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

Abstract

Abstract. Emissions of nitrogen oxide (NO x = NO + NO 2 ) from the photolysis of nitrate (NO − 3 ) in snow affect the oxidising capacity of the lower troposphere especially in remote regions of high latitudes with little pollution. Current air-snow exchange models are limited by poor understanding of processes and often require unphysical tuning parameters. Here, two multiphase models were developed from physically based parameterisations to describe the interaction of nitrate between the surface layer of the snowpack and the overlying atmosphere. The first model is similar to previous approaches and assumes that below a threshold temperature, T o , the air-snow grain interface is pure ice and above T o a disordered interface (DI) emerges covering the entire grain surface. The second model assumes that air-ice interactions dominate over all temperatures below melting of ice and that any liquid present above the eutectic temperature is concentrated in micropockets. The models are used to predict the nitrate in surface snow constrained by year-round observations of mixing ratios of nitric acid in air at a cold site on the Antarctic Plateau (Dome C; 75 • 06 S, 123 • 33 E; 3233 m a.s.l.) and at a relatively warm site on the Antarctic coast (Halley; 75 • 35 S, 26 • 39 E; 35 m a.s.l). The first model agrees reasonably well with observations at Dome C (C v (RMSE) = 1.34) but performs poorly at Halley (C v (RMSE) = 89.28) while the second model reproduces with good agreement observations at both sites (C v (RMSE) = 0.84 at both sites). It is therefore suggested that in winter air-snow interactions of nitrate are determined by non-equilibrium surface adsorption and cocondensation on ice coupled with solid-state diffusion inside the grain, similar to Bock et al. (2016). In summer, however, the air-snow exchange of nitrate is mainly driven by solvation into liquid micropockets following Henry's law with contributions to total surface snow NO − 3 concentrations of 75 and 80 % at Dome C and Halley, respectively. It is also found that the liquid volume of the snow grain and airmicropocket partitioning of HNO 3 are sensitive to both the total solute concentration of mineral ions within the snow and pH of the snow. The second model provides an alternative method to predict nitrate concentration in the surface snow layer which is applicable over the entire range of environmental conditions typical for Antarctica and forms a basis for a future full 1-D snowpack model as well as parameterisations in regional or global atmospheric chemistry models.

Deposition, recycling and archival of nitrate stable isotopes between the air-snow interface: comparison between Dronning Maud Land and Dome C, Antarctica

“…Recent modelling suggests that co-condensation is the most important process explaining NO3incorporation in snow undergoing temperature gradient metamorphism at Dome C (Bock et al, 2016). Diamond dust can also scavenge high concentrations of HNO3 at Dome C (Chan et al, 2018). Furthermore, the top layer of the snow pack is only 1 mm thick in the TRANSITS model, which is where we would expect the highest concentrations due to the exponential decay of NO3with depth (Fig.…”

“…In addition, the uptake is not dependent on the HNO3 concentration in the air (Abbatt, 1997). However, the seasonal temperature difference at an individual site (i.e., DML or Dome C) is far greater, which could allow a seasonal dependence on the uptake and loss of NO3in the skin layer, which results in the retention of a greater proportion of NO3in summer (Chan et al, 2018).…”

“…;Chan et al, 2018) and chemical (NO3re-emission;Erbland et al(2015)) processes explain the cycling of NO3between the air and snow. The lowest NO3mass concentrations in the skin layer are found in winter.…”

New Estimation of the NOx Snow‐Source on the Antarctic Plateau

“…However, the model does not reproduce the summer observations at the coast, where the temperature, relative humidity and concentration of aerosol in air and snow are much higher than that on the Plateau, and where snow surface melt is possible, strengthening previous theory of several photolytic domains aforementioned. Nonetheless, Chan et al (2018) developed a new model and concluded that winter air-snow interactions of nitrate between the air and skin layer snow can be described as a combination of non-equilibrium surface adsorption and co-condensation on ice, coupled with solid-state diffusion inside the grain, similar to Bock et al (2016). In addition, Chan et al (2018) were able for the first time to reproduce the summer observations on the Antarctic Plateau and at the Coast, concluding that it is the equilibrium solvation into liquid micro-pockets, based on Henry's solubility law, that dominates the exchange of nitrate between air and snow at warmer sites, that is, where the temperatures are above the eutectic temperature.…”

“…photolysis in snow (Chan et al, 2018). Therefore, a robust quantification of the NO x E sources on a continental scale over Antarctica is still lacking (Frey et al, 2013(Frey et al, , 2015Legrand et al, 2014;Savarino et al, 2016).…”