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Since the emergence of the Antarctic 'ozone hole' in the 1980s (1) and elucidation of the chemical mechanisms (2-5) and meteorological conditions (6) involved in its formation, the likelihood of [...], Chemical ozone destruction occurs over both polar regions in local winter-spring. In the Antarctic, essentially complete removal of lower-stratospheric ozone currently results in an ozone hole every year, whereas in the Arctic, ozone loss is highly variable and has until now been much more limited. Here we demonstrate that chemical ozone destruction over the Arctic in early 2011 was--for the first time in the observational record--comparable to that in the Antarctic ozone hole. Unusually long-lasting cold conditions in the Arctic lower stratosphere led to persistent enhancement in ozone-destroying forms of chlorine and to unprecedented ozone loss, which exceeded 80 per cent over 18-20 kilometres altitude. Our results show that Arctic ozone holes are possible even with temperatures much milder than those in the Antarctic. We cannot at present predict when such severe Arctic ozone depletion may be matched or exceeded.

A new approach to calculate chemical ozone destruction rates that facilitate comparisons of ozone concentrations in specific air parcels at different times has been described. During arctic winter of 1991-92 the frequency of the formation of polar stratospheric clouds (PSCs) increased between early December until mid January. A minor warming occurred at this time which inhibited further large scale formation of PSCs. CIO measurements by the Upper Atmosphere Research Satellite Microwave Limb Sounder instrument had showed a high level of CIO (up to 2.5 p.p.b.v. at 19 km altitude) in the vortex.

Extensive ozone depletion due to heterogenous reactions on polar stratospheric clouds (PSCs) occurs in the Arctic and Antarctic lower stratosphere. The ozone loss over the Arctic region during the unusually cold 1995-96 Arctic winter, was the highest recorded. The continued loss could be due to the removal of the nitrogen species that slow down the rate of chemical ozone depletion.