In this paper, results from a photochemical model integrated along ensembles of 8-day air parcel trajectories are used to simulate the latitude and vertical composition gradients observed from the ER-2 aircraft during the 1987 Airborne Arctic Ozone Experiment (AAOE). The photochemical model used includes heterogeneous chemical reactions when polar stratospheric clouds are inferred, from local temperature and pressure, to be present. The model results were found to be very sensitive to NOy mixing ratios and the frequency of polar stratospheric clouds (PSCs). Trajectories often showed instantaneous (adiabatic) cooling rates of up to 30 K d-1, with air parcels even at high latitudes spending substantial periods outside modeled clouds. This leads to a photochemical balance in the model calculations between the perturbations to the composition caused by the heterogeneous reactions and the restoring effects of HNO3 destruction. For the period of interest (early September 1987), the model was able to simulate well both the latitude gradient of ClO on the 428 K potential temperature surface and the ClO vertical gradient at 72¿S. The model simulated well the NO and NOy concentrations at high latitudes inside the dehydrated, denitrified region, although NO values outside were significantly overestimated. There is evidence from the chlorine and the nitrogen species partitioning in the model that even outside the denitrified, dehydrated region the chemical composition is perturbed. It is argued that heterogeneous processing on type I PSCs has occurred in this outer region, but without denitrification or dehydration. Model results imply that the BrO observations made from the Er-2 within the dehydrated, denitrified region are consistent with there being approximately 5 parts per trillion by volume of BrOy at 428 K in spring. Within the high ClO region, ozone destruction rates are calculated to exceed 2% d-1 with approximately 80% due to the ClO dimer mechanism. ¿ American Geophysical Union 1989