Multiscale Ice Fluidity in NOx Photodesorption from Frozen Nitrate Solutions

The temperature programmed desorption of nitric oxide, NO, and nitrogen dioxide, NO2, during the 302 nm photolysis of KNO3-doped, spray-frozen ice layers was investigated using two-photon laser-induced NOx fluorescence detection in the range – 35 ≤ T/oC ≤ 0. Upon applying steady illumination, and a 0.67 °C min heating ramp to frozen KNO3 solutions, NO2 begins to evolve at increasing rates, while NO emissions plateau soon after until, at ∼ – 8 o C, both species surge abruptly. Although the primary photoproduct NO2 avoids geminate recombination by escaping from a permeable molecular cage throughout, NO2(g) levels are controlled by desorption from the outermost ice layers rather than by NO3 photolysis rates. The NOx accumulated in the deeper layers bursts when the solid undergoes a sintering transition following the onset of surface melting at – 10 o C. Since elementary photochemical events occur in a communal fluid phase of molecular dimensions at temperatures far below the KNO3/H2O eutectic (Teutectic = – 2.88 °C), we infer that doped polycrystalline ice contains operationally distinguishable fluid phases of low dimensionality over various length scales and