Uptake Mechanism of Atmospheric Hydrogen Chloride Gas in Ice Crystals via Hydrochloric Acid Droplets

Surfaces of ice have attracted considerable attention as “reaction sites” where atmospheric gases cause various chemical reactions in nature. Hence, revealing the uptake mechanism of atmospheric gases on/in ice remains an experimental challenge. Here we show the direct observation of ice crystal surfaces by advanced optical microscopy in the presence of hydrogen chloride (HCl) gas, which triggers a series of chemical reactions that cause ozone depletion. We found that the HCl gas induced the appearance of droplets of HCl solution on ice crystal surfaces. Under supersaturated water vapor pressure, the HCl droplets were quickly embedded in the ice crystals during the growth of the ice. In contrast, under undersaturated conditions, the embedded HCl droplets reappeared on the ice crystal surfaces during the evaporation of the ice. We estimated that the mole fraction of HCl incorporated into the ice as the HCl droplets (0.19% at −15 °C) was ten times larger than the solubility of HCl gas in an ice crystal (0.017%). This picture of the uptake of HCl gas in ice is quite different from the conventional speculation in which HCl gas is confined to ice surfaces.