Impact of mesospheric intrusions on ozone--tracer relations in the stratospheric polar vortex

Ozone--tracer relations are used to quantify chemical ozone loss in the polar vortices. The underlying assumptions for the application of this technique have been extensively discussed in recent years. However, is has hitherto not been studied, which impact the occurrence of intrusions of mesospheric air into the polar lower stratosphere has on estimating chemical ozone loss using the ozone--tracer technique. Here, we revisit observations of an intrusion of mesospheric air down to altitudes of about 25~km (or 600~K potential temperature) in the Arctic vortex in early 2003. In the ozone--N2O relations, the measurements influenced by mesospheric air do not show unusually low ozone mixing ratios in any of the three balloon profiles measured in the vortex in early 2003. Rather, ozone mixing ratios in these air masses range between 3.6--5.6 ppm clearly greater than those used for the `early vortex' reference relation employed to deduce chemical ozone loss. Using ILAS-II satellite measurements we deduce an average chemical ozone loss in the vortex core for the partial column 380--550~K of 37 Dobson units in March and of 50 Dobson units in April 2003. We conclude that the impact of intrusions of mesospheric air into the polar vortex on chemical ozone loss estimates based on ozone--tracer relations are likely small; the correlations cannot be affected in a way that would lead to an overestimate of ozone depletion.