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. Organosulfates (OSs) are ubiquitous in the atmosphere and serve as importanttracers for secondary organic aerosols (SOAs). Despite intense research overthe years, the abundance, origin, and formation mechanisms of OSs in ambientaerosols, particularly in regions with severe anthropogenic pollution, arestill not well understood. In this study, we collected filter samples ofambient fine particulate matter (PM2.5) over four seasons in both2015–2016 and 2018–2019 at an urban site in Shanghai, China, andcomprehensively characterized the OS species in these PM2.5 samplesusing an ultra-performance liquid chromatography quadrupole time-of-flightmass 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, mainlyas a result of the reduction of anthropogenic pollutant emissions in easternChina, the annually averaged concentrations of 35 quantified OSs were similar in both years (65.5 ± 77.5 ng m−3, 0.57 % ± 0.56 % of OA in 2015–2016 vs. 59.4 ± 79.7 ng m−3,0.66 % ± 0.56 % of OA in 2018–2019), suggesting an increasedcontribution of SOAs to OAs in 2018–2019 compared to 2015–2016. Isoprene- andmonoterpene-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 importantcontribution of biogenic emissions to the production of OSs and SOAs inShanghai. The abundance of biogenic OSs, particularly those arising fromisoprene, exhibited strong seasonality (peaked in summer) but no significantinterannual variability. In contrast, the quantified anthropogenic OSs hadlittle seasonal variability and declined in 2018–2019 compared with those in2015–2016. The C2 and C3 OS species that have both biogenic andanthropogenic origins contributed, on average, 19.0 % of the quantifiedOSs, with C2H3O6S−, C3H5O5S−, andC3H5O6S− being the most abundant species, togetheraccounting for 76 % of the C2 and C3 OS concentrations in 2015–2016and 2018–2019. 2-Methyltetrol sulfate (2-MTS,C5H11O7S−) and monoterpene-derivedC10H16NO7S− were the most abundant OSs and nitrooxy OSs insummer, on average, contributing 31 % and 5 % of the quantified OSs,respectively, during the summertime of the sampling years. The substantiallylarger concentration ratio of 2-MTS to 2-methylglyceric acid sulfate(2-MAS, C4H7O7S−) in summer (6.8–7.8) compared to the otherseasons (0.31–0.78) implies that low-NOx oxidation pathways played adominant role in isoprene-derived SOA formation in summer, whilehigh-NOx reaction pathways were more important in other seasons. Wefurther find that the production of OSs was largely controlled by the levelof Ox (Ox= O3+ NO2), namely the photochemistry ofOS precursors, particularly in summer, though sulfate concentration,aerosol acidity, and aerosol liquid water content (ALWC) that couldaffect the heterogeneous chemistry of reactive intermediates leading to OSformation also played a role. Our study provides valuable insights into thecharacteristics and mechanisms of OS formation in a typical Chinese megacityand implies that the mitigation of Ox pollution can effectively reduce theproduction of OSs and SOAs in eastern China.

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