1Institut für Energie- und Klimaforschung – Stratosphäre (IEK-7), Forschungszentrum Jülich, Germany
2Laboratoire d'Aérologie, CNRS/INSU-Université de Toulouse, Toulouse, France
3Groupe d'étude de l'Atmosphère Météorologique, CNRM-GAME, Météo-France, Toulouse, France
4Department of Atmospheric and Oceanic Science, University of Colorado, Boulder, CO, USA
Abstract. Balloon-borne observations of ozone from Antarctic stations have been reported to reach ozone mixing ratios as low as about 10 ppbv at the 70 hPa level by late September. After reaching a minimum, ozone mixing ratios then increase to the ppmv level by late December. While the basic mechanisms causing the ozone hole have been known for more than 20 yr, the detailed chemical processes controlling how low the local concentration can fall, and how it recovers from the minimum have not been explored so far. Both of these aspects are investigated here by analysing results from the Chemical Lagrangian Model of the Stratosphere (CLaMS). We discuss the processes responsible for stopping of the catalytic ozone depletion. We show that an irreversible chlorine deactivation into HCl can occur either when ozone drops to very low values or by temperatures increasing above the PSC threshold in these simulations. As a consequence, the timing and mixing ratio of the minimum depends sensitively on model parameters including the ozone initialisation. The subsequent observed ozone increase between October and December is linked not only to transport, but also to photochemical ozone production, caused by oxygen photolysis and by the oxidation of carbon monoxide and methane.