Photochemical production and release of gaseous NO2 from nitrate-doped water ice

Temperature-programmed NO2 emissions from frozen aqueous NaNO3 solutions irradiated at 313 nm were monitored as function of nitrate concentration and heating rate, H, above -30 degrees C. Emissions increase nonmonotonically with temperature, displaying transitions suggestive of underlying metamorphic transformations. Thus, NO2 emissions surge at ca. -8 degrees C in frozen [NO3-] > 200 mu M samples warmed at H = 0.70 degrees C min(-1) under continuous irradiation, and also in the dark from samples that had been photolyzed at -30 degrees C. The amounts of NO2 released in individual thermograms, Sigma(N), increase less than linearly with [NO3-] or the duration of experiments, revealing the significant loss of photogenerated NO2. The actual Sigma(N) proportional to [NO3-](1/2) dependence (at constant H) is consistent with NO2 hydrolysis: 2NO(2) + (HO)-O-2 -> NO3- + NO2- + 2H(+), overtaking NO2 desorption, even below the eutectic point (-18 degrees C for aqueous NaNO3). The increasingly larger NO2 losses detected in longer experiments (at constant [NO3-]) are ascribed to secondary photolysis of trapped NO2. The relevance of present results to the interpretation of polar NO2 measurements is briefly analyzed.