Superoxide and Nitrous Acid Production from Nitrate Photolysis Is Enhanced by Dissolved Aliphatic Organic Matter

Nitrate anion (NO3) is ubiquitous in the environment, and its photochemistry produces nitrous acid (HONO), a major source of tropospheric hydroxyl radical (OH). Enhanced HONO (g) emissions have been observed from NO3(aq)- photolysis in field studies, although the underlying reasons for this enhancement are debated. Here, we show that the enhancement is in part caused by changes in secondary nitrate anion photochemistry due to dissolved aliphatic organic matter (DAOM). Increased yields of superoxide radical (O-2(-)) and HONO were observed when NO3- solutions (pH 6) were photolyzed in the presence of DAOM surrogates of varying solubility. In an additional experiment, nitrate titrated with additional DAOM showed a further simultaneous increase in the levels of O-2((aq))- and HONO(g) with decreased yields of gaseous nitric oxide (NO) and nitrogen dioxide (NO2). To the best of our knowledge, this is the first time that superoxide was directly observed as an intermediate in nitrate photolysis experiments, produced through DOAM oxidation by OH(aq). Herein, we suggest that enhanced HONO(g) emissions from NO3 (aq) photolysis result from the reaction of O-2((aq))- with NO(aq) and NO2(aq) to form perox-ynitrate (OONO2-) and peroxynitrite (OONO-), respectively, which are precursors to nitrite (NO2-). Overall, this points to an important role of O-2((aq)) in aqueous aerosol chemistry, which is currently underappreciated.

Automated summary

The enhancement of HONO(g) emissions from NO3(aq) photolysis is partially caused by changes in secondary nitrate anion photochemistry due to dissolved aliphatic organic matter (DAOM). Superoxide radical (O-2(-)) produced through DAOM oxidation by OH(aq) reacts with NO(aq) and NO2(aq) to form peroxynitrate (OONO2-) and peroxynitrite (OONO-), leading to an overall increase in nitrite (NO2-) precursors and enhanced HONO(g) emissions. This indicates an underappreciated role of O-2((aq)) in aqueous aerosol chemistry.