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 less than or equal to T/degreesC less than or equal to 0. Upon applying steady illumination and a 0.67 degreesC min(-1) heating ramp, frozen KNO3 solutions begin to evolve NO2 at increasing rates, while NO emissions plateau soon after until, at similar to -8 degrees 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 degreesC. Since elementary photochemical events occur in a communal fluid phase of molecular dimensions at temperatures far below the KNO3/H2O eutectic (T-eutectic = - 2.88 degreesC), we infer that doped polycrystalline ice contains operationally distinguishable fluid phases of low dimensionality over various length scales and temperature ranges.