Large loss of total ozone during the Arctic winter of 1999/2000

Three-dimensional model calculations are used together with total ozone obs ervations from the Global Ozone Monitoring Experiment (GOME) and ozone sond e measurements at Ny-Alesund, Spitsbergen to quantify the chemical ozone lo ss inside the Arctic polar vortex in winter 1999/2000. GOME shows March 200 0 mean Arctic total ozone values of 365 DU, about 100 DU less than the 1980 -1989 mean from TOMS data, well reproduced by the model calculations. A com parison of the modeled ozone with a passive ozone tracer and ozone sonde ob servations at Ny-Alesund shows that by the end of March 2000 about 2.5 ppmv of ozone are chemically depleted in the lower stratosphere, corresponding to more than 70% ozone loss. At the same time, the inferred loss in total o zone inside or at the edge of the polar vortex is between 90 and 140 DU. Th e large ongoing loss during March 2000 is likely to be due to widespread de nitrification, which maintains high chlorine activation during this period.