Although it is well known that temperature affects the heterogeneous reaction probability (&ggr;) of ClONO2 and H2O on sulfuric acid aerosol surfaces by determining the amount of water which will co-condense with H2SO4 vapor, the effect of the ambient water vapor concentration, which also affects the aerosol composition, has not been investigated in detail. A nearly constant stratospheric water vapor mixing ratio dictates that the water vapor partial pressure vary proportional to atmospheric pressure and thus increase rapidly in the lower stratosphere. In addition, the water vapor mixing ratio increases dramatically in the lower stratosphere following tropospheric-stratospheric exchange. The increase in water vapor partial pressure results in a large increase in &ggr; and may be a factor contributing to ozone destruction in the lower stratosphere. Comparison of the lifetime of ClONO2 to heterogeneous reactions with that expected from gas phase photochemistry is used to estimate the relative importance of these processes and indicates that winter and spring polar regions are the most likely to be affected.