Exchange of gases across an interface involves mass transfer in both gas and condensed phase and transfer across the interface. The fraction of gas-kinetic collisions that results in transfer of a molecule across an interface, the mass accommodation coefficient (MAC), is an inherently kinetic quantity which is central to consideration of the rate of evaporation (sublimation) of liquids and solids generally and of volatile solute gases. In atmospheric science the MAC is pertinent in the first instance to the rate of condensation/evaporation (sublimation) of water (ice) and in the second instance to the rate of reactive uptake of gaseous solutes by tropospheric liquid-water clouds or polar stratospheric clouds. Whether or not the MAC controls the rate of uptake in a particular situation depends strongly on particle dimension, as interfacial mass transport inherently occurs in series with diffusive mass transport in the gas and/or aqueous phase. Whether or not mass transport is limiting depends additionally on the solubility and reaction rate in the condensed phase. Measurement of MACs is inherently difficult because of the need to account for other resistances to mass transfer and any return of material from condensed phase back to the gas phase as the surface of the solution becomes locally saturated in the dissolved gas. In the case of water, the large latent heat of condensation provides a further apparent resistance to interfacial mass transport. For quite some time failure fully to appreciate these considerations led to the view that MACs were low (10-4 or lower) both for pure water and for solute gases on aqueous solution. Such low values would have implications for cloud activation kinetics and for reactive uptake of gases such as SO2, which undergo reaction in cloudwater. Work since the mid eighties has resulted in rather precise measurements of MACs of and their dependence on such variables as solution composition and temperature, as will be discussed at this Workshop, yielding values of 0.05 or greater for many gases of interest in cloudwater chemistry. Based on these laboratory studies it appears that mass accommodation is not limiting in atmospheric systems of interest either for water or for solute gases on aqueous solution.
This page was last updated 2002-02-15.
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