Efficient Production of Reactive Oxidants by Atmospheric Bacterial-Derived Organic Matter in the Aqueous Phase

Hydroxyl radicals (•OH), singlet oxygen (¹O₂*), and organic triplet excited states (³C*) play key roles as oxidants ("reactive intermediates (RIs)") in forming and oxidizing aqueous organic aerosols. Bioaerosols are ubiquitous in the atmosphere, but little is known about their photochemical behavior and contributions to atmospheric photochemistry. We investigated the photochemical behavior of aqueous-phase cellular organic matter (COM) and extracellular polymeric substances (EPS) from cultured bacteria isolated from atmospheric PM_2.5, focusing on their photochemical production of ³C*, ¹O₂*, and •OH. The molecular size and aromaticity of chromophores and fluorophores in COM and EPS increased with molecular weight (MW). Apparent quantum yields (Φ_RI) of up to 10% and 5% were measured for ¹O₂* and ³C*, respectively, which are in the upper range of previously reported values. This indicated that COM and EPS contain photosensitizers that are highly efficient at producing ¹O₂* and ³C*. Φ_RI and concentrations ([RI]_ss) decreased with MW due to higher-MW molecules engaging in charge-transfer interactions that disrupt photochemical processes and oxidant production. Machine learning models were used to understand and predict oxidant production based on measurable optical and chemical properties of COM and EPS. This study provides new insights into the roles that bioaerosols can play in atmospheric aqueous photochemistry.