Measurement report: Seasonal dynamics and driving factors of aqueous-phase photooxidants in atmospheric particles: Implications for wintertime SOA formation

Abstract. Aqueous-phase oxidation processes significantly promote secondary organic aerosol (SOA) formation, driven primarily by photooxidants including hydroxyl radical (·OH), singlet oxygen (¹O₂*), and organic triplet excited states (³C*). However, seasonal variations and driving factors of these oxidants in atmospheric aqueous phases remain poorly understood. In this study, we quantified the steady-state concentrations of ·OH, ¹O₂*, and ³C* in PM_2.5 extracts under simulated solar irradiation, and estimated their ranges in ambient aerosol water. The results show that [·OH] exhibited no significant seasonal variation, whereas [¹O₂*] and [³C*] displayed distinct seasonal variations of winter > autumn > summer. All three oxidants correlated strongly with water-soluble organic compounds (WSOC), especially biomass burning-derived WSOC. By extrapolating the fitted relationships between oxidant concentrations ([·OH], [¹O₂*], and [³C*]) and extract concentrations to ambient conditions, their ranges in ambient aerosol water were estimated as [·OH] = (1.0–5.4) × 10⁻¹⁴ M, [¹O₂*] = (2.3–61.9) × 10⁻¹¹ M, and [³C*] = (1.6–35.8) × 10⁻¹² M. The relative contributions of the three photooxidants to aqueous-phase oxidation of typical organic precursors revealed the dominant role of ³C*-mediated reactions in ambient aerosol water, even at low temperatures. This work thus resolves the winter SOA underestimation in current model studies by demonstrating the critical yet overlooked role of ³C*-mediated aqueous-phase oxidation.