17:15 〜 18:45
[MIS18-P03] 大気中の微量酸性ガス成分が氷の気相成長ステップへ与える影響
キーワード：氷、氷と大気の相互作用、気相成長、ステップ、オゾン層
Surfaces of ice act as sites of various chemical reactions of atmospheric acidic gases, which cause serious environmental issues, such as catalytic ozone depletion by hydrogen chloride (HCl) gas. 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). By ultrahigh height resolution, LCM-DIM can directly visualize 0.37-nm-thick elementary steps [2] and quasi-liquid layers (QLLs) on ice crystal surfaces [3].
In the presence of HCl gas, we found that HCl droplets appear on ice basal faces at temperatures lower than -10°C [1]. The ice basal faces grow 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, resulting in bunched steps forming. In addition, acidic droplets were found in the presence of HNO3 and CO2 gases. Interestingly, the appearance temperatures of acidic droplets were -10°C, like HCl droplets.
We hypothesized that acidic droplets appear at the same temperature regardless of the type of acidic gas because the ice surface structure changes. Therefore, we have tried to measure the step velocities sensitive to surface structure at various temperatures. Because we can visualize individual steps by LCM-DIM, we measured the step velocities by eliminating the effects of volume diffusion of water vapor and surface diffusion of the H2O molecule, and so on. In addition, we determined the step kinetic coefficient from the supersaturation dependence of step velocities. According to the results of the step kinetic coefficient, it was found that the step velocity in the presence of HCl gas was reduced to 1/10 to 1/100 of that in the absence of HCl gas [4]. This is thought to be due to the adsorption of chloride ions on the ice surface.
We also found that the value of the step kinetic coefficient peaks at T ∼ -15°C. In principle, the step kinetic coefficient should decrease monotonically with decreasing temperature. At present, the cause of the peak is still unclear. However, the peak was also reported without HCl gas [4]. Therefore, the cause of the peak may not be due to HCl gas but may be due to changes in the surface structure of the ice.
In the presence of HCl gas, we found that HCl droplets appear on ice basal faces at temperatures lower than -10°C [1]. The ice basal faces grow 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, resulting in bunched steps forming. In addition, acidic droplets were found in the presence of HNO3 and CO2 gases. Interestingly, the appearance temperatures of acidic droplets were -10°C, like HCl droplets.
We hypothesized that acidic droplets appear at the same temperature regardless of the type of acidic gas because the ice surface structure changes. Therefore, we have tried to measure the step velocities sensitive to surface structure at various temperatures. Because we can visualize individual steps by LCM-DIM, we measured the step velocities by eliminating the effects of volume diffusion of water vapor and surface diffusion of the H2O molecule, and so on. In addition, we determined the step kinetic coefficient from the supersaturation dependence of step velocities. According to the results of the step kinetic coefficient, it was found that the step velocity in the presence of HCl gas was reduced to 1/10 to 1/100 of that in the absence of HCl gas [4]. This is thought to be due to the adsorption of chloride ions on the ice surface.
We also found that the value of the step kinetic coefficient peaks at T ∼ -15°C. In principle, the step kinetic coefficient should decrease monotonically with decreasing temperature. At present, the cause of the peak is still unclear. However, the peak was also reported without HCl gas [4]. Therefore, the cause of the peak may not be due to HCl gas but may be due to changes in the surface structure of the ice.