The effect of stratospheric water vapor on the heterogeneous reaction rate of ClONO₂ and H₂O for sulfuric acid aerosol

Although it is well known that temperature affects the heterogeneous reaction probability (γ) of ClONO 2 and H 2 O on suite acid aerosol surfaces by determining the amount of water which will co‐condense with H 2 SO 4 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 γ and may be a factor contributing to ozone destruction in the lower stratosphere. Comparison of the lifetime of ClONO 2 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.