Photochemical Interactions of Nitrate Photolysis and Photosensitization of BBOA: Enhanced HONO Production and the Formation of Nitrophenols

Renoxification of nitrate to reactive nitrogen species (RNS) can sustain atmospheric oxidant production. Because nitrate and light-absorbing brown carbon can coexist in biomass burning (BB) plumes, their photochemical interaction may alter RNS cycling. Furthermore, nitration of phenols in biomass burning organic aerosol (BBOA) can form nitrophenols, potentially increasing the warming effect of aged BBOA. Yet the mechanisms of renoxification and nitration in irradiated nitrate-BBOA mixtures remain poorly understood. We experimentally investigate these coupled processes over a wide range of fir-wood BBOA concentrations during aqueous nitrate photolysis (10 mM) under acidic conditions using H2SO4. At low BBOA concentration (TOCBBOA/NO3– ≤ 0.0036 mg L–1/mg L–1), the steady-state production rate of HONO (Pss_HONO) increased as BBOA concentration increased, reaching a maximum enhancement factor of 1.8 driven primarily by triplet-mediated photosensitized reduction of NO2 to HONO. At higher BBOA concentration (TOCBBOA/NO3– > 0.0036 mg L–1/mg L–1), Pss_HONO progressively decreased and was nearly fully suppressed at TOCBBOA/NO3– = 0.7 mg L–1/mg L–1. This inhibition is attributed to reactions of aqueous HNO2/NO2 with phenols, accompanied by formation of nitrophenols. These findings point to a potentially overlooked daytime HONO source and an atmospheric source of nitrophenols, a key class of BrC chromophores, via photochemical interactions between nitrate photolysis and photosensitization of BBOA.