Underestimation of anthropogenic organosulfates in atmospheric aerosols in urban regions

Abstract. Organosulfates (OSs) are important components of organic aerosols, which serve as critical tracers of secondary organic aerosols (SOA). However, molecular composition, the relationship between OSs and their precursors, and formation driving factors of OSs at different atmospheric conditions have not been fully constrained. In this work, we integrated OS molecular composition, precursor-constrained positive matrix factorization (PMF) source apportionment, and OS-precursor correlation analysis to classify OS detected from PM2.5 samples according to their volatile organic compounds (VOCs) precursors collected from three different cities (Beijing, Taiyuan, and Changsha) in China. This new approach enables the accurate classification of OSs from molecular perspective. Compared with conventional classification methods, we found the mass fraction of Aliphatic OSs (including nitrooxy OSs; NOSs) increased by 22.0 %, 17.8 %, and 10.3 % in Beijing, Taiyuan, and Changsha, respectively, highlighting the underestimation of Aliphatic OSs in urban regions. The formation driving factors of Aliphatic OSs during the field campaign were further investigated. We found that elevated aerosol liquid water content promoted the formation of Aliphatic OSs only when aerosols transition from non-liquid state to liquid state. In addition, enhanced inorganic sulfate mass concentrations, and Ox (Ox= NO2+ O3) concentrations, as well as decreased aerosol pH commonly facilitated the formation of Aliphatic OSs. These results reveal a significant underestimation of OSs derived from anthropogenic emissions in wintertime, particularly Aliphatic OSs, highlighting the need for a deeper understanding of SOA formation and composition in urban environments.