Photochemical production of gas phase NOx from ice crystal NO3-

Recent measurements have demonstrated that sunlight irradiation of snow res ults 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 hyp othesis that the observed NOx production is the result of nitrate photolysi s. Snow produced from deionized water with and without the addition of nitr ate was exposed to natural sunlight in an outdoor flow chamber. While NOx r elease from snow produced without added NO3- was minimal, the addition of 1 00 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; th e rate of release was highly correlated with solar radiation. Further addit ion 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 plu s sodium nitrate, production of NO2 dominated that of NO. The reverse was t rue 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 p resence of free radical scavengers. A mechanism far NOx release from NO3- p hotolysis consistent with these observations is presented. These results su pport previous suggestions that surface NOx release may have a significant impact on boundary layer photochemistry in snow-covered regions and that ni trate 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 oxi dants and oxidized compounds is also suggested.