Model molecular dynamics simulation of hydrochloric acid ionization at the surface of stratospheric ice

The issue of the acid ionization of HCl at the surface of stratospheric ice particles, which is relevant in connection with Antarctic stratospheric ozone depletion, is studied theoretically. This study extends that of Gertner and Hynes (Science, 1996, 271, 1563) by examination of 12 new sites and by quantization of the transferring proton's nuclear motion. Fourteen initial adsorption sites for the HCl are selected via arguments based on HCl incorporation mechanisms for a dynamic ice surface, for the basal-plane and prism face surfaces of ice. Acid ionization at 190 K in those sites is examined via combined electronic structure and classical molecular dynamics methodologies to estimate the reaction Gibbs energies and barriers. In each case examined,;the acid ionization step of proton transfer from the HCl to a neighboring water molecule is estimated to be thermodynamically feasible and kinetically rapid.