A MODEL FOR HETEROGENEOUS CHEMICAL PROCESSES ON THE SURFACES OF ICE AND NITRIC-ACID TRIHYDRATE PARTICLES

A model is developed that incorporates the physics and physical chemistry of ice surfaces relevant to polar stratospheric clouds. The Langmuir and Brunauer, Emmett, and Teller (BET) adsorption isotherms are used to compute surface concentrations of H2O, HCl, HOCl, ClONO2 and N2O5 on ice and nitric acid trihydrate (NAT) crystals. Assuming pseudo-first-order kinetics with respect to adsorbed HOCl, ClONO2 and N2O5, surface reaction rates and reaction probabilities (sticking coefficients) are determined. The model parameters (surface morphology and energies) are extracted from measured uptake coefficients and reaction probabilities. For gas pressures of about 10(-7) torr and temperatures in the range of 180-200 K, HCl completely coats ice and water-rich NAT surfaces, while HOCl, ClONO2 and N2O5 may cover 0.01-1% of these surfaces. The model is applied to analyze laboratory data, leading to estimates of adsorption free energies, enthalpies and entropies for HCl, HOCl, ClONO2 and N2O5 on ice and NAT surfaces, and activation energies for the heterogeneous reactions of HCl and H2O with HOCl, ClONO2 and N2O5 on these surfaces. The energy parameters are used to calculate surface parameters such as adsorption and desorption constants, surface coverages, reaction rate coefficients, surface diffusion coefficients and reaction probabilities for varies species and chemical interactions on ice and NAT surfaces. Implications for chemical processing on polar stratospheric clouds are discussed.