Comparative Study of Particulate Organosulfates in Contrasting Atmospheric Environments: Field Evidence for the Significant Influence of Anthropogenic Sulfate and NOx

Organosulfates (OSs) are an important group of secondary organic aerosols, but the key influential factors of their formation in polluted atmospheres are not well understood. In this study, we monitored particulate OSs (carboxy OSs, hydroxyacetone sulfate, and isoprene- and monoterpene-derived OSs) at an urban site and a regional site in Beijing and examined their compositions and formation pathways under contrasting atmospheric conditions. The quantified OSs were most abundant in the summer at the regional site due to higher biogenic emissions and favorable formation conditions (higher aerosol acidity and humidity), followed by urban summer and winter conditions. Larger fractions of inorganic sulfate were converted to organosulfur when sulfate was less abundant. This implies that OSs would play more important roles in aerosol properties as the decline of sulfate. Monoterpene-derived nitrooxy-OSs were enhanced via NO3 oxidation in the summer under high-NOx conditions at night, while the day-night variations in the winter were not as obvious. Among isoprene-OSs, IEPOX (isoprene epoxydiols)-OS formation was clearly suppressed under high-NOx conditions, while other isoprene-OSs that are favored under high-NOx conditions showed increasing formation with NOx. The results highlight that isoprene-OS formation pathways in polluted atmospheres could be different from the IEPOX-dominated regions reported for the low-NOx environments in the literature.