A photochemical two-dimensional model including a grid-point temperature distribution calculated from long-term circulation statistics has been used to assess the relative role of heterogeneous reactions of chlorine nitrate on background sulfate aerosols. The sulfuric acid weight percentage is calculated explicitly as a function of temperature and water vapor content. It is shown that the contribution of these reactions to the ozone trend in the 1980's accounts for about only 10% the ozone loss due to N2O5+H2O during the Northern Hemisphere winter months at mid-latitude and between 50--200 mb. The weight of these reactions, however, may substantially increase in the presence of larger aerosol loading. The reaction ClONO2+HCl on sulfate aerosols is found to be of comparable importance with respect to ClONO2+H2O, and it is found to represent a significant source of active chlorine in the Antarctic spring. ¿ American Geophysical Union 1993