Technical Note: A Technique to Convert NO₂ to NO₂^– with S(IV) and its Application to Measuring Nitrate Photolysis

Abstract. Nitrate photolysis is a potentially significant mechanism for “renoxifying” the atmosphere, i.e., converting nitrate into nitrogen oxides (nitrogen dioxide (NO₂) and nitric oxide (NO)) and nitrous acid (HONO). Nitrate photolysis in the environment occurs through two channels, which produce: (1) NO₂ and hydroxyl radical (^•OH) and (2) nitrite (NO₂^–) and an oxygen atom (O(³P)). Although the aqueous quantum yields and photolysis rate constants of both channels have been established, field observations suggest that nitrate photolysis is enhanced in the environment. Laboratory studies investigating these enhancements typically only measure one of the two photo-channels, since measuring both channels generally requires separate analytical methods and instrumentation. However, measuring only one channel makes it difficult to assess whether secondary chemistry is enhancing one channel at the expense of the other, or if there is an overall enhancement of nitrate photochemistry. Here, we show that the addition of S(IV), i.e., bisulfite and sulfite, can convert NO₂ to NO₂^–, allowing measurement of both nitrate photolysis channels with the same equipment. By varying the concentration of S(IV) and exploring method parameters, we determine the experimental conditions that quantitatively convert NO₂ and accurately quantify the resulting NO₂^–. We then apply the method to a test case, showing how an ^•OH scavenger in solution prevents the oxidation of NO₂^– to NO₂ but does not enhance the overall photolysis efficiency of nitrate.