Environmental context. Nitrite is an important nutrient in surface waters, a key intermediate in the interconversion of nitrate into ammonium, and a considerable photochemical source of reactive species such as the hydroxyl radical. We have found that scavengers of hydroxyl radicals such as dissolved organic matter, which are usually supposed to inhibit the photodegradation of dissolved compounds, are able on the contrary to enhance the phototransformation of nitrite. The three weeks’ lifetime of nitrite in the surface layer of lakes, derived from the results of the present work, would make photochemistry an important issue in determining the concentration of nitrite in lake water.

Abstract. Here we studied the degradation rate of nitrite (NO₂^–), added to lake water at sub-micromolar levels, upon ultraviolet (UV) irradiation. NO₂^– photodegradation was considerably faster in lake water compared with ultra-pure water. A key issue was the presence in lake water of hydroxyl radical (^•OH) scavengers that inhibited the reaction between NO₂^– and ^•OH. Such a reaction, while causing additional NO₂^– transformation, produced nitrogen dioxide (NO₂^•) that was subsequently involved into the regeneration of NO₂^– by dimerisation or the reaction with nitric oxide (NO^•). The scavenging of ^•OH by compounds different from NO₂^– (mainly dissolved organic matter, DOM) prevented the regeneration reactions from taking place, and enhanced the phototransformation of NO₂^–. Model calculations for the direct photolysis of NO₂^–, applied to the lake water samples, yielded a NO₂^– half-life time of around three weeks in the mixing layer of the lakes because of photodegradation. Therefore, we conclude that photodegradation is a potentially important process to control the concentration of NO₂^– in shallow lakes, or in deeper ones under stratification conditions.