title: Photochemical production and release of gaseous NO2 from nitrate-doped water ice.
abstract: 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 microM 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, SigmaN, increase less than linearly with [NO3-] or the duration of experiments, revealing the significant loss of photogenerated NO2. The actual SigmaN proportional, variant [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 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.
authors.affiliation: W. M. Keck Laboratories, California Institute of Technology, Pasadena, California 91125, USA.
authors.forename: C S
authors.initials: CS
authors.lastname: Boxe
authors.forename: A J
authors.initials: AJ
authors.lastname: Colussi
authors.forename: M R
authors.initials: MR
authors.lastname: Hoffmann
authors.forename: J G
authors.initials: JG
authors.lastname: Murphy
authors.forename: P J
authors.initials: PJ
authors.lastname: Wooldridge
authors.forename: T H
authors.initials: TH
authors.lastname: Bertram
authors.forename: R C
authors.initials: RC
authors.lastname: Cohen