Model molecular dynamics simulation of hydrochloric acid ionization at thesurface 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 oz one 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 incor poration mechanisms for a dynamic ice surface, for the basal-plane and pris m face surfaces of ice. Acid ionization at 190 K in those sites is examined via combined electronic structure and classical molecular dynamics methodo logies to estimate the reaction Gibbs energies and barriers. In each case e xamined,;the acid ionization step of proton transfer from the HCl to a neig hboring water molecule is estimated to be thermodynamically feasible and ki netically rapid.