Molecular insight into aqueous-phase photolysis and photooxidation of water-soluble organic matter emitted from biomass burning and coal combustion

Abstract. Biomass and coal combustion represent substantial contributors to atmospheric water-soluble organic matter (WSOM). It experienced intense photochemical oxidation once entered atmospheric environment, but the resulting changes in WSOM are largely unclear. This study examines the changes in the optical properties, fluorophores, and molecular composition of WSOM derived from the combustion of biomass (specifically rice straw, RS) and coal (from Yulin, YL) during aqueous photolysis and hydroxyl radical (·OH) photooxidation. The results indicate that photochemical aging induces distinct changes in the light-absorbing properties of RS and YL WSOM, characterized by pronounced photobleaching in RS WSOM and photoenhancement in YL WSOM. Additionally, more pronounced alterations were observed during ·OH photooxidation than direct photolysis, for both RS and YL WSOM. Furthermore, a greater proportion of molecules in both RS (61.6 %) and YL (65.0 %) WSOM were degraded during ·OH photooxidation compared to photolysis (14.9 % and 23.1 %, respectively), resulting in products with larger molecular weight and higher oxidation levels, including tannin-like substances and newly formed black carbon-like compounds, whereas the products of photolysis were characterized by relative minor alteration. These findings provide new insights into the photochemical evolution of combustion-derived WSOM and help to predict its effects in environmental and climate changes.