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

Nitrogen dioxide ((NO2)-N-center dot) is produced in sunlit natural surface waters by the direct photolysis of nitrate, together with (OH)-O-center dot, and upon the oxidation of nitrite by (OH)-O-center dot itself. (NO2)-N-center dot is mainly scavenged by dissolved organic matter, and here, it is shown that (NO2)-N-center dot 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)-N-center dot hydrolysis, but with a very low concentration, i.e., several orders of magnitude below (NO2)-N-center dot, and even below (OH)-O-center dot. Therefore, at most, N2O4 would only be involved in the transformation (nitration/nitrosation) of electron-poor compounds, which would not react with (NO2)-N-center dot. Although it is known that nitrite oxidation by CO3 center dot- in high-alkalinity surface waters gives a minor-to-negligible contribution to (NO2)-N-center dot formation, it is shown here that NO2- oxidation by Br-2(center dot)- can be a significant source of (NO2)-N-center dot in saline waters (saltwater, brackish waters, seawater, and brines), which offsets the scavenging of (OH)-O-center dot by bromide. As an example, the anti-oxidant tripeptide glutathione undergoes nitrosation by (NO2)-N-center dot preferentially in saltwater, thanks to the inhibition of the degradation of glutathione itself by (OH)-O-center dot, which is scavenged by bromide in saltwater. The enhancement of (NO2)-N-center dot 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 Rhone river delta (S. France), and that the laboratory irradiation of phenol-spiked seawater yields nitrophenols in a significant amount.