Can We Model Snow Photochemistry? Problems with the Current Approaches

Abstract

Snow is a very active photochemical reactor that considerably affects the composition and chemistry of the lower troposphere in polar regions. Snow photochemistry models have therefore been recently developed to describe these processes. In all those models, the chemically active medium is a brine formed at the surface of snow crystals by impurities whose presence cause surface melting. Reaction and photolysis rate coefficients are those measured in dilute liquid solutions. Here, we critically examine the basis for these models by considering the structure of ice crystal surfaces, the processes involved in the interactions between impurities and ice crystals, the location of impurities in snow, and the reactivity of impurities in the various media present in snow. We conclude that the brine formed by impurities can only be present in grooves at grain boundaries and cannot cover ice crystal surfaces because of insufficient ice wettability. It is then very likely that most reactions in snow do not take place in liquids, but rather either on an actual ice surface highly different from a liquid or in particulate matter contained in snow, such as organic particles that are thought to contain most snow chromophores. We discuss why some snow models appear to adequately reproduce some observations, concluding that they are insufficiently constrained and that the use of adjustable parameters allows acceptable fits. We discuss the complexity of developing a snow model without adjustable parameters and with a predictive value. We conclude that reaching this goal in the near future is a tremendous challenge. Modeling attempts focused on snow where the impact of organic particles is minimal, such as on the east Antarctic plateau, represents the best chance of midterm success.

Modelling the physical multiphase interactions of HNO<sub>3</sub> between snow and air on the Antarctic Plateau (Dome C) and coast (Halley)

“…Only Model 2 provides a reasonable estimate at both sites year-round, which suggests that in the summer the major interface between snow grain and surrounding air is still air-ice, but it is the equilibrium solvation into liquid micropockets that dominates the exchange of nitrate between air and snow. Despite the simplified parameterisation of processes in Model 2, it provided a new parameterisation to describe the interaction of nitrate between air and snow as air-ice with a liquid formed by impurities present as micropockets as suggested by Domine et al (2013) instead of an air-DI interface assumed by most models developed previously. Moreover, the non-equilibrium boundary between air and snow grain allows the models to work at sites with a high rate of accumulation such that the snow layer might be buried by fresh snowfall before reaching equilibrium.…”

“…The liquid forms micropockets and is assumed to be located in grooves at grain boundaries or triple junctions due to the limited wettability of ice (Domine et al, 2013). Therefore, at all temperatures below melting the major interface between air and snow grain is assumed to be pure ice and the concentration of NO − 3 in ice is defined by non-equilibrium surface adsorption and co-condensation followed by solidsate diffusion within the grain.…”

“…Most studies assume the liquid solution forms a thin layer covering the whole grain surface (e.g. Kuo et al, 2011) while Domine et al (2013) suggested the liquid is located in grooves at grain boundaries and triple junctions. The arguments of the latter study were as follows: (1) the ionic concentration is so low in natural snow that only a small amount of liquid can be formed and (2) the wettability of ice by liquid water is imperfect, preventing the liquid drop from spreading out across the entire solid surface.…”

“…The threshold temperature, T o , is a value based on lab experiments. The temperature at which a disordered interface is detected on pure ice varies between 238 and 270 K depending on the measurement technique (Domine et al, 2013, and references therein). Here, T o , is set to 238 K, the lower end of the range.…”

“…non-equilibrium adsorption and co-condensation. At T ≥ T e , liquid is assumed to coexist with ice and the liquid fraction is in the form of micropockets that are located at grain boundaries and triple junctions (Domine et al, 2013). where V (n) is the volume of the nth layer of the concentric shell; V (n) is the total volume of the grain, V grain ; and the concentration of nitrate in the nth layer can be determined by re-substituting U such that [NO…”