Calculations of ozone depletion during the 1988/89 Arctic winter using a Lagrangian coupled photochemical-microphysical model are presented in this paper. Abundances of ClO in excess of 1 ppbv were observed at the end of the Airborne Arctic Stratospheric Expedition (AASE) on February 10, 1989. These are shown to be consistent with the removal of more than 90% of the reactive nitrogen and the conversion of more than 80% of reservoir chlorine to active forms. This chemical state implies that ozone losses of more than 20 ppbv per day can be sustained in heavily denitrified air throughout much of February according to current photochemistry. As much as 74% of the loss is calculated to be due to ClO dimer photolysis. Following the warming of the vortex in mid February 1989, ozone loss through ClO dimer photolysis becomes less effective as the rate of thermal decomposition of the ClO dimer increases. Thus, model results suggest that thermal decomposition of the dimer plays an important role in limiting ozone loss in the Arctic spring. ¿ American Geophysical Union 1990