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)-O-center dot) and hydrogen peroxide (H2O2) are regarded as major oxidants associated with secondary organic aerosols (SOA), the kinetics and reaction mechanisms of superoxide (O-2(center dot-)) formation are rarely quantified and poorly understood. Here, we demonstrate a dominant formation of O-2(center dot-) with molar yields of 0.01-0.03% from aqueous reactions of biogenic SOA generated by (OH)-O-center dot photooxidation of isoprene, beta-pinene, alpha-terpineol, and D-limonene. The temporal evolution of (OH)-O-center dot and O-2(center dot-) formation is elucidated by kinetic modeling with a cascade of aqueous reactions including the decomposition of organic hydroperoxides, (OH)-O-center dot oxidation of primary or secondary alcohols, and unimolecular decomposition of alpha-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.