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

PM_2.5 and its major chemical components, and light scattering (σ_scat) and absorption (σ_abs) coefficients were measured in Chengdu (CD) and Chongqing (CQ) in Sichuan Basin, from October 2014 to July 2015. Annual mean PM_2.5, σ_scat and σ_abs were 67.0±43.4μgm^-3, 421.4±290.1Mm^-1 and 36.7±26.4Mm^-1, respectively, in CD, and annual mean PM_2.5 and σ_abs were 70.9±41.4μgm^-3 and 45.4±24.5Mm^-1, respectively, in CQ. PM_2.5, σ_scat and σ_abs 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 σ_scat and σ_abs exhibited a peak value around 7:00-8:00 LT and a valley value around 17:00-18:00 LT. High levels of PM_2.5 accompanied with low wind speed and high relative humidity conditions were the major causes of visibility impairment in Sichuan Basin. Both σ_scat and σ_abs 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 (b_ext) 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 b_ext, 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, NO_x, NH₃ and VOCs systemically may be efficient to improve the air quality and visibility simultaneously in Sichuan Basin.