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

Abstract

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 (O2–) 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 O2–(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 O2–(aq) with NO2(aq) and NO(aq) to form peroxynitrate (OONO2–) and peroxynitrite (OONO–), respectively, which are precursors to nitrite (NO2–). Overall, this points to an important role of O2–(aq) in aqueous aerosol chemistry, which is currently underappreciated.