Kinetics of NO and NO ₂ evolution from illuminated frozen nitrate solutions

The release of NO and NO ₂ from frozen aqueous NaNO ₃ irradiated at 313 nm was studied using time-resolved spectroscopic techniques. The kinetic behavior of NO and NO ₂ signals during on-and-off illumination cycles confirms that NO ₂ is a primary photoproduct evolving from the outermost ice layers and reveals that NO is a secondary species generated deeper in the ice, whence it eventually emerges due to its inertness and larger diffusivity. NO is shown to be more weakly held than NO ₂ by ice in thermal desorption experiments on preirradiated samples. The partial control of gaseous emissions by mass transfer, and hence by the morphology and metamorphisms of polycrystalline ice, is established by (1) the nonmonotonic temperature dependence of NO and NO ₂ signals upon stepwise warming under continuous illumination, (2) the fact that the NO, NO ₂ or NO _x (NO _x ≡ NO + NO ₂) amounts released in bright thermograms performed under various heating ramps fail to scale with photon dose, due to irreversible losses in the adsorbed state. Because present NO/NO ₂ ratios are up to 10-fold smaller than those determined over sunlit snowpacks, we infer that the immediate precursors to NO mostly absorb at λ > λ _max (NO ₃ ^-) - 302 nm.