Multiscale Ice Fluidity in NO_x Photodesorption from Frozen Nitrate Solutions

Abstract

The temperature-programmed desorption of nitric oxide, NO, and nitrogen dioxide, NO 2 , during the 302 nm photolysis of KNO 3 -doped, spray-frozen ice layers was investigated using two-photon laser-induced NO x fluorescence detection in the range -35 e T/°C e 0. Upon applying steady illumination and a 0.67 °C min -1 heating ramp, frozen KNO 3 solutions begin to evolve NO 2 at increasing rates, while NO emissions plateau soon after until, at ∼ -8 °C, both species surge abruptly. Although the primary photoproduct NO 2 avoids geminate recombination by escaping from a permeable molecular cage throughout, NO 2 (g) levels are controlled by desorption from the outermost ice layers rather than by NO 3photolysis rates. The NO x accumulated in the deeper layers bursts when the solid undergoes a sintering transition following the onset of surface melting at -10 °C. Since elementary photochemical events occur in a communal fluid phase of molecular dimensions at temperatures far below the KNO 3 /H 2 O eutectic (T eutectic ) -2.88 °C), we infer that doped polycrystalline ice contains operationally distinguishable fluid phases of low dimensionality over various length scales and temperature ranges.

A mechanism for the photochemical transformation of nitrate in snow

“…Similarly, previous studies of the NO 3 − photolysis using natural or artificial snow samples [3,5,18,21] or thin ice layers [23,26] have also indicated that the NO x flux to the gas phase was dominated by NO 2 , although in most cases the emission of NO was not negligible with ratios of the NO 2 to NO production rates between 2 and 8 in the snow experiments [5,18,21] and ratios between 10 and 23 in the experiments with the ice films [23,26]. However, the formation of NO possibly indicates further reactions in the gas phase like the photolysis of NO 2 and HONO, which can also lead to the formation of NO.…”

“…Several laboratory studies have provided evidence that the photolysis of nitrate and the subsequent reactions in ice and snow take place in a so-called quasi-liquid layer (QLL) on the surface of ice crystals [19,20,22,23]. Although the properties of the QLL are not well established, it is now well known that at temperatures close to the melting point and/or in the presence of impurities, the QLL shows a strongly enhanced disorder compared to the highly ordered interior of the ice crystal (e.g.…”

“…The photolysis of nitrate (NO 3 − ) was first identified as an important photochemical reaction in this environment [2]. This reaction is considered as one of the key reactions in surface-snow and has been the subject of a series of field [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] and laboratory studies [18][19][20][21][22][23][24][25][26][27]. Nitrate photoly-sis has an impact on the composition of and processes in the surface-snow.…”

“…Previous laboratory studies have focused on different aspects, like the photolytic decomposition of NO 3 − [24,27], the formation of products like OH [20,22] and nitrite (NO 2 − ) [19,20,27] and the release of NO x to the gas phase [18,19,21,23,25,26].…”

The physics of premelted ice and its geophysical consequences

“…Research into the adsorption of gaseous impurities on ice and snow has been carried out in laboratory studies by many groups ͑Clapsaddle and Lamb, 1989; Valdez et al, 1989;Mitra et al, 1990;Conklin and Bales, 1993;Chen and Crutzen, 1994;Tabazadeh et al, 1999;Domine et al, 2000;Huthwelker et al, 2001Huthwelker et al, , 2004Cho et al, 2002;Boxe et al, 2003;Khvorostyanov and Curry, 2004;Sadtchenko et al, 2004͒. Particular emphasis is on the uptake of SO 2 , which is a significant component of volcanic emissions and of the combustion of fossil fuels.…”