Surface chemistry on ice can play an important role in atmospheric composition, but uncertainties in the chemical partitioning within ice samples has hindered the development of accurate models of these systems. Using Raman microscopy, we examine the nitrate distribution in ice and liquid samples containing environmentally relevant concentrations of sodium chloride, sodium bromide, and sea salt analogue Instant Ocean. Nitrate is enhanced at the surface compared to in the bulk in all frozen samples but to different degrees depending on the added salt. A large variation of nitrate in the horizontal dimension of frozen samples provides direct evidence that it congregates in pockets or grain boundaries within the crystal. In these pockets, nitrate experiences a solution-like environment, whereas at the surface nitrate sometimes forms solid crystals.

Nitrate photolysis from snow can have a significant impact on the oxidative capacity of the local atmosphere, but the factors affecting the release of gas-phase products are not well understood. Here, we report a systematic study of the amounts of NO, NO, and total nitrogen oxides (NOy) emitted from illuminated snow samples as a function of both nitrate and total salt (NaCl and Instant Ocean) concentration. The results provide experimental evidence that the release of nitrogen oxides to the gas phase is directly related to the expected nitrate concentration in the brine at the surface of the snow crystals. With no added salts, steady-state release of gas-phase products increases to a plateau value with increasing prefreezing nitrate concentration; with the addition of salts, the steady-state gas-phase nitrogen oxides generally decrease with increasing prefreezing NaCl or Instant Ocean concentration. In addition, for these frozen mixed nitrate (25 mM)-salt (0-500 mM) solutions, there is an increase in gas-phase NO seen at low added salt amounts, with NO production enhanced by up to 42% at low prefreezing [NaCl] (≤25 mM) and by up to 89% at prefreezing Instant Ocean concentrations lower than 200 mM [Cl]. This enhancement may be important to the atmospheric oxidative capacity in polar regions.

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