Aerosol pollution in a megacity of southwest China inferred from variation characteristics of sulfate-δ34S and water-soluble inorganic compositions in TSP

Abstract

Daytime and nighttime aerosol samples for total suspended particles (TSP) were collected in Chengdu from 12 to 23 January 2013 (winter) and 10 to 21 August 2014 (summer). The mass concentrations of TSP, major inorganic ions, and stable sulfur isotope ratios were determined. Clear winter–summer variation in TSP was observed; the mass concentrations of TSP in summer were below levels defined by the Chinese National Ambient Air Quality Standard (GB3095-2012), whereas winter TSP levels exceeded the standard by 2–4 times. Overall, the sum of SO42−, NO3−, and NH4+ concentrations represented the highest contribution to water-soluble ionic components, which showed similar trends to TSP mass. According to correlations among major ions, the major compounds in TSP were likely to be (NH4)2SO4, NH4NO3, CaCl2, and KCl in both sampling times, and CaSO4 and Ca(NO3)2 were also present in summer. The NO3−/SO42− ratios ranged from 0.57 to 1.28 (average 0.85±0.16) in winter and 0.5 to 0.93 (average 0.66±0.11) in summer, indicating stationary source emissions (coal burning) were more important sources than the vehicle exhaust. The relationship between SO42− and δ34S indicated the sulfate sources were affected by the low sulfur isotope value of biogenic sulfur release in summer, and the high sulfur isotope value of coal in winter. As a whole, the mixture of coal burning and vehicle exhaust represent the major TSP sources in Chengdu in the study period, and biogenic sulfur also contributes to TSP in summer.