A review of air–ice chemical and physical interactions (AICI): liquids, quasi-liquids, and solids in snow

Abstract

Abstract. Snow in the environment acts as a host to rich chemistry and provides a matrix for physical exchange of contaminants within the ecosystem. The goal of this review is to summarise the current state of knowledge of physical processes and chemical reactivity in surface snow with relevance to polar regions. It focuses on a description of impurities in distinct compartments present in surface snow, such as snow crystals, grain boundaries, crystal surfaces, and liquid parts. It emphasises the microscopic description of the ice surface and its link with the environment. Distinct differences between the disordered air–ice interface, often termed quasi-liquid layer, and a liquid phase are highlighted. The reactivity in these different compartments of surface snow is discussed using many experimental studies, simulations, and selected snow models from the molecular to the macro-scale. Although new experimental techniques have extended our knowledge of the surface properties of ice and their impact on some single reactions and processes, others occurring on, at or within snow grains remain unquantified. The presence of liquid or liquid-like compartments either due to the formation of brine or disorder at surfaces of snow crystals below the freezing point may strongly modify reaction rates. Therefore, future experiments should include a detailed characterisation of the surface properties of the ice matrices. A further point that remains largely unresolved is the distribution of impurities between the different domains of the condensed phase inside the snowpack, i.e. in the bulk solid, in liquid at the surface or trapped in confined pockets within or between grains, or at the surface. While surface-sensitive laboratory techniques may in the future help to resolve this point for equilibrium conditions, additional uncertainty for the environmental snowpack may be caused by the highly dynamic nature of the snowpack due to the fast metamorphism occurring under certain environmental conditions. Due to these gaps in knowledge the first snow chemistry models have attempted to reproduce certain processes like the long-term incorporation of volatile compounds in snow and firn or the release of reactive species from the snowpack. Although so far none of the models offers a coupled approach of physical and chemical processes or a detailed representation of the different compartments, they have successfully been used to reproduce some field experiments. A fully coupled snow chemistry and physics model remains to be developed.

Surface-Enhanced Nitrate Photolysis on Ice

“…22 Furthermore, while recent reports have shown that only at temperatures very near the melting point of ice (T>-1,5 o C) is a QLL present at the air/single crystalline ice interface, 23,24 it is clear that strong photochemical NOx fluxes are observed to emanate from the snowpack even at temperatures (i.e., 240 K-260 K) where the extent (i.e., coverage and thickness) of the QLL is expected to be very limited. 25 Finally, while the conditions required to favor the occurrence of a QLL on ice are still being debated, it has been recently advocated, based on simple physical considerations, that the extent of the QLL coverage at the air-ice interface on snow at the temperature of the polar boundary layer was much overestimated by these approximations. 26 At these much colder temperatures (i.e., T<263 K), a molecularly-thin disordered/defective interfacial layer must nonetheless exist at the interstitial air/ice interface whose properties must be better understood as it mediates pollutants adsorption and the heterogeneous reactivity of snow.…”

X-ray computed microtomography of sea ice – comment on &quot;A review of air–ice chemical and physical interactions (AICI): liquids, quasi-liquids, and solids in snow&quot; by Bartels-Rausch et al. (2014)

“…Recently, Bartels-Rausch et al (2014) published a review of the recent literature on air-ice interactions. In it, they state that "using XMT is difficult when liquid is present, due to the small difference in absorption of liquid solutions and of solid ice.…”

“…Chem. Phys., 14, 1587-1633, doi:10.5194/acp-14-1587-2014, 2014). Here we rebut the assertion that X-ray computed microtomography of sea ice fails to reveal liquid brine inclusions by discussing the phases present at the analysis temperature.…”

Surface Snow, Firn and Ice Core Composition in Polar Areas in Relation to Atmospheric Aerosol and Gas Concentrations: Critical Aspects

“…Many studies have observed significant changes in the concentration of gases contained in the deposited snow due to metamorphism, photolysis, diffusion out of the snow crystals, etc. [104]- [106] In addition, the distribution of impurities between the different domains of the condensed phase inside the snowpack (at the grain surface, in the bulk of the solid, in liquid trapped in confined pockets between grains), the presence of a multiphase system where liquid solutions and ice coexist, and changes in snow structure and properties affect both physical processes and Snow-grain metamorphism ATMOSPHERE chemical reactivity [107].…”