Heterogeneous chemistry could have a very significant effect on the predicted impact of engine exhaust from high speed civil transport (HSCT) aircraft on atmospheric ozone. Two-dimensional models including only gas phase chemistry indicate that deposition of nitrogen oxides from aircraft exhaust in the lower stratosphere would significantly perturb the natural nitrogen budget, most likely resulting in ozone depletion. The model calculates that an injection of 1 megaton of NO2 per year at 17--20 km would decrease the column ozone by 3--6% at northern mid-latitudes using gas phase chemistry only. Inclusion of the reaction N2O5+H2O → 2HNO3 acting on the global sulfate aerosol layer greatly decreases the sensitivity of ozone to injection of nigroten oxides, with the calculated ozone column increasing by up to 0.4% for the same emission scenario. Additional simulations showed that this response is only slightly sensitive to changes in chlorine content and aerosol loading. ¿ American Geophysical Union 1991