Biogenic and Anthropogenic Contributions to Atmospheric Organosulfates in a Typical Megacity in Eastern China

Organosulfates (OSs) are important components of secondary organic aerosols (SOA) in the atmosphere. Despite the frequent observation of OSs in field and laboratory studies, the sources and detailed formation pathways of these species, particularly in polluted regions, are still not well understood. In this study, a total of 156 ambient PM2.5 filter samples were collected in a Chinese megacity (Shanghai) over four seasons in both 2015-2016 and 2018-2019. More than 270 OS formulas were detected in ambient aerosols using high-resolution liquid chromatography-mass spectrometry. Interestingly, a series of alkyl OSs, CnH(2n+1)O(4)S- (C5-C11), were observed, which are most likely to be produced from the heterogeneous reactions of organic peroxides with sulfur dioxide or sulfate. The observation of these compounds provides the first field evidence for atmospheric OS formation involving organic peroxides. By employing a positive matrix factorization model, the possible precursors of OSs were classified into six groups, that is, isoprene-derived, monoterpene-derived, aromatic, short-chain aliphatic, long-chain aliphatic, and anthropogenic C-2/C-3 OSs. Biogenic (isoprene and monoterpenes) and anthropogenic sources (the remaining factors) contributed to 57.2% and 42.8% of OS formation in 2015-2016, respectively, and their contributions just slightly changed in 2018-2019. In addition, the OS sources exhibited strong seasonality. Isoprene-derived OSs contributed to the largest fraction of total OSs in summer, while anthropogenic sources dominated OS formation in other seasons. Our findings provide valuable insight into OS sources and formation mechanisms in the atmosphere and highlight the importance of both biogenic and anthropogenic emissions for the formation of OSs in a typical Chinese megacity.