PM_2.5 Chemical compositions and aerosol optical properties in Beijing during the late fall

Daily PM_2.5 mass concentrations and chemical compositions together with the aerosol optical properties were measured from 8-28 November 2011 in Beijing. PM_2.5 mass concentration varied from 15.6-237.5 μg.m^-3 and showed a mean value of 111.2 ± 73.4 μg.m^-3. Organic matter, NH₄NO₃ and (NH₄)₂SO₄ were the major constituents of PM_2.5, accounting for 39.4%, 15.4%, and 14.9% of the total mass, respectively, while fine soil, chloride salt, and elemental carbon together accounted for 27.7%. Daily scattering and absorption coefficients (σ_sc and σ_ap) were in the range of 31.1-667 Mm^-1 and 8.24-158.0 Mm^-1, with mean values of 270 ± 200 Mm^-1 and 74.3 ± 43.4 Mm^-1. Significant increases in σ_sc and σ_ap were observed during the pollution accumulation episodes. The revised IMPROVE algorithm was applied to estimate the extinction coefficient (b_ext). On average, organic matter was the largest contributor, accounting for 44.6% of b_ext, while (NH₄)₂SO₄, NH₄NO₃, elemental carbon, and fine soil accounted for 16.3% 18.0%, 18.6%, and 2.34% of b_ext, respectively. Nevertheless, the contributions of (NH₄)₂SO₄ and NH₄NO₃ were significantly higher during the heavy pollution periods than those on clean days. Typical pollution episodes were also explored, and it has been characterized that secondary formation of inorganic compounds is more important than carbonaceous pollution for visibility impairment in Beijing.