A Model Assessment of the Occurrence and Reactivity of the Nitrating/Nitrosating Agent Nitrogen Dioxide (NO) in Sunlit Natural Waters.

Nitrogen dioxide (NO) is produced in sunlit natural surface waters by the direct photolysis of nitrate, together with OH, and upon the oxidation of nitrite by OH itself. NO is mainly scavenged by dissolved organic matter, and here, it is shown that NO levels in sunlit surface waters are enhanced by high concentrations of nitrate and nitrite, and depressed by high values of the dissolved organic carbon. The dimer of nitrogen dioxide (NO) is also formed in the pathway of NO hydrolysis, but with a very low concentration, i.e., several orders of magnitude below NO, and even below OH. Therefore, at most, NO would only be involved in the transformation (nitration/nitrosation) of electron-poor compounds, which would not react with NO. Although it is known that nitrite oxidation by CO in high-alkalinity surface waters gives a minor-to-negligible contribution to NO formation, it is shown here that NO oxidation by Br can be a significant source of NO in saline waters (saltwater, brackish waters, seawater, and brines), which offsets the scavenging of OH by bromide. As an example, the anti-oxidant tripeptide glutathione undergoes nitrosation by NO preferentially in saltwater, thanks to the inhibition of the degradation of glutathione itself by OH, which is scavenged by bromide in saltwater. The enhancement of NO reactions in saltwater could explain the literature findings, that several phenolic nitroderivatives are formed in shallow (i.e., thoroughly sunlit) and brackish lagoons in the Rhône river delta (S. France), and that the laboratory irradiation of phenol-spiked seawater yields nitrophenols in a significant amount.