A three-dimensional chemistry-transport model has been used to investigate N2O and O3 distributions in the lowermost stratosphere at middle and high latitudes in the Northern Hemisphere. The model results are compared with detailed in situ aircraft observations, performed in the winters of 1994--1995 and 1996--1997 and spring 1996, as part of the Stratosphere Troposphere Experiment by Aircraft Measurements (STREAM) II and III. In addition, observations performed earlier in the winter of 1997 during the Polar Stratospheric Aerosol Experiment (POLSTAR) I mission have been included. It is shown that slopes from the observed N2O-O3 relationships can be used to characterize air masses in the lowermost stratosphere and to test a global tropospheric-stratospheric chemistry-transport model. The calculated slopes are consistent with the general view of the N2O-O3 distribution in the lower stratosphere. However, examining the lowermost stratosphere in detail, the model occasionally calculates significantly steeper slopes than observed. The observed shallower slopes reflect the presence of polar vortex air. Depending on the strength and the persistence of the polar vortex during winter, confined polar stratospheric air masses reach the tropopause, even during late spring at midlatitudes. This is consistent with the view that stratosphere-troposphere exchange and vortex erosion take place along isentropic surfaces from the polar reservoir. ¿ 2000 American Geophysical Union