Photochemical production of gas phase NOx from ice crystal NO-3

Recent measurements have demonstrated that sunlight irradiation of snow results in the release of significant amounts of gas phase NOx (NO + NO2). We report here the results of a series of experiments designed to test the hypothesis that the observed NOx 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 NOx release from snow produced without added NO3- was minimal, the addition of 100 muM NO3- resulted in the release of >500 pptv NOx 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 NOx production (to >8 ppbv in a flow of 20 sLpm). In snow produced from deionized water plus sodium nitrate, production of NO2 dominated that of NO. The reverse was true in the presence of radical trap reagents; this suggests sensitivity of the NOx release mechanism to pH, as a basic compound was added, or to the presence of free radical scavengers. A mechanism far NOx release from NO3- photolysis consistent with these observations is presented. These results support previous suggestions that surface NOx 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 NOx. A potential for pH-dependent impacts on ice core records of oxidants and oxidized compounds is also suggested.