[1] The Arctic polar vortex exhibited widespread regions of low temperatures during the winter of 2005, resulting in significant ozone depletion by chlorine and bromine species. We show that chemical loss of column ozone (DO 3 ) and the volume of Arctic vortex air cold enough to support the existence of polar stratospheric clouds (V PSC ) both exceed levels found for any other Arctic winter during the past 40 years. Cold conditions and ozone loss in the lowermost Arctic stratosphere (e.g., between potential temperatures of 360 to 400 K) were particularly unusual compared to previous years. Measurements indicate DO 3 = 121 ± 20 DU and that DO 3 versus V PSC lies along an extension of the compact, near linear relation observed for previous Arctic winters. The maximum value of V PSC during five to ten year intervals exhibits a steady, monotonic increase over the past four decades, indicating that the coldest Arctic winters have become significantly colder, and hence are more conducive to ozone depletion by anthropogenic halogens.

Evidence of mid-latitude ozone depletion and proof that the Antarctic ozone hole was caused by humans spurred policy makers from the late 1980s onwards to ratify the Montreal Protocol and subsequent treaties, legislating for reduced production of ozone-depleting substances. The case of anthropogenic ozone loss has often been cited since as a success story of international agreements in the regulation of environmental pollution. Although recent data suggest that total column ozone abundances have at least not decreased over the past eight years for most of the world, it is still uncertain whether this improvement is actually attributable to the observed decline in the amount of ozone-depleting substances in the Earth's atmosphere. The high natural variability in ozone abundances, due in part to the solar cycle as well as changes in transport and temperature, could override the relatively small changes expected from the recent decrease in ozone-depleting substances. Whatever the benefits of the Montreal agreement, recovery of ozone is likely to occur in a different atmospheric environment, with changes expected in atmospheric transport, temperature and important trace gases. It is therefore unlikely that ozone will stabilize at levels observed before 1980, when a decline in ozone concentrations was first observed.

[1] We present validation studies of MLS version 2.2 upper tropospheric and stratospheric ozone profiles using ozonesonde and lidar data as well as climatological data. Ozone measurements from over 60 ozonesonde stations worldwide and three lidar stations are compared with coincident MLS data. The MLS ozone stratospheric data between 150 and 3 hPa agree well with ozonesonde measurements, within 8% for the global average. MLS values at 215 hPa are biased high compared to ozonesondes by $20% at middle to high latitude, although there is a lot of variability in this altitude region. Comparisons between MLS and ground-based lidar measurements from Mauna Loa, Hawaii, from the Table Mountain Facility, California, and from the Observatoire de HauteProvence, France, give very good agreement, within $5%, for the stratospheric values. The comparisons between MLS and the Table Mountain Facility tropospheric ozone lidar show that MLS data are biased high by $30% at 215 hPa, consistent with that indicated by the ozonesonde data. We obtain better global average agreement between MLS and ozonesonde partial column values down to 215 hPa, although the average MLS values at low to middle latitudes are higher than the ozonesonde values by up to a few percent. MLS

We estimate the tropospheric column ozone using a forward trajectory model to increase the horizontal resolution of the Aura Microwave Limb Sounder (MLS) derived stratospheric column ozone. Subtracting the MLS stratospheric column from Ozone Monitoring Instrument total column measurements gives the trajectory enhanced tropospheric ozone residual (TTOR). Because of different tropopause definitions, we validate the basic residual technique by computing the 200‐hPa‐to‐surface column and comparing it to the same product from ozonesondes and Tropospheric Emission Spectrometer measurements. Comparisons show good agreement in the tropics and reasonable agreement at middle latitudes, but there is a persistent low bias in the TTOR that may be due to a slight high bias in MLS stratospheric column. With the improved stratospheric column resolution, we note a strong correlation of extratropical tropospheric ozone column anomalies with probable troposphere‐stratosphere exchange events or folds. The folds can be identified by their colocation with strong horizontal tropopause gradients. TTOR anomalies due to folds may be mistaken for pollution events since folds often occur in the Atlantic and Pacific pollution corridors. We also compare the 200‐hPa‐to‐surface column with Global Modeling Initiative chemical model estimates of the same quantity. While the tropical comparisons are good, we note that chemical model variations in 200‐hPa‐to‐surface column at middle latitudes are much smaller than seen in the TTOR.

We have studied the interannual and longer‐term variations in ozone profiles over the Arctic from 1989 to 2003 using ozonesonde observations from seven northern high‐latitude stations. The ozonesonde data have been carefully examined and made as internally consistent as possible. The homogenized measurements are analyzed with a statistical model. In both the stratosphere and troposphere the largest long‐term changes have occurred in the late winter/spring period (January–April), the period of greatest interannual variability. In the stratosphere, layer ozone amounts correlate highly with proxies for the stratospheric circulation (100 hPa eddy heat flux averaged over 45–70°N), polar ozone depletion (the calculated volume of polar stratospheric clouds combined with the effective equivalent stratospheric chlorine) and tropopause height. At altitudes between 50 and 70 hPa, we estimate that chemical polar ozone depletion accounted for up to 50% of the March ozone variability. Negative trends in the lower stratosphere prior to 1997 can be attributed to the combined effect of dynamical changes, the impact of aerosols from the Mt. Pinatubo eruption and winters of relatively large chemical ozone depletion. Since 1996–1997 the observed increase in lower stratospheric ozone can be attributed primarily to dynamical changes. In the free troposphere, a statistically significant increase of 11.3 ± 1.8% over 15 years is observed which also maximizes in the January to April period (16.0 ± 3.1% over 15 years). The model suggests that this can be attributed to the effects of changes in the Arctic Oscillation.