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 PM2.5, 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 (ΦRI) 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*. ΦRI 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.