Photochemical Production and Release of Gaseous NO2 from Nitrate-Doped Water Ice

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2005-09, Vol.109 (38), p.8520-8525
• Main Authors: Boxe, C. S, Colussi, A. J, Hoffmann, M. R, Murphy, J. G, Wooldridge, P. J, Bertram, T. H, Cohen, R. C
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp0518815

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 °C. Emissions increase nonmonotonically with temperature, displaying transitions suggestive of underlying metamorphic transformations. Thus, NO2 emissions surge at ca. −8 °C in frozen [NO3 -] > 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 NO2 released in individual thermograms, ΣN, increase less than linearly with [NO3 -] or the duration of experiments, revealing the significant loss of photogenerated NO2. The actual ΣN ∝ [NO3 -]1/2 dependence (at constant H) is consistent with NO2 hydrolysis:  2NO2 + H2O → NO3 - + NO2 - + 2H+, overtaking NO2 desorption, even below the eutectic point (−18 °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.