Effect of competitive adsorption on polar stratospheric cloud reactions

Existing laboratory data for reaction rates on ice and nitric acid trihydrate are interpreted in terms of a simple, speculative mechanism for these reactions. The mechanism assumes that the solid phase is either pure H 2 O or pure HNO 3 (H 2 O) 3 and that the state of the surface depends largely on the gas phase composition. This dependence is described by Langmuir adsorption isotherms. The adsorption of a nonreactive species can affect the rate of reaction by inhibiting the adsorption of reactive species. In particular, differences in reaction rates observed on ice and HNO 3 (H 2 O) 3 are attributed to the adsorption of gas phase HNO 3 produced by the decomposition of HNO 3 (H 2 O) 3 . The interpretation of heterogeneous reaction rates requires detailed measurements of the rates as a function of gas phase composition.