Elucidating the role of brown carbon in HONO and NOx production from renoxification of nitrate-containing aerosol

Oxides of nitrogen (NO_x = NO + NO₂) and nitrous acid (HONO) play crucial roles in forming tropospheric ozone (O₃), hydroxyl radicals (·OH), and secondary aerosols. The photochemical reactions of nitrate aerosol are of significant atmospheric interest as they produce HONO and NO_x, a process termed renoxification. Light-absorbing organic species, particularly chromophoric Brown Carbon (BrC) predominantly derived from biomass burning, are suggested to be key players in renoxification, though the mechanism remains controversial. Here, we investigate BrC-associated renoxification upon irradiation of films containing BrC extracts from authentic biomass-burning aerosols and BrC model compounds using the coated wall flow tube (CWFT) technique. We mimic real-world aerosol conditions by adjusting the pH, nitrate concentration, and relative humidity of the CWFT films, ensuring atmospheric relevance. We show that the renoxification rate is enhanced in the presence of BrC. This is likely due to the photosensitizing effect of BrC, which enhances the reduction of nitrate, rather than the previously proposed surface-enhanced direct photolysis of adsorbed nitrate. Given the efficient use of the solar spectrum from UV to visible light by this photosensitized mechanism and the widespread coexistence of nitrates and BrC in various environmental systems, we suggest BrC-photosensitized renoxification could be a substantial source of HONO and NO_x. This process may significantly influence the trends and distributions of tropospheric O₃, ·OH and secondary aerosols, marking an important, yet largely unexplored, area in atmospheric chemistry.