| Summary: | An unprecedented stratospheric ozone loss has been reported in the Arctic from all ground-based, sondes and satellites observations during the winter 2010-11. It was attributed to unusually long-lasting cold conditions allowing Polar Stratospheric Clouds to form until at least mid-March. Chlorine activation and ozone loss amplitudes are found comparable to that in the Antarctic ozone hole, but the warmer conditions, the lesser reduction of nitric acid, and the absence of dehydration, indicate that Arctic ozone holes are possible even with temperature much milder than those in the Antarctic (Manney et al., 2011, Arnone et al, 2011). Here the ozone loss in and out vortex is evaluated by the "passive" method by comparison between total ozone measured at height SAOZ stations and that simulated by the REPROBUS Chemistry Transport Model (CTM) ignoring chemistry. Most unprecedented feature is the loss rate of 0.5-0.6% per day after February 20, similar to that observed in the Antarctic but never seen before in the Arctic. The explanation, demonstrated by the simultaneous NO2 SAOZ measurements, is the late renoxification of the vortex after the photolysis of HNO3 starting on March 20 only, later than all other years in the Arctic, particularly in 1997, the year of the longer lasting cold vortex. This implies strong denoxification (heterogeneous conversion of N2O5 in HNO3) and at least partial denitrification by sedimentation of NAT particles, that is temperature far below that of NAT formation in the early winter as reported. In summary, it is shown that the unprecedented loss amplitude in the winter 2011 was due to a cold vortex until mid-March, but also to extremely cold temperatures below TICE, in the mid stratosphere in January. Shown in the presentation will be the evolution of the ozone loss and the NO2 column in and out vortex during the 2010-2011winter, the contribution of gas phase chemistry, the relationship with sunlit VPSC, and comparisons with previous years, particularly those of largest losses.
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