Mass and thermal accommodation coefficients of H2O(g) on liquid water as afunction of temperature

The probabilities that a water vapor molecule striking a liquid water surfa ce will (1) thermally equilibrate with the liquid surface and (2) penetrate that surface and be incorporated into the bulk liquid are parameters of fu ndamental importance to both chemical physics and atmospheric science. Here we report values for these parameters as a function of temperature in the range of 258-280 K, measured with a droplet train flow reactor under condit ions that circumvent difficulties encountered in earlier studies. The mass accommodation coefficient (alpha) of H2O(g) on water was determined by meas uring the uptake of O-17 labeled gas-phase water under near equilibrium con ditions. The mass accommodation coefficient has a negative temperature depe ndence, with the magnitude ranging from 0.17 +/- 0.03 at 280 K to 0.32 +/- 0.04 at 258 K. The temperature dependence and the magnitude of a are consis tent with the critical complex theory of mass accommodation, previously app lied to the uptake of other gas-phase species by aqueous surfaces. Experime nts with D2O(g) on liquid water show that D-H isotope exchange on the liqui d surface proceeds with unit probability, independent of temperature. This result implies that the thermal accommodation coefficient of H2O(g) on liqu id water is I in the temperature range studied.