1.2) and lowest in summer (0.2-0.6). Over north, northeast and east China, AOD is highest in summer while lowest in winter. The high value of EXT under 850 hPa which is the reflection of low visibility ranges from 0.4-0.8 km^-1 and the high value area shifts to north during winter, spring and summer, then back to south in autumn. SSA is 0.92-0.94 in winter and 0.94-0.96 for the other three seasons because of highest BC concentration in winter over south China. Over east China, SSA is highest (0.92-0.96) in summer, and 0.88-0.92 during winter, spring and autumn as the concentration of scattering aerosol is highest while BC concentration is lowest in summer over this region. Over north China, SSA is highest (0.9-0.94) in summer and lowest (0.82-0.86) in winter due to the significant variation of aerosol concentration. The SO₄^2- induced EXT increases about 5%-55% and the impacts of BC on EXT is much smaller (2%-10%). The SO₄^2--induced increase in SSA is 0.01-0.08 and the BC-induced SSA decreases 0.02-0.18.