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

摘要

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 ± 43.4 μg m~(-3),421.4 ± 290.1 Mm~(-1) and 36.7 ± 26.4 Mm~(-1), respectively, in CD, and annual mean PM_(2.5) and σ_(abs) were 70.9 ± 41.4 μg m~(-3) and 45.4 ± 24.5 Mm~(-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, (NH_4)_2SO_4 contributed the most to b_(ext), accounting for 34.4% and 31.5% in CD and CQ, respectively, followed by NH_4NO_3 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 SO_2, NO_x NH_3 and VOCs systemically may be efficient to improve the air quality and visibility simultaneously in Sichuan Basin.