Chemical characteristics and sources of organosulfates, organosulfonates, and carboxylic acids in aerosols in urban Xi'an, Northwest China

Abstract

We investigated speciation and levels of organosulfates, organosulfonates as well as carboxylic acids in aerosol samples collected during summer (2014) and winter (2014/15) in Xi'an, Northwest China, to improve understanding of composition and sources of organic aerosols in this region heavily affected by air pollution. Organosulfates are formed from reactive gas-phase organic compounds and acidic sulfate aerosols, contributing to secondary organic aerosols, SOA. The aerosol samples show a large diversity in organosulfur species in line with other regions of China, reflecting the high levels and complexity of SOA precursors. In summer samples, organosulfates from isoprene are prevalent due to transport of air masses from southern regions with isoprene-emitting mountain forests. During winter, air masses are local or from areas north of the city with low population density and very low temperatures. The estimated levels of organosulfates and organosulfonates in summer (768 ± 346 ng m−3) and winter samples (938 ± 374 ng m−3) are more similar than expected given the high levels of sulfate and organic carbon in winter, indicating the complexity of organosulfur formation processes. We observed an organosulfonate with molecular weight 214 (C6H14O6S) at high estimated levels (254 ± 232 ng m−3) in winter, but much lower concentrations (12 ± 13 ng m−3) in summer. High levels of organosulfur compounds were mainly observed at aerosol pH below about 2.5. Concentrations of carboxylic acids from oxidation of monoterpenes were low (5.2 ± 2.7 ng m−3 in summer). Phthalic acid was as high as 90 ± 29 ng m−3 during winter and correlated highly with organic carbon, chloride and potassium, indicating a common origin, most likely burning of biomass and plastic-containing waste. Further research is needed to elucidate formation and sources of organosulfates and organosulfonates, as well as the impact on aerosol properties affecting e.g. health effects.