Investigating the photosensitized chemistry of proxies for tropospheric brown carbon

Brown carbon (BrC) profoundly influences secondary organic aerosol (SOA) formation via photosensitized reactions in atmospheric condensed phases, yet the intrinsic oxidative power of its excited triplet state remains mechanistically unresolved. We reveal that the BrC proxy 1,4-naphthoquinone (NQ) undergoes dual photochemical pathways in deoxygenated aqueous systems, simultaneous self-photoconversion and substrate oxidation, enabled by two distinct, long-lived triplet states (³NQ* and ³OH-NQ*) with ³(n, π*) configurations generated under 355 nm irradiation. Crucially, ³NQ* acts as an endogenous oxidant, driving efficient hydrogen abstraction from key organic classes without requiring external oxidants. Oxidation kinetics exhibit a striking hierarchy governed by bond dissociation energies and one-electron oxidation potentials: alkanes (10⁴ M^-1s^-1) < oxygenated alkanes (10⁵ M^-1s^-1) < phenols (10⁶ M^-1s^-1). Radical intermediates (alkyl, alkoxyl, phenoxy) dehydrogenate into stable products (e.g., benzoquinone from phenol), while the self-photoconversion product OH-NQ regenerates a secondary triplet (³OH-NQ*), amplifying oxidative cascades. This establishes ³BrC* as the dominant sink for phenols in BrC-rich haze, particularly under acidic, O₂-limited conditions. Our findings provide kinetic and mechanistic benchmarks for integrating triplet-state chemistry into atmospheric models, advancing predictions of SOA formation in regions plagued by severe particulate pollution.