Photochemical Production and Release of Gaseous NO_2 from Nitrate-Doped Water Ice

Abstract

Temperature-programmed NO_2 emissions from frozen aqueous NaNO_3 solutions irradiated at 313 nm were monitored as function of nitrate concentration and heating rate, H, above −30 °C. Emissions increase nonmonotonically with temperature, displaying transitions suggestive of underlying metamorphic transformations. Thus, NO_2 emissions surge at ca. −8 °C in frozen [NO_3^-] > 200 μM samples warmed at H = 0.70 °C min^(-1) under continuous irradiation, and also in the dark from samples that had been photolyzed at −30 °C. The amounts of NO_2 released in individual thermograms, Σ_N, increase less than linearly with [NO_3^-] or the duration of experiments, revealing the significant loss of photogenerated NO_2. The actual Σ_N ∝ [NO_3^-]^(1/2) dependence (at constant H) is consistent with NO_2 hydrolysis:  2NO_2 + H_2O → NO_3^- + NO_2^- + 2H^+, overtaking NO_2 desorption, even below the eutectic point (−18 °C for aqueous NaNO_3). The increasingly larger NO2 losses detected in longer experiments (at constant [NO_3^-]) are ascribed to secondary photolysis of trapped NO_2. The relevance of present results to the interpretation of polar NO_2 measurements is briefly analyzed.

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