MOLECULAR-DYNAMICS SIMULATION OF HYDROCHLORIC-ACID IONIZATION AT THE SURFACE OF STRATOSPHERIC ICE

Molecular dynamics simulations were used to study the acid ionization of hydrochloric acid (HCl) at the basal plane surface of ice at 190 ke lvin, as a model for the acid ionization process in Antarctic polar st ratospheric clouds (PSCs). Initial conditions for HCl placement within the top bilayer of the ice lattice were selected on the basis of rele vant dynamic equilibrium adsorption-desorption conditions. Free energy changes calculated for the first step in the stepwise acid ionization mechanism ranged from -5.8 to -6.7 kilocalories per mole for various likely initial conditions. These results indicate that acid ionization is thermodynamically favorable and that this process has important im plications for ozone depletion mechanisms involving PSCs.