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

Abstract

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.

Interactions of organosulfates with water vapor under sub- and supersaturated conditions

“…Significant differences between κ gf and κ ccn were reported in previous studies (Petters et al, 2009;Wex et al, 2009;Hansen et al, 2015), attributed to several factors discussed below. Petters and Kreidenweis (2008) demonstrated that cloud droplet activation was highly sensitive to the solubility for sparingly soluble compounds in the range of 5 × 10 −4 -2 × 10 −1 , expressed as volume of solute per unit volume of water (Petters and Kreidenweis, 2008).…”

“…For example, the mass concentration of sodium methyl sulfate, the smallest organosulfate, was found to be 0.2-9.3 ng m −3 in Centreville, Alabama (Hettiyadura et al, 2017). Hydroxyacetone sulfate, which may originate from both biogenic (Surratt et al, 2008) and anthropogenic emissions (Hansen et al, 2014), has been detected at various locations, such as the Arctic (1.27-9.56 ng m −3 ) (Hansen et al, 2014), Beijing (0.5-7.5 ng m −3 ) (Wang et al, 2018), Shanghai (1.8-2.3 ng m −3 ) (Wang et al, 2021), Xi'an (0.9-2.6 ng m −3 ) (Huang et al, 2018), 3 ng m −3 ) (Hettiyadura et al, 2017) and Iowa City (4.8 ± 1.1 ng m −3 ) (Hughes and Stone, 2019). In addition, benzyl and phenyl sulfates were also ubiquitous in the troposphere, with reported concentrations of up to almost 1 µg m −3 (Kundu et al, 2013;Ma et al, 2014;Staudt et al, 2014;Huang et al, 2018).…”

Abundance and Fractional Solubility of Aerosol Iron During Winter at a Coastal City in Northern China: Similarities and Contrasts Between Fine and Coarse Particles

“…In our calculations NH 3 and HNO 3 in the gas phase were implicitly assumed to be negligible when compared to NH 4 + and NO 3 − in aerosol particles, and this would lead to uncertainties in calculated aerosol acidity. The same method was also used in a recent study (Wang et al, 2021) to calculate aerosol pH for PM 2.5 samples in Shanghai. We also tried to use the iteration process (Fang et al, 2017) to calculate aerosol pH, but the results failed to converge.…”

Importance of Oxidants and Temperature in the Formation of Biogenic Organosulfates and Nitrooxy Organosulfates

“…Organosulfates (OSs) are key contributors of atmospheric secondary organic aerosol and have been detected in ambient aerosols in both clean and polluted sites around the world. − OSs have been shown to make up significant portions of organic aerosol (OA) and PM 2.5 (particulate matter less than 2.5 μm in diameter) mass, − with isoprene derived OSs accounting for up to 8% of organic matter . Radiocarbon measurements have shown that OA is dominated by modern carbon even in urban areas, − suggesting that biogenic sources could be playing a key role in OA and PM 2.5 formation.…”

“…The formation mechanisms of OSs have been studied extensively, with OS formation through an acid catalyzed ring opening of epoxides considered to be the most important route, ,,− especially for those derived from isoprene. However, several other routes have been proposed such as sulfate radical addition to an unsaturated precursor in the aqueous phase, , direct sulfate esterification of an alcohol precursor, or the replacement of nitrate groups within organonitrate species with sulfate. , Recent studies have reported the formation of OSs both directly and indirectly from the reaction of unsaturated species with SO 2 . − Nitrooxy-OSs (NOSs) are a sub class of OS and have been observed in several locations, ,,,− but their formation pathways are less well-studied. ,− NOS species have been shown to form from the reaction of sulfate with organic nitrate species, formed during OH photo-oxidation of VOCs via a reaction of the peroxy radicals with NO. Organic nitrates can also be formed from nitrate radical initiated oxidation of VOCs, , which becomes increasingly important at night when the concentration of NO 3 radicals is higher and concentrations of OH are reduced .…”