Reactive Oxygen Species Production from Secondary Organic Aerosols: The Importance of Singlet Oxygen

Organic aerosols are subjected to atmospheric processes driven by sunlight, including the production of reactive oxygen species (ROS) capable of transforming their physicochemical properties. In this study, secondary organic aerosols (SOA) generated from aromatic precursors were found to sensitize singlet oxygen (¹O₂), an arguably underappreciated atmospheric ROS. Specifically, we quantified ¹O₂, OH radical, and H₂O₂ quantum yields within photoirradiated solutions of laboratory-generated SOA from toluene, biphenyl, naphthalene, and 1,8-dimethylnaphthalene. At 5 mg_C L^-1 of SOA extracts, the average steady-state concentrations of ¹O₂ and of OH radicals in irradiated solutions were 3 ± 1 × 10^-14 M and 3.6 ± 0.9 × 10^-17 M, respectively. Furthermore, ROS quantum yields of irradiated ambient PM₁₀ extracts were comparable to those from laboratory-generated SOA, suggesting a similarity in ROS production from both types of samples. Finally, by using our measured ROS concentrations, we predict that certain organic compounds found in aerosols, such as amino acids, organo-nitrogen compounds, and phenolic compounds have shortened lifetimes by more than a factor of 2 when ¹O₂ is considered as an additional sink. Overall, our findings highlight the importance of SOA as a source of ¹O₂ and its potential as a competitive ROS species in photooxidation processes.