Measurements of ClO (Brune et al., 1990) acquired during the Airborne Arctic Stratospheric Expedition are used to infer concentrations of reactive chlorine (ClO+2¿Cl2O2). Observed fields of potential temperature and potential vorticity are used to extrapolate in situ data to larger regions of the vortex. Calculated values of the loss rate of O3, based on estimates of reactive chlorine and measurements of BrO (Toohey et al., 1990), suggest that the loss of O3 was about 12% for levels of the atmosphere with potential temperatures between 440 and 470 K over the 39 day duration of the ER-2 flights into the polar vortex. Calculated loss rates agree with observed rates of removal of O3, although significant uncertainties exist for each. ¿ American Geophysical Union 1990