Biomass burning (BB) is the second largest global source of organic aerosol (OA), with its impact on climate and air quality expected to rise with global warming. Aircraft measurements of BB plumes show a rapid increase in the OA oxidation state, which could not be reproduced in simulation chambers based on gas-phase oxidation. Here, we investigate isolation key aging processes of BB emissions in controlled chamber experiments, including the effects of ultraviolet-A irradiation, OH-driven secondary OA formation, heterogeneous OH oxidation or ozonolysis, and evaporation. We find that ultraviolet-A irradiation can drive rapid intraparticle OA aging within hours, proceeding an order of magnitude faster than gas-phase oxidation. Other mechanisms were even slower in producing oxygenated OA (OOA). Based on our experiments, we estimate that this process proceeds with a quantum yield of ∼0.4%, meaning that four in every thousand photons absorbed by organic chromophores in BBOA generate radicals in the particle phase that rapidly oxidize the particulate organics. We further analyzed aircraft measurements of wildfire and prescribed burn plumes, showing that photoinduced processes can account for much of the rapid OOA formation. We estimate that, in BB-influenced regions, over half of OOA may originate from photo-oxidized primary OA within the first day. Our findings challenge the prevailing paradigm that gas-phase oxidation dominates BBOA aging and highlight a previously underappreciated, but dominant role of intraparticle photochemistry.
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