A Model Assessment of the Occurrence and Reactivity of the Nitrating/Nitrosating Agent Nitrogen Dioxide (^•NO₂) in Sunlit Natural Waters

Nitrogen dioxide (^•NO₂) 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. ^•NO₂ is mainly scavenged by dissolved organic matter, and here, it is shown that ^•NO₂ 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 (N₂O₄) is also formed in the pathway of ^•NO₂ hydrolysis, but with a very low concentration, i.e., several orders of magnitude below ^•NO₂, and even below ^•OH. Therefore, at most, N₂O₄ would only be involved in the transformation (nitration/nitrosation) of electron-poor compounds, which would not react with ^•NO₂. Although it is known that nitrite oxidation by CO₃^•− in high-alkalinity surface waters gives a minor-to-negligible contribution to ^•NO₂ formation, it is shown here that NO₂⁻ oxidation by Br₂^•− can be a significant source of ^•NO₂ 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 ^•NO₂ 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 ^•NO₂ 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.