Revealing the Underestimation of Anthropogenic Organosulfates in Atmospheric Aerosols in Urban Regions

Abstract. Organosulfates (OSs) are important component of organic aerosols, which serve as critical tracers of secondary organic aerosols (SOA). However, molecular composition, classification, and formation driving factors of OSs at different atmospheric conditions have not been fully constrained. In this work, we integrated OSs molecular composition, precursor-constrained positive matrix factorization (PMF) source apportionment, and OSs-precursor correlation analysis to classify OSs detected from PM_2.5 samples collected from three different cities (Beijing, Taiyuan, and Changsha). 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 and nitrooxy OSs (NOSs) increased by 22.0 %, 17.8 %, and 10.3 % in Beijing, Taiyuan, and Changsha, respectively, highlighting the underestimation of Aliphatic OSs and NOSs in urban regions. The formation driving factors of Aliphatic OSs and NOSs were further investigated. We found that elevated aerosol liquid water content promoted the formation of Aliphatic OSs and NOSs only when aerosols transition from non-liquid state to liquid state. In addition, enhanced inorganic sulfate mass concentrations, and O_x (O_x = NO₂ + O₃) concentrations, as well as decreased aerosol pH commonly facilitated the formation of Aliphatic OSs and NOSs. These results reveal the underestimation of OSs derived from anthropogenic emission, highlighting the potential indicative role of Aliphatic OSs and NOSs in urban SOA.