Superoxide Formation from Aqueous Reactions of Biogenic Secondary Organic Aerosols

Reactive oxygen species (ROS) play a central role in aqueous-phase processing and health effects of atmospheric aerosols. Although hydroxyl radical (^•OH) and hydrogen peroxide (H₂O₂) are regarded as major oxidants associated with secondary organic aerosols (SOA), the kinetics and reaction mechanisms of superoxide (O₂^•-) formation are rarely quantified and poorly understood. Here, we demonstrate a dominant formation of O₂^•- with molar yields of 0.01-0.03% from aqueous reactions of biogenic SOA generated by ^•OH photooxidation of isoprene, β-pinene, α-terpineol, and d-limonene. The temporal evolution of ^•OH and O₂^•- formation is elucidated by kinetic modeling with a cascade of aqueous reactions including the decomposition of organic hydroperoxides, ^•OH oxidation of primary or secondary alcohols, and unimolecular decomposition of α-hydroxyperoxyl radicals. Relative yields of various types of ROS reflect a relative abundance of organic hydroperoxides and alcohols contained in SOA. These findings and mechanistic understanding have important implications on the atmospheric fate of SOA and particle-phase reactions of highly oxygenated organic molecules as well as oxidative stress upon respiratory deposition.