A model is presented to simulate heterogeneous reactions of nitrogen, chlorine, and bromine compounds on and in sulfate aerosols under conditions encountered in the lowermost stratosphere and upper troposphere. Box model calculations were performed to investigate the impact of these heterogeneous reactions on the chemistry of the midlatitude tropopause region. The simulations reveal that in the lowermost stratosphere the considered heterogeneous conversions effectively dilute NOx and cause a strong enhancement of the HOx and ClOx mixing ratios. Lowermost stratospheric NOx/NOy and OH/HO2 ratios can efficiently be reduced by the heterogeneous reactions. The model calculations further suggest that also in the upper troposphere the heterogeneous transformations lead to denoxification. The modeled upper tropospheric OH abundance is reduced when the heterogeneous reactions are taken into account. The upper troposphere appears to show a large potential for heterogeneous activation of ClOx and BrOx. The simulations further indicate that the considered heterogeneous chemistry induces ozone loss in the entire tropopause region. This modeled ozone loss is governed by the hydrolysis reactions of N2O5 and BrONO2. In the simulations of the upper troposphere, also heterogeneous reactions of chlorine compounds on and in sulfate aerosols have a major impact on ozone chemistry when an enhanced aerosol loading is assumed. ¿ 1999 American Geophysical Union