Abstract:

Photochemical aging alters the physical and chemical properties of atmospheric aerosols, influencing their climate and health impacts, yet the underlying mechanisms remain poorly understood. A central mystery is the role of short-lived free radicals that escape most direct detection methods. This work provides new insights into radical chemistry during photochemical aging of simplified biomass burning organic aerosol (BBOA) systems. Radicals were detected in situ during irradiation using electron paramagnetic resonance (EPR) spectroscopy with a spin-trapping compound. Timeresolved EPR data were incorporated into kinetic models to identify reactions leading to reactive oxygen species (ROS) formation, which drive atmospheric oxidation and influence particle toxicity. We also developed a method to identify organic radicals adducts using liquid chromatography-electrospray ionization-mass spectrometry (LCESI-MS). Their radical nature produced unusual oxidized [M]+ and reduced [M+2H]+ ions that complicated interpretation but using simplified systems we established a framework for using collision induced fragmentation mass spectra to differentiate between adduct types. These methods were applied to examine radical formation during irradiation of solutions containing benzoquinone and levoglucosan, key BBOA tracer molecules. EPR analysis revealed formation of hydroxyl radicals (•OH), known products of the reaction between triplet-state benzoquinone and water, accompanied by organic carbon- and oxygen-centered radicals that became more prominent with increasing levoglucosan content. LC-ESI-MS confirmed semiquinone formation and detected additional radicals derived from benzoquinone and levoglucosan oxidation. We also observed the first experimental evidence of hydrogen radical (H•) formation, likely from semiquinone decomposition. Kinetic modeling reproduced the time evolution of BMPO adducts observed by EPR and predicted the evolution of species in atmospherically relevant systems. These findings indicate that photoirradiation of aerosols containing photosensitizers induces radical formation and secondary radical chemistry that drives BBOA aging in the atmosphere. We next investigated radicals formed during irradiation of secondary organic aerosols from biomass burning precursors (BBSOA). Through a combination of laboratory measurements and kinetic modeling we determined that superoxide generation is driven by photosensitization reactions in aromatic SOA, while the dominant source of superoxide in biogenic SOA is photo-induced decomposition of carbonyls with minor contributions from peroxide decomposition. This was supported by measurements of the radical formation by model compounds including carbonyls and organic peroxides. The superoxide burst serves 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 work elucidated mechanisms of radical formation during photochemical aging, advancing understanding of oxidant budgets and health impacts of aged atmospheric systems.