Photochemical production and release of gaseous NO ₂ from nitrate-doped water ice

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 NO ₂ 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.