An overview of snow photochemistry: evidence, mechanisms and impacts

Abstract

Abstract. It has been shown that sunlit snow and ice plays an important role in processing atmospheric species. Photochemical production of a variety of chemicals has recently been reported to occur in snow/ice and the release of these photochemically generated species may significantly impact the chemistry of the overlying atmosphere. Nitrogen oxide and oxidant precursor fluxes have been measured in a number of snow covered environments, where in some cases the emissions significantly impact the overlying boundary layer. For example, photochemical ozone production (such as that occurring in polluted mid-latitudes) of 3-4 ppbv/day has been observed at South Pole, due to high OH and NO levels present in a relatively shallow boundary layer. Field and laboratory experiments have determined that the origin of the observed NO x flux is the photochemistry of nitrate within the snowpack, however some details of the mechanism have not yet been elucidated. A variety of low molecular weight organic compounds have been shown to be emitted from sunlit snowpacks, the source of which has been proposed to be either direct or indirect photo-oxidation of natural organic materials present in the snow. Although myriad studies have observed active processing of species within irradiated snowpacks, the fundamental chemistry occurring remains poorly understood. Here we consider the nature of snow at a fundamental, physical level; photochemical processes within snow and the caveats needed for comparison to atmospheric photochemistry; our current understanding of nitrogen, oxidant, halogen and organic photochemistry within snow; the current limitations faced by the field and implications for the future.

Spatial and temporal variability of snow chemical composition and accumulation rate at Talos Dome site (East Antarctica)

“…The ions mainly related to secondary aerosol (nitrate, MSA, ammonium) have gas-phase precursors and could be contaminated by uptaking gas-phase compounds from the storehouse or laboratory atmosphere (ammonium) or could be affected by post-depositional processes, such as movement in the firn lattice and re-emission into the atmosphere by acid-base exchanges for MSA and nitrate (Grannas et al, 2007 andreferences therein, Wagnon et al, 1999;Traversi et al, 2009). Fig.…”

Using Singlet Molecular Oxygen to Probe the Solute and Temperature Dependence of Liquid-Like Regions in/on Ice

“…[12][13][14][15][16][17] Understanding the composition of LLRs is essential, in part, because values of the freeze-concentration factor can significantly influence the (photo)chemistry of trace species on ice and snow. 2,18 For some reactions the chemical behavior in LLRs is similar to liquid-phase chemistry. For example, liquid-phase kinetics and mechanisms can explain the direct photolysis of nitrate, nitrite, and hydrogen peroxide on/in ice made from slowly frozen solutions, conditions where we expect solutes to be in LLRs.…”

Ice sheets and nitrogen

“…It has been shown that snowpacks are a strong source of NO x to the atmosphere [56][57][58]. The mechanism has been shown to be photolysis of nitrate in the snow.…”