Superoxide formation upon photochemical aging of secondary organic aerosols derived from biomass burning precursors.

Photochemical oxidation in the aqueous phase is critical for chemical evolution of secondary organic aerosols (SOA), but it represents one of the most uncertain processes in aerosols and clouds. Here, we show that photoirradiation of SOA derived from biomass burning precursors leads to substantial formation of superoxide. The combination of laboratory measurements and kinetic modeling reveals that superoxide generation is driven by photosensitization reactions in aromatic SOA, while the dominant source of superoxide in biogenic SOA is photoinduced decomposition of carbonyls with minor contributions from peroxide decomposition. These processes serve as a substantial source of reactive oxygen species (ROS) including hydrogen peroxide and hydroxyl radical in cloud droplets and deliquesced particles, competing with traditional sources such as uptake from the gas phase. This previously unrecognized source of ROS will enhance the impact of aqueous-phase processing on composition and properties of aerosols and clouds.