The condensation and evaporation coefficients for H2O on ice surfaces were measured using optical interference techniques. The condensation coefficient, alpha, was determined at ice surface temperatures from 20 to 185 K. For H2O vapor at 300 K, the condensation coefficient decreased as a function of surface temperature from alpha = 1.06 +/- 0.10 at 20 K to alpha = 0.65 +/-0.08 at 185 K. The temperature dependence of the condensation coefficient could be fit by a precursor-mediated adsorption model. The evaporation coefficient, gamma was obtained at various surface temperatures using isothermal desorption measurements. The evaporation coefficient was observed to be constant at gamma = 0.63 +/- 0.15 for ice surface temperatures from 173 to 205 K. Over the temperature range whom the condensation and evaporation coefficients could both be measured, alpha and gamma were equivalent within the experimental error limits. This equivalence indicates that evaporation or condensation rates are dictated only by temperature and pressure and can be treated individually during net condensation, not evaporation, or steady-state equilibrium. An Arrhenius analysis of the H2O isothermal desorption rates from ice at different temperatures revealed zero-order desorption kinetics expected for multilayer desorption. The activation barrier for desorption was E(d) = 11.9 +/- 0.2 kcal/mol with a preexponential of upsilon0 = 2.8 x 10(30) +/- 1.0 x 10(30) molecules/(cm2 s). Quasi-equilibrium experiments also determined an enthalpy of sublimation for H2O from ice of DELTAH(sub) = 11.8 +/- 0.2 kcal/mol and an entropy of sublimation of DELTAS(sub) = 31.0 cal/(K mol). The equivalency of the kinetic desorption barrier and the quasi-equilibrium enthalpy of sublimation indicates that there is no barrier for H2O adsorption on ice surfaces. The measured condensation and evaporation coefficients predict the presence of polar stratospheric clouds over the Antarctic pole at 10-20 km. These measurements also reveal that ice surfaces in the polar stratosphere are very dynamic with H2O condensation and evaporation rates of 10-1000 ML/s (1 ML = 9.8 x 10(14) molecules.cm2) for equilibrium conditions between 180 and 210 K.
