Nitrate Photolysis at the Air–Ice Interface of Nature-Identical Snow

Abstract

Nitrate in snowpacks can photolyze to give NO x and HONO, which can affect the processing of organic pollutants and impact the oxidant budget of the boundary layer. Recent work found that nitrate photolysis at the air−ice interface is enhanced by a factor of 3.4 ± 0.3 relative to aqueous solution, but this past work used water ice pellets with surface concentrations of nitrate much higher than environmental levels. Here, we dope nature-identical snow with nitric acid (and ammonia for pH control) to achieve nitrate surface concentrations orders of magnitude lower than in previous work and more similar to natural conditions. We then illuminate the nitrate-doped snow at −20 °C with 313 nm irradiation and measure the formation of nitrite. Our results are roughly comparable to previous ice pellet work, with nitrite formation enhanced by a factor of 6.4 ± 4.7 relative to the predicted aqueous result at the same temperature; this result is larger than the ice pellet outcome by a factor of 1.8 ± 1.3 but is not statistically different. Our findings indicate nitrate at the air−ice interface is more reactive than in liquid-like regions or in aqueous solution but suggest that higher nitrate concentrations on ice surfaces only reduce this reactivity slightly, if at all.