Photochemical Processing of Dissolved Organic Matter in Fog Water: Oxidation and Functionalization Pathways Driving Organic Aerosol Evolution

Abstract. Photochemical reactions of dissolved organic matter (DOM) in atmospheric waters can alter the composition and properties organic aerosols (OA), with implications for climate and air quality. In this study, we investigated the aqueous-phase transformation of fog DOM under simulated sunlight using online aerosol mass spectrometry (AMS), offline Orbitrap mass spectrometry with electrospray ionization, UV-vis spectroscopy, and aerosol volatility measurements. Irradiation increased the mass concentration of DOM-derived OA (DOM_OA), defined as the low-volatility fraction of DOM that forms OA upon water evaporation. This increase was primarily driven by functionalization reactions that added oxygen- and nitrogen-containing groups, as indicated by a stable C mass, rising oxygen-to-carbon (O/C) and nitrogen-to-carbon (N/C) ratios, and enhanced signals of heteroatom-containing compounds over the course of irradiation. Despite evidence of fragmentation, spectral features associated with oligomerization, such as phenolic dimers, were also observed. To characterize chemical aging of fog DOM, we applied positive matrix factorization to the AMS spectra and identified three distinct factors representing progressive stages of aqueous-phase aging: initial DOM_OA, more oxidized intermediates, and highly oxidized products, characterized by progressively increasing O/C and N/C ratios. These findings demonstrate that sunlight-induced aqueous-phase oxidation and functionalization of fog DOM drive the formation and aging of secondary OA, altering its composition, volatility, and light-absorbing properties with potential atmospheric consequences.