Heterogeneous reactions on and in sulfate aerosols: Implications for the chemistry of the midlatitude tropopause region

A model is presented to simulate heterogeneous reactions of nitrogen, chlor ine, and bromine compounds on and in sulfate aerosols under conditions enco untered in the lowermost stratosphere and upper troposphere. Box model calc ulations were performed to investigate the impact of these heterogeneous re actions on the chemistry of the midlatitude tropopause region. The simulati ons reveal that in the lowermost stratosphere the considered heterogeneous conversions effectively dilute NOx and cause a strong enhancement of the HO x and ClOx mixing ratios. Lowermost stratospheric NOx/NOy and OH/HO2 ratios can efficiently be reduced by the heterogeneous reactions. The model calcu lations further suggest that also in the upper troposphere the heterogeneou s transformations lead to denoxification. The modeled upper tropospheric OH abundance is reduced when the heterogeneous reactions are taken into accou nt. The upper troposphere appears to show a large potential for heterogeneo us activation of ClOx and BrOx. The simulations further. indicate that the considered heterogeneous chemistry induces ozone loss in the entire tropopa use region. This modeled ozone loss is governed by the hydrolysis reactions of N2O5 and BrONO2. In the simulations of the upper troposphere, also hete rogeneous reactions of chlorine compounds on and in sulfate aerosols have a major impact on ozone chemistry when an enhanced aerosol loading is assume d.