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[MIS12-05] HCl droplets induced bunched steps on ice crystal surfaces under atmospheric-concentration HCl gas
Keywords:ice crystal, air-snow interaction, atmospheric-concentration HCl gas, bunched step, HCl uptake, advanced optical microscopy
Surfaces of ice act as sites of various chemical reactions of atmospheric acidic gases, which cause serious environmental issues, such as the catalytic ozone depletion by hydrogen chloride (HCl) gas. As a means of direct observation of QLLs on ice surfaces, we and Olympus Engineering Co. Ltd., have developed laser confocal microscopy combined with differential interference contrast microscopy (LCM-DIM), which can directly visualize 0.37-nm-thick elementary steps and QLLs on ice crystal surfaces [1].
Even in the presence of HCl gas, we found liquid droplets on ice basal faces [2-3]. In addition, the droplets were not pure water but an HCl aqueous solution, whose HCl concentration was very close to that of a liquidus line of a binary phase diagram of water and HCl. The ice basal faces mainly grew by the spiral growth mechanism under supersaturated conditions. When the lateral growth of the spiral steps collided with the HCl droplets, advancement of the steps was pinned by the droplets [4]. When a laterally growing step passes through two adjacent pining centers, the step is gradually bent. With decreasing curvature radius, a curved step shows a slower growth rate because of the Gibbs-Thomson effect, resulting in the formation of bunched steps. Finally, the HCl droplets were embedded in the ice crystals by the lateral growth of the bunched steps with enough height.
These results indicate that a bulk of an ice crystal plays an important role in the uptake of HCl. So far, it has been believed that the contribution of a bulk of an ice crystal to the HCl uptake is less important than that of an ice surface, because of the small solubility of HCl and the slow diffusion of chloride ions in an ice crystal. Hence, the results found in this study suggest that the uptake of HCl in an ice crystal can be promoted via the bunching and embedding mechanism at temperatures.
[1] Sazaki et al., Prog. Cryst. Growth Charact. Mater. 67 (2021) 100550.
[2] Nagashima et al., Cryst. Growth Des. 16 (2016) 2225.
[3] Nagashima et al., Cryst. Growth Des. 18 (2018) 4117.
[4] Nagashima et al., Cryst. Growth Des. 21 (2021) 2508.
Even in the presence of HCl gas, we found liquid droplets on ice basal faces [2-3]. In addition, the droplets were not pure water but an HCl aqueous solution, whose HCl concentration was very close to that of a liquidus line of a binary phase diagram of water and HCl. The ice basal faces mainly grew by the spiral growth mechanism under supersaturated conditions. When the lateral growth of the spiral steps collided with the HCl droplets, advancement of the steps was pinned by the droplets [4]. When a laterally growing step passes through two adjacent pining centers, the step is gradually bent. With decreasing curvature radius, a curved step shows a slower growth rate because of the Gibbs-Thomson effect, resulting in the formation of bunched steps. Finally, the HCl droplets were embedded in the ice crystals by the lateral growth of the bunched steps with enough height.
These results indicate that a bulk of an ice crystal plays an important role in the uptake of HCl. So far, it has been believed that the contribution of a bulk of an ice crystal to the HCl uptake is less important than that of an ice surface, because of the small solubility of HCl and the slow diffusion of chloride ions in an ice crystal. Hence, the results found in this study suggest that the uptake of HCl in an ice crystal can be promoted via the bunching and embedding mechanism at temperatures.
[1] Sazaki et al., Prog. Cryst. Growth Charact. Mater. 67 (2021) 100550.
[2] Nagashima et al., Cryst. Growth Des. 16 (2016) 2225.
[3] Nagashima et al., Cryst. Growth Des. 18 (2018) 4117.
[4] Nagashima et al., Cryst. Growth Des. 21 (2021) 2508.