A prominent source of hydroxyl radicals (•OH), nitrous acid (HONO) plays a key role in tropospheric chemistry. Apart from direct emission, HONO (or its conjugate base nitrite, NO2 –) can be formed secondarily in the atmosphere. Yet, how secondary HONO forms requires elucidation, especially for heterogeneous processes involving numerous organic compounds in atmospheric aerosols. We investigated nitrite production from aqueous photolysis of nitrate for a range of conditions (pH, organic compound, nitrate concentration, and cation). Upon adding small oxygenates such as ethanol, n-butanol, or formate as •OH scavengers, the average intrinsic quantum yield of nitrite [Φ­(NO2 –)] was 0.75 ± 0.15%. With near-UV-light-absorbing vanillic acid (VA), however, the effective Φ­(NO2 –) was strongly pH-dependent, reaching 8.0 ± 2.1% at a pH of 8 and 1.5 ± 0.39% at a more atmospherically relevant pH of 5. Our results suggest that brown carbon (BrC) may greatly enhance the nitrite production from the aqueous nitrate photolysis through photosensitizing reactions, where the triplet excited state of BrC may generate solvated electrons, which reduce nitrate to NO2 for further conversion to nitrite. This photosensitization process by BrC chromophores during nitrate photolysis under mildly acidic conditions may partly explain the missing HONO in urban environments.

Brown carbon (BrC) absorbs radiation in the near-UV and visible ranges, affecting atmospheric photochemistry and radiative forcing. Our understanding on the photochemical transformation of BrC is still limited, especially when mixed with the abundant and photochemically labile inorganic salt, nitrate. Herein, we investigate the photochemical reactions of four BrC chromophores, including two methoxyphenols and two nitrophenols. Experiments were conducted in the absence and presence of different concentrations of H2O2 and nitrate with lights of 254 and 313 nm. The results show that the pseudo-first-order decay rate constants (k) of these four BrC compounds at 313 nm illumination were approximately 10 times lower than those at 254 nm, demonstrating longer lifetimes of these BrC chromophores under tropospherically relevant irradiation. Photo-enhancement in the visible range was observed in most experiments, with those under 313 nm illumination lasting longer, indicating the prolonged effects of nascent and transformed BrC chromophores on radiative forcing. Methoxyphenols had higher averaged k values than nitrophenols during direct photolysis with 254 or 313 nm lights, but the k values for nitrophenols under high-nitrate (or high-H2O2) conditions approached those of methoxyphenols. The photo-enhancement in the visible range for methoxyphenols in the presence of nitrate was substantially contributed by nitro products, while that for nitrophenols was mainly contributed by hydroxylated and/or dimerized products. Our results reveal the similarity and difference between the photolysis of methoxyphenols and nitrophenols, which may help better understand the aging of different types of BrC for better model representation of their effects on radiative forcing.