Nitrate Photolysis in Salty Snow

Abstract

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.

Chemical Morphology of Frozen Mixed Nitrate–Salt Solutions

“…However, the question of whether such effects might change the partitioning of nitrate within a frozen matrix has not been addressed. As we suggested recently, a difference in the partitioning of nitrate in salty frozen matrices could significantly change the local atmospheric oxidation environment in polar regions influenced by sea ice.…”

“…However, very recently we have reported that the additional presence of halide salts at environmentally relevant concentrations can drastically alter this correlation . The amount of gas-phase nitrogen oxide production generally decreases with an increase in the prefreezing concentration of added halide salt, consistent with a dilution of the nitrate concentration in the liquid-like surface region due to an increased brine volume.…”

“…The amount of gas-phase nitrogen oxide production generally decreases with an increase in the prefreezing concentration of added halide salt, consistent with a dilution of the nitrate concentration in the liquid-like surface region due to an increased brine volume. Interestingly, at the lowest added concentrations of NaCl and especially Instant Ocean, a sea salt analogue, there is an increase in nitrogen oxide formation over that seen in the absence of the salts, followed by the decreasing trend as the prefreezing added salt concentration is increased . We could explain this behavior by considering that halide anions can react efficiently with OH, a product of aqueous nitrate photolysis, thereby preventing the recombination of this radical with the NO 2 product and allowing NO 2 escape to the gas phase.…”

Particulate nitrate photolysis in the atmosphere

“…94 Highly variable quantum yields of nitrate photolysis on ice and snow are reported because they are strongly inuenced by the location of nitrate anions in an ice grain, 95,96 and the co-existing species (e.g., Cl À ). 97 Meusinger et al have proposed two photochemical domains of nitrate photolysis: photolabile nitrate anion and nitrate anion buried within the ice grain. 95 Photoproducts produced from photolabile nitrate anion can escape the ice grain and hence the quantum yields are higher than those of nitrate anion buried within the ice grain.…”

Chemical Morphology Controls Reactivity of OH Radicals at the Air–Ice Interface

“…Water ice contains liquid inclusions and liquid-filled grain boundaries, which may trap reagents within them. Solutes excluded from the ice crystal during a slow freezing process also create a concentrated solution at the air–ice interface, which can lead to altered reaction mechanisms. − Moreover, recent work has shown that the distribution of excluded molecules in the quasi-liquid layer at the ice surface is not always uniform but that solutes may be arranged in uneven patches and self-associate, with liquid or frozen regions between them. − Thus, reagents may either be solvated or exist in solid form at the ice surface, even in the case of highly soluble species like nitrate; this inhomogeneous distribution may play a crucial role in the reaction dynamics at the surface.…”