Observation Of Particulate Organosulfates In Urban Shanghai And Preliminary Studies On Formation Mechanisms Of Aromatic Organosulfates

Organosulfates (OSs) are important components of secondary organic aerosols (SOAs). Studies on identification and formation mechanisms of OSs can help further our current understanding on SOA formation and close the gap between measured and modeled SOA mass concentrations. Biogenic volatile organic compounds (VOCs) have been accepted as precursors of OSs, whereas the role of anthropogenic VOCs are yet to be elucidated. Hence, there are critical needs to identify and quantify OSs in the urban aerosols, and to explore the formation mechanism and precursors of identified organosulfates.In this study, we identified particulate OSs in urban Shanghai and investigated their temporal variations. PM2.5 samples have been collected in urban Shanghai, China, during spring (April), summer (July), autumn (October) 2012, and winter (January) 2013, respectively. Seventeen OSs derived from isoprene, α-/β-pinene, and presumably aromatic precursors were quantified using a high performance liquid chromatogram (HPLC) coupled to a triple quadrupole mass spectrometer (TQ-MS/MS). During our sampling days, the total concentration of detected OSs ranged between 0.0209 and 60.8 ng m-3 (sodium octyl sulfate and benzyl sulfate as standards). The seasonal average concentration of OSs was the highest (26.2 ng m-3) in summer 2012 and the lowest (0.510 ng m-3) in spring 2012, accounting for 0.471%o and 0.00965%o of the corresponding PM2.5 mass, respectively. Isoprene-and α-/β-pinene-derived OSs showed a clear seasonal fluctuation with a peak in summer, whereas OS derived from aromatic precursors was less variable in most seasons. In addition, a number of OSs demonstrated distinct diurnal profiles, reflecting diverse formation mechanisms with multiple atmospheric oxidants in an urbanized area like Shanghai.We further investigated the potential and formation mechanism of anthropogenic VOCs derived-organosulfates using an ultra-resolution orbitrap mass spectrometer. Three aromatic organosulfates with m/z 187, m/z 201 and m/z 215 were tentatively identified in a Nanjing aerosol sample, with speculated precursors of toluene, xylene, and trimethylbenzene, respectively. In addition, chamber experiments of OH radical initiated-reaction of toluene under high NOx conditions with the presence of seed particles showed a 3.22×10-3‰～3.89×10-2‰ yield of benzyl sulfate. The mass concentrations of benzyl sulfate calculated from this yield and ambient abundance of toluene are at the same order of magnitude as that of measured benzyl sulfate, reflecting that reaction between toluene and OH is likely the source of the benzyl sulfate in the atmospheric aerosols of Shanghai. Aerosol acidity seems to have little effect on the formation of benzyl sulfate, whereas ionic strength of the seed particles can promote the yield. The chamber experiments of xylenes showed that xylenes may serve as the precursors of m/z 201.