Fast generation of peroxides via atmospheric particulate photosensitization

Peroxide species, including hydrogen peroxide (H₂O₂) and organic peroxides, are key oxidants in shaping the atmospheric oxidative capacity. However, their formation pathways remain elusive under high-NO_x conditions, where gas-phase mechanisms are suppressed. Herein, we report an "in-particle" peroxide formation pathway driven by photosensitization reactions in biomass burning organic aerosol. This mechanism remains highly efficient even in polluted, high-NO_x environments, leading to orders-of-magnitude increase in particulate H₂O₂ concentrations in the presence of sunlight than that expected from gas-phase partitioning. These findings suggest that intensifying wildfires in our warming world, beyond their primary emissions, may substantially alter the atmospheric oxidation chemistry and exacerbate air quality degradation.