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.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry