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AIM: We have recently developed a microscopic Monte Carlo approach to studysurface chemistry on interstellar grains and the morphology of ice mantles. Themethod is designed to eliminate the problems inherent in the rate-equationformalism to surface chemistry. Here we report the first use of this method ina chemical model of cold interstellar cloud cores that includes both gas-phaseand surface chemistry. The surface chemical network consists of a small numberof diffusive reactions that can produce molecular oxygen, water, carbondioxide, formaldehyde, methanol and assorted radicals. METHOD: The simulationis started by running a gas-phase model including accretion onto grains but nosurface chemistry or evaporation. The starting surface consists of either flator rough olivine. We introduce the surface chemistry of the three species H, Oand CO in an iterative manner using our stochastic technique. Under theconditions of the simulation, only atomic hydrogen can evaporate to asignificant extent. Although it has little effect on other gas-phase species,the evaporation of atomic hydrogen changes its gas-phase abundance, which inturn changes the flux of atomic hydrogen onto grains. The effect on the surfacechemistry is treated until convergence occurs. We neglect all non-thermaldesorptive processes. RESULTS: We determine the mantle abundances of assortedmolecules as a function of time through 2x10^5 yr. Our method also allowsdetermination of the abundance of each molecule in specific monolayers. Themantle results can be compared with observations of water, carbon dioxide,carbon monoxide, and methanol ices in the sources W33A and Elias 16. Other thana slight underproduction of mantle CO, our results are in very good agreementwith observations.

Gas-grain chemistry in cold interstellar cloud cores with a microscopic Monte Carlo approach to surface chemistry