Abstract. 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 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 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 C2 and C3 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 C2 and C3 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 Ox (Ox= O3+ 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 Ox pollution can effectively reduce the
production of OSs and SOAs in eastern China.
