Molecular characteristics and formation pathways of organosulfur compounds: a comparative field study across contrasting atmospheric environments

Abstract. Organosulfur compounds (OrgSs), especially organosulfates (OSs), are ubiquitous aerosol components. However, the spatial, seasonal, and diurnal variations of OrgS formation in polluted atmospheres remain poorly understood. Here, we monitored particulate OrgSs at an urban site and a suburban site in Shanghai and examined their molecular composition and formation pathways under contrasting atmospheric conditions. A total of 1964, 1914, and 2689 OrgS molecular formulas were detected in suburban summer, urban summer, and urban winter, respectively. More than 79 % of sulfur-containing molecular formulas had (4s + 3n)/o ≤ 1, indicating that OrgSs were dominated by OSs and nitrooxy-OSs (NOSs). Compared with summer, wintertime OrgSs exhibited lower O/C ratios but higher double-bond equivalence and aromaticity, suggesting a stronger influence of anthropogenic emissions and more unsaturated molecular structures. Although OrgSs were mostly present in aliphatic molecular structures, an increase in the number of aromatic OSs in winter revealed an enhanced contribution from anthropogenic sources. Isoprene/monoterpene-derived OSs peaked during the daytime due to photochemical oxidation in summer, whereas monoterpene-derived NOSs were markedly enhanced at night via nighttime NO₃-initiated oxidation. Non-metric multidimensional scaling analysis further revealed that OrgS composition in summer was associated with temperature and O₃ during the day but shifted toward RH-driven processing at night. In winter, inorganic nitrogen and sulfur species, aerosol liquid water content, and particle acidity became more important in shaping OrgS composition, suggesting enhanced aqueous-phase and acid-catalyzed formation. These findings provide molecular-level insights into the sources and formation of atmospheric OrgSs across contrasting environments.