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

Abstract

Abstract. Organosulfates (OS) are ubiquitous in the atmosphere and serve as important tracers for secondary organic aerosols (SOA). Despite intense research over years, the abundance, origin, and formation mechanisms of OS in ambient aerosols, in particular 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 a liquid chromatography coupled to a high resolution mass spectrometer (UPLC-ESI-QToF-MS). We find that while the concentration of organic aerosol (OA) decreased by 29 % in 2018/2019, compared to that in 2015/2016, the annually averaged concentrations of 35 quantified OS were similar in two years (65.5 ± 77.5 ng m−3 in 2015/2016 versus 59.4 ± 79.7 ng m−3 in 2018/2019), suggesting an increased contribution of SOA to OA in 2018/2019 than in 2015/2016. Isoprene- and monoterpene-derived OS are the two most abundant OS families, on average accounting for 36.3 % and 31.0 % of the quantified OS concentrations, respectively, suggesting an important contribution of biogenic emissions to the production of OS and SOA in Shanghai. The abundance of biogenic OS, particularly those arising from isoprene, exhibited strong seasonality (peaked in summer) but no significant interannual variability. In contrast, anthropogenic OS such as diesel-derived ones had little seasonal variability and declined obviously in 2018/2019 compared with that in 2015/2016. This reflects a significant change in precursor emissions in eastern China in recent years. The C2/C3 OS species that have both biogenic and anthropogenic origins averagely contributed to 19.0 % of the quantified OS, with C2H3O6S−, C3H5O5S−, and C3H5O6S− being the most abundant ones, together accounting for 76 % of C2/C3 OS concentrations. 2-Methyltetrol sulfate (2-MT-OS, C5H11O7S−) and monoterpene-derived C10H16NO7S− were the most abundant OS and nitrooxy-OS in summer, contributing to 31 % and 5 % of the quantified OS, respectively. The substantially larger concentration ratio of 2-MT-OS to 2-methylglyceric acid sulfate (2-MA-OS, C4H7O7S−) in summer (6.8–7.8) than in 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 OS was largely controlled by the level of Ox (Ox = O3 + NO2), namely, the photochemistry of OS precursors, in particular in summer, though sulfate concentration, aerosol acidity, as well as 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 mitigation of Ox pollution can effectively reduce the production of OS and SOA in eastern China.

Chemical Transformation of α-Pinene derived Organosulfate via Heterogeneous OH Oxidation: Implications for Sources and Environmental Fates of Atmospheric Organosulfates

“…As shown in Fig. S1, MS 2 spectra show these 30 ions fragmented into sulfur containing ions (SO3 −• (m/z = 79.9574), HSO3 − (m/z = 80.9651), and HSO4 − (m/z = 96.9601)) and further confirmed the identity of these ions are OSs (Surratt et al, 2008;Hettiyadura et al, 2017;Wang et al, 2017Wang et al, , 2021. It may not be surprising that SO4 −• (m/z = 95.9523) was not observed in these MS 2 spectra since it is likely originated from the fragmentation of tertiary OSs (Surratt et al, 2008).…”

“…Extracts were then 35 Spectrometer (HPLC/ESI-QToF-MS). Experimental details have been given elsewhere (Wang et al, 2017(Wang et al, , 2021. Sample injections were first separated using an Acquity UPLC 10 HSS T3 column (2.1 mm × 100 mm, 1.8 µm; Waters, Milford, MA) with mobile phase consisting of water (H2O) (eluent A) and methanol (eluent B), each containing 0.1 % formic acid, at a flow rate of 0.3 mL min −1 .…”

“…To quantify the amount of αpOS-249 before and after 25 oxidation, 5 µL of reconstituted extract was injected into an Agilent 1260 LC system (Palo Alto, CA) interfaced with a QTRAP 4500 mass spectrometer (AB Sciex, Toronto, Ontario, Canada) and a TurbolonSpray source operated in multiple reaction monitoring (MRM) mode (Wang et al, 2017(Wang et al, , 2021. LC separation was preformed using the same column and mobile phase adopted for HPLC/ESI-QToF-MS.…”

“…On the basis of reaction products, we attempt to examine whether the heterogeneous OH oxidation of αpOS-249 freshly formed from the photooxidation of α-pinene could explain the formation of OSs detected in laboratory generated α-pinene SOA and ambient aerosols. As shown in Table 2, five out of six product ions (m/z = 247, 263, 265, 279, and 281) have been detected in 20 ambient aerosols (Iinuma et al, 2005;Surratt et al, 2008;Ma et al, 2014;Hettiyadura et al, 2019;Wang et al, 2021) while four out of six product ions (m/z = 247, 265, 279, and 281) have been observed for SOA formed from the photooxidation of α-pinene in the presence of acidic sulfate aerosols in laboratory studies (Liggio et al, 2006;Surratt et al, 2008;Ma et al, 2014;Zhang et al, 2015;Hettiyadura et al, 2019). Reaction mechanisms have been proposed for the formation of 25 some of these products such as m/z = 265 (C10H17O6S − ) and 279 (C10H15O7S − ) in the literature.…”

“…OSs have been found to be among important constituents of α-pinene derived SOA in chamber studies and ambient aerosols (Surratt et al, 2008;Stone et al, 2012;Ma et al, 2014). Field studies reported that a variety of α-5 pinene derived OSs can contribute 0.6-7.7 % of total sulfate in atmospheric aerosols (Huang et al, 2015(Huang et al, , 2018Wang et al, 2018Wang et al, , 2021. Here, we conducted a laboratory study to investigate the chemical transformation of a model α-pinene derived OS C10H17O5SNa,6, heptan-3-yl sulfate) through heterogeneous OH oxidation (Table 1).…”