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[MIS14-P08] VLS growth and bunched steps induced by HCl droplets on ice crystal surfaces
Keywords:ice crystal, air-snow interaction, VLS growth, 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. However, the reported amounts of the uptake of HCl by ice surfaces exhibited considerable variations from 0.01 to 1 monolayer of HCl, showing that the uptake mechanisms of HCl by ice are still unclear. One of the plausible causes for the considerable variations of the uptake amounts is the presence/absence of thin liquid water layers, so-called quasi-liquid layers (QLLs), on ice crystal surfaces at temperatures below the melting point. Hence, we performed direct observations of ice basal faces under atmospheric-concentration HCl gas (~10-4 Pa) [1] by laser confocal microscopy combined with differential interference contrast microscopy (LCM-DIM), which can directly visualize 0.37-nm-thick elementary steps [2] and QLLs on ice crystal surfaces [3].
Even in the presence of HCl gas, we found that liquid droplets appeared on ice basal faces at temperatures lower than -10 °C. However, the droplets were not pure water but an HCl aqueous solution, whose HCl concentration was the same as those of a liquidus line of a binary phase diagram of water and HCl. We also found that the vapor-liquid-solid (VLS) growth [4] of ice crystals occurred utilizing the HCl droplets. However, because the normal growth rate of the VLS growth was slower than that of the spiral growth, the ice basal faces grew mainly by the spiral growth mechanism. When a spiral step collided with the HCl droplets, the lateral advancement of the step was pinned by the HCl droplets. Therefore, the spiral step was gradually bent during the passage through the two adjacent HCl droplets. With decreasing curvature radius, the growth of the curved step became slower because of the Gibbs-Thomson effect, resulting in the formation of bunched steps. Eventually, by the lateral growth of the bunched steps, which had enough height, the HCl droplets were embedded in the ice crystal. These results indicate that a bulk of an ice crystal plays an important role in the uptake of HCl.
So far, the contribution of a bulk ice to the HCl uptake was thought to be less important than that of an ice surface due to the small solubility of HCl and the slow diffusion of chloride ions in an ice crystal. However, the results found in this study demonstrate that the uptake of HCl in an ice crystal can be significantly promoted via the bunching and embedding mechanism at temperatures lower than -10 °C.
References
[1] Nagashima K. et al., HCl droplets induced bunched steps on ice crystal surfaces under atmospheric-concentration HCl gas. Cryst. Growth Des. 21(4), 2508-2515 (2021).
[2] Sazaki G. et al., In-situ optical microscopy observation of elementary steps on ice crystals grown in vapor and their growth kinetics. Prog. Cryst. Growth Charact. Mater 67, 100550 (2021).
[3] Sazaki G. et al., Quasi-liquid layers on ice crystal surfaces are made up of two different phases. Proc Nat Acad Sci USA 109(4), 1052–1055 (2012).
[4] Wagner R.S. and Ellis W.C., Vapor-liquid-solid mechanism of single crystal growth. Applied Physics Letters 4(5), 89–90 (1964).
Even in the presence of HCl gas, we found that liquid droplets appeared on ice basal faces at temperatures lower than -10 °C. However, the droplets were not pure water but an HCl aqueous solution, whose HCl concentration was the same as those of a liquidus line of a binary phase diagram of water and HCl. We also found that the vapor-liquid-solid (VLS) growth [4] of ice crystals occurred utilizing the HCl droplets. However, because the normal growth rate of the VLS growth was slower than that of the spiral growth, the ice basal faces grew mainly by the spiral growth mechanism. When a spiral step collided with the HCl droplets, the lateral advancement of the step was pinned by the HCl droplets. Therefore, the spiral step was gradually bent during the passage through the two adjacent HCl droplets. With decreasing curvature radius, the growth of the curved step became slower because of the Gibbs-Thomson effect, resulting in the formation of bunched steps. Eventually, by the lateral growth of the bunched steps, which had enough height, the HCl droplets were embedded in the ice crystal. These results indicate that a bulk of an ice crystal plays an important role in the uptake of HCl.
So far, the contribution of a bulk ice to the HCl uptake was thought to be less important than that of an ice surface due to the small solubility of HCl and the slow diffusion of chloride ions in an ice crystal. However, the results found in this study demonstrate that the uptake of HCl in an ice crystal can be significantly promoted via the bunching and embedding mechanism at temperatures lower than -10 °C.
References
[1] Nagashima K. et al., HCl droplets induced bunched steps on ice crystal surfaces under atmospheric-concentration HCl gas. Cryst. Growth Des. 21(4), 2508-2515 (2021).
[2] Sazaki G. et al., In-situ optical microscopy observation of elementary steps on ice crystals grown in vapor and their growth kinetics. Prog. Cryst. Growth Charact. Mater 67, 100550 (2021).
[3] Sazaki G. et al., Quasi-liquid layers on ice crystal surfaces are made up of two different phases. Proc Nat Acad Sci USA 109(4), 1052–1055 (2012).
[4] Wagner R.S. and Ellis W.C., Vapor-liquid-solid mechanism of single crystal growth. Applied Physics Letters 4(5), 89–90 (1964).