Aerosol optical properties and chemical composition apportionment in Sichuan Basin, China.

PM2.5 and its major chemical components, and light scattering (σscat) and absorption (σabs) coefficients were measured in Chengdu (CD) and Chongqing (CQ) in Sichuan Basin, from October 2014 to July 2015. Annual mean PM2.5, σscat and σabs were 67.0±43.4μgm-3, 421.4±290.1Mm-1 and 36.7±26.4Mm-1, respectively, in CD, and annual mean PM2.5 and σabs were 70.9±41.4μgm-3 and 45.4±24.5Mm-1, respectively, in CQ. PM2.5, σscat and σabs were all evidently higher in winter than in other seasons mainly due to the unfavorable meteorological conditions for dispersion of local pollutants. Diurnal patterns of σscat and σabs exhibited a peak value around 7:00-8:00 LT and a valley value around 17:00-18:00 LT. High levels of PM2.5 accompanied with low wind speed and high relative humidity conditions were the major causes of visibility impairment in Sichuan Basin. Both σscat and σabs were remarkably higher under calm wind condition, indicating that local emissions were largely responsible for the aerosol pollutions in this region. High relative humidity enhanced extinction coefficient (bext) by up to around 1.6 and 1.4 times in CD and CQ, respectively, due to the hygroscopic growth of water soluble components. On annual basis, (NH4)2SO4 contributed the most to bext, accounting for 34.4% and 31.5% in CD and CQ, respectively, followed by NH4NO3 and organic matter, 28.1% and 17.5%, respectively, in CD, and 20.1% and 26.8%, respectively, in CQ. EC contributed about 10% and the rest contributed to <12% at both urban sites. Therefore, reducing emissions of the precursor gases such as SO2, NOx, NH3 and VOCs systemically may be efficient to improve the air quality and visibility simultaneously in Sichuan Basin.