Organosulfates in atmospheric aerosols in Shanghai, China: seasonal and interannual variability, origin, and formation mechanisms

Organosulfates (OSs) are ubiquitous in the atmosphere and serve as important tracers for secondary organic aerosols (SOAs). Despite intense research over the years, the abundance, origin, and formation mechanisms of OSs in ambient aerosols, particularly in regions with severe anthropogenic pollution, are still not well understood. In this study, we collected filter samples of ambient fine particulate matter (PM2.5) over four seasons in both 2015-2016 and 2018-2019 at an urban site in Shanghai, China, and comprehensively characterized the OS species in these PM2.5 samples using an ultra-performance liquid chromatography quadrupole time-of-flight mass spectrometer equipped with an electrospray ionization (ESI) source (UPLC-ESI-QToFMS). Overall, we find that while the concentration of organic aerosols (OAs) decreased by 29% in 2018-2019 compared to that in 2015-2016, mainly as a result of the reduction of anthropogenic pollutant emissions in eastern China, the annually averaged concentrations of 35 quantified OSs were similar in both years (65.5 +/- 77.5 ngm(-3), 0.57% +/- 0.56% of OA in 2015-2016 vs. 59.4 +/- 79.7 ngm(-3), 0.66% +/- 0.56% of OA in 2018-2019), suggesting an increased contribution of SOAs to OAs in 2018-2019 compared to 2015-2016. Isoprene- and monoterpene-derived OSs were the two most abundant OS families, on average, accounting for 36.3% and 31.0% of the quantified OS concentrations, respectively, during both sampling years, suggesting an important contribution of biogenic emissions to the production of OSs and SOAs in Shanghai. The abundance of biogenic OSs, particularly those arising from isoprene, exhibited strong seasonality (peaked in summer) but no significant interannual variability. In contrast, the quantified anthropogenic OSs had little seasonal variability and declined in 2018-2019 compared with those in 2015-2016. The C-2 and C-3 OS species that have both biogenic and anthropogenic origins contributed, on average, 19.0% of the quantified OSs, with C2H3O6S-, C3H5O5S-, and C3H5O6S- being the most abundant species, together accounting for 76% of the C-2 and C-3 OS concentrations in 2015-2016 and 2018-2019. 2-Methyltetrol sulfate (2-MTS, C5H11O7S-) and monoterpene-derived C10H16NO7S- were the most abundant OSs and nitrooxy OSs in summer, on average, contributing 31% and 5% of the quantified OSs, respectively, during the summertime of the sampling years. The substantially larger concentration ratio of 2-MTS to 2-methylglyceric acid sulfate (2-MAS, C4H7O7S-) in summer (6.8-7.8) compared to the other seasons (0.31-0.78) implies that low-NOx oxidation pathways played a dominant role in isoprene-derived SOA formation in summer, while high-NOx reaction pathways were more important in other seasons. We further find that the production of OSs was largely controlled by the level of O-x (O-x = O-3 + NO2), namely the photochemistry of OS precursors, particularly in summer, though sulfate concentration, aerosol acidity, and aerosol liquid water content (ALWC) that could affect the heterogeneous chemistry of reactive intermediates leading to OS formation also played a role. Our study provides valuable insights into the characteristics and mechanisms of OS formation in a typical Chinese megacity and implies that the mitigation of O-x pollution can effectively reduce the production of OSs and SOAs in eastern China.