Chemical and optical properties of atmospheric aerosols during the polluted periods in a megacity in the Yangtze river delta, China

Abstract

The chemical composition and optical properties of particulate matter (PM) were characterized in an urban-industrial area of Nanjing, China, in January 2015, when heavily polluted periods frequently occurred. Only 30% of the days fulfilled the National Ambient Air Quality Standards of China. The average scattering and absorption coefficients at 532 nm during the polluted periods were 620 ± 320 Mm–1 and 102 ± 57 Mm–1. An increasing relative fraction of the large size PM during the polluted periods can be deduced from the variations of the scattering Ångström exponent, backscattering ratio, and asymmetry factor. The mean mass concentrations of NO3–, SO42– and NH4+ in PM2.5 during the polluted periods were 32.87 ± 17.76 µg m–3, 23.6 ± 13.2 µg m–3, and 19.4 ± 10.1 µg m–3, respectively. NO3–, SO42– and NH4+ were the dominant water-soluble ions (WSIs) and accounted for 87% of the total ion concentration. Nitrate and organic matter (OM) dominated the aerosol composition during the polluted periods. The averaged organic carbon/elementary carbon ratios during the polluted and the cleaner periods were 3.6 and 4.3, respectively, consistent with a mix of primary emissions and secondary organic aerosol formation. Organic matter and ammonium nitrate (AN) were the dominant species contributing to light extinction during the polluted periods, contributing values of 159 ± 63 Mm–1 and 156 ± 91 Mm–1, respectively.