Photochemical production of gas phase NO_<i>x</i> from ice crystal NO₃⁻

Abstract

Recent measurements have demonstrated that sunlight irradiation of snow results in the release of significant amounts of gas phase NO x (NO+NO 2 ). We report here the results of a series of experiments designed to test the hypothesis that the observed NO x production is the result of nitrate photolysis. Snow produced from deionized water with and without the addition of nitrate was exposed to natural sunlight in an outdoor flow chamber. While NO x release from snow produced without added NO − 3 was minimal, the addition of 100 µ M NO − 3 resulted in the release of &gt;500 pptv NO x in a 9 standard liter per minute (sLpm) flow of synthetic air exposed to the snow for 10–20 s; the rate of release was highly correlated with solar radiation. Further addition of radical trap reagents resulted in greatly increased NO x production (to &gt;8 ppbv in a flow of 20 sLpm). In snow produced from deionized water plus sodium nitrate, production of NO 2 dominated that of NO. The reverse was true in the presence of radical trap reagents; this suggests sensitivity of the NO x release mechanism to pH, as a basic compound was added, or to the presence of free radical scavengers. A mechanism for NO x release from NO − 3 photolysis consistent with these observations is presented. These results support previous suggestions that surface NO x release may have a significant impact on boundary layer photochemistry in snow‐covered regions and that nitrate photolysis on cirrus cloud particles may result in the release of gas phase NO x . A potential for pH‐dependent impacts on ice core records of oxidants and oxidized compounds is also suggested.