Field Evidence for Constraints of Nearly Dry and Weakly Acidic Aerosol Conditions on the Formation of Organosulfates

A global perspective on the abundance and formation of organosulfates (OSs) during field studies (relative humidity of 53% to 77%) suggested that the investigated particles are generally nondry and acidic (pH < 6). However, the key factors affecting OS formation in nearly dry and weakly acidic aerosol conditions remain elusive. This topic was resolved by examining the composition and formation of OSs in PM_2.5 collected in Urumqi (dry and dusty) over a one-year period. Anthropogenic OSs accounted for 49 ± 8% of the total OSs, indicating a large anthropogenic contribution to OS formation in Urumqi (particularly in winter). The low aerosol liquid water (ALW) concentration (2 ± 2 μg m^–3) and weak particle acidity (pH = 7 ± 2) during the summer were important factors limiting anthropogenic OS formation. However, increased ALW (100 ± 70 μg m^–3) and particle acidity (pH = 5 ± 1) during the winter significantly promoted anthropogenic OS production. The formation of most of isoprene- and monoterpene-derived OSs during summer was also constrained by unfavorable ALW concentration and particle acidity, resulting in biogenic OS levels being lower in summer than in winter. This study provides observational evidence on OS formation constraints by dry and dusty atmospheric conditions.