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Novel airborne in situ measurements of inorganic chlorine, nitrogen oxide species, and ozone were performed inside the lower Antarctic polar vortex and at its edge in September 2012. We focus on one flight during the Transport and Composition of the LMS/Earth System Model Validation (TACTS/ESMVal) campaign with the German research aircraft HALO (High-Altitude LOng range research aircraft), reaching latitudes of 65 degrees S and potential temperatures up to 405 K. Using the early winter correlations of reactive trace gases with N2O from the Atmospheric Chemistry Experiment-Fourier Transform Spectrometer (ACE-FTS), we find high depletion of chlorine reservoir gases up to similar to 40% (0.8 ppbv) at 12 km to 14 km altitude in the vortex and 0.4 ppbv at the edge in subsided stratospheric air with mean ages up to 4.5 years. We observe denitrification of up to 4 ppbv, while ozone was depleted by 1.2 ppmv at potential temperatures as low as 380 K. The advanced instrumentation aboard HALO enables high-resolution measurements with implications for the oxidation capacity of the lowermost stratosphere.

Plain Language Summary Chemistry climate models reveal large uncertainties in the future ozone projection until the end of the century in the lower polar and midlatitude stratosphere. One process that impacts the ozone lifetime during the polar winter is the formation of active chlorine from chlorine reservoir species. Here we present high-resolution measurements performed aboard the new German research aircraft HALO (High-Altitude LOng range research aircraft) in the lower Antarctic vortex in winter 2012. We find significant amounts of active chlorine in the lower vortex that has been transported from higher altitudes and latitudes to the flight altitude of HALO (similar to 14 km). This enhanced activated chlorine content has implications on the ozone lifetime in this region. Our measurements complement satellite observations but feature higher-altitude resolution and extend to lower altitudes. With our case study we investigate the intersection of midlatitude and polar air as well as the effects of transport from the upper to the lower polar vortex. These process studies help to improve chemistry climate models.