Stratospheric mixing is studied using a chemical transport model simulating the evolution of 12 long-lived tracers at high resolution. A model integration is performed covering the period from March 1999 to August 2001. From the tracer relationships in the lower stratosphere we infer relative tracer lifetimes. Where the model captures the species' sinks properly and where the tracers are largely inert in the lower stratosphere, the model tracer lifetimes compare favorably with literature values. The breakup phases of the polar vortices in the two boreal winters covered by the simulation are studied in some detail. In both winters, before the final warming, separate canonical correlations appear. Mixing manifests itself in a progressive merger of the correlation curves marking the polar vortex and middle latitudes. We introduce a method to quantify the origins of fractions of simulated air masses based on tracer relationships describing the situation prior to the final warming. In both years, several weeks after the final vortex breakdown of the polar vortex, the method retrieves realistic quantities of former polar vortex air. The findings are verified using idealized tracer advection experiments. Some important differences between the two winters emerge with regard to the size and longevity of the polar vortex.