N2O and O-3 relationship in the lowermost stratosphere: A diagnostic for mixing processes as represented by a three-dimensional chemistry-transport model

A three-dimensional chemistry-transport model has been used to investigate N2O and O-3 distributions in the lowermost stratosphere at middle and high latitudes in the Northern Hemisphere. The model results are compared with d etailed in situ aircraft observations, performed in the winters of 1994-199 5 and 1996-1997 and spring 1996, as part of the Stratosphere Troposphere Ex periment by Aircraft Measurements (STREAM) II and III. In addition, observa tions performed earlier in the winter of 1997 during the Polar Stratospheri c Aerosol Experiment (POLSTAR) I mission have been included. It is shown th at slopes from the observed N2O-O-3 relationships can be used to characteri ze air masses in the lowermost stratosphere and to test a global tropospher ic-stratospheric chemistry-transport model. The calculated slopes are consi stent with the general view of the N2O-O-3 distribution in the lower strato sphere. However, examining the lowermost stratosphere in detail, the model occasionally calculates significantly steeper slopes than observed. The obs erved shallower slopes reflect the presence of polar vortex air. Depending on the strength and the persistence of the polar vortex during winter, conf ined 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 surface s from the polar reservoir.