Description: Large losses of Arctic ozone occur during winters with cold, stable stratospheric circulations that result in the extensive occurrence of polar stratospheric clouds (PSCs). Reactions on the surface of PSCs lead to elevated abundances of chlorine monoxide (ClO) that, in the presence of sunlight, destroys ozone. Here we show that PSCs were more widespread during the 1999/2000 Arctic winter than for any other winter in the past two decades. We have used three fundamentally different approaches to derive the degree of chemical ozone loss from ozone sonde, balloon, aircraft and satelite instruments. We show that the ozone losses derived from these different instruments and approaches agree very well, resulting in a high level of confidence in the results. Chemical processes led to a 70% reduction of ozone for a ~1 km thick region of the lower stratosphere, the largest degree of local loss ever reported for the Arctic. The chemical loss of ozone in the total column amounted to about 100 DU by the end of the winter. This total column loss was balanced by transport, resulting in relatively constant total ozone between early January and late March, which is in contrast to the climatological increase of the total ozone column during this period, that is observed during most years.

Description: A number of studies have reported empirical estimates of ozone loss in the Arctic vortex.They have used satellite and in situ measurements and have principally covered the Arcticwinters in the 1990s. While there is qualitative consistency between the patterns of ozone loss, aquantitative comparison of the published values shows apparent disagreements. In this paper weexamine these disagreements in more detail. We choose to concentrate on the five maintechniques (Match, Système d'Analyse par Observation Zénithale (SAOZ)/REPROBUS,Microwave Limb Sounder (MLS), vortex average descent, and the Halogen OccultationExperiment (HALOE) ozone tracer approach). Estimates of the ozone losses in three winters(1994/1995, 1995/1996 and 1996/1997) are recalculated so that the same time periods, altituderanges, and definitions of the Arctic vortex are used. This recalculation reveals a remarkably goodagreement between the various estimates. For example, a superficial comparison of results fromMatch and from MLS indicates a big discrepancy (2.0 ± 0.3 and 0.85 ppmv, respectively, for airending at ~460 K in March 1995). However, the more precise comparisons presented here revealgood agreement for the individual MLS periods (0.5 ± 0.1 versus 0.5 ppmv; 0.4 ± 0.2 versus0.3-0.4 ppmv; and 0.16 ± 0.09 ppmv versus no significant loss). Initial comparisons of the columnlosses derived for 1999/2000 also show good agreement with four techniques, giving 105 DU(SAOZ/REPROBUS), 80 DU (380-700 K partial column from Polar Ozone and Aerosol Monitoring(POAM)/REPROBUS), 85 ± 10 DU (HALOE ozone tracer), and 88 ± 13 (400-580 partial columnfrom Match). There are some remaining discrepancies with ozone losses calculated using HALOEozone tracer relations; it is important to ensure that the initial relation is truly representative of thevortex prior to the period of ozone loss.

Description: Over the past decade tremendous progress has been made toward quantifying and understanding ozoneloss in the Arctic stratosphere. Today a variety of approaches exist to quantify the degree of chemicalloss over the course of an Arctic winter. Some have been used in a consistent way for many years andhave produced a record of the interannual variability. On the other hand a wide range of chemicalmodels have been used to understand the processes in the wintertime Arctic stratosphere and tocalculate the degree of ozone loss.〈br〉〈br〉An active scientific discussion has started about the level of maturity of up to date chemical models ofthe polar stratospheric chemistry. How well are observations of the ozone loss rate reproduced? Howcomplete is our current quantitative understanding of the involved chemical and microphysicalprocesses? Are discrepancies between model results and observations larger than the combined knownuncertainties?〈br〉〈br〉In the published literature the answers to these questions are controversial. Over the next few yearsone of the major challenges for the stratospheric research community will be to predict the future of theArctic ozone layer in a scenario of decreasing stratospheric halogen loading and possible changes inclimate. A solid assessment of our ability to reproduce currently observed ozone losses with modelcalculations is indispensable to determine the requirements for future research and to correctly interpretthe reliability of model based predictions.〈br〉〈br〉To address these issues the Arctic Ozone Loss Workshop was held on 4-6 March in Potsdam,Germany, hosted by the Alfred-Wegener-Institute for Polar and Marine Research. The conveners (theauthors of this summary) were joined in a program committee by Georgios Amanatidis, Mike Kurylo, PaulNewman, and John Pyle. About 70 scientists from Europe, the US, Japan, Russia and New Zealandparticipated. The workshop was mainly based on poster presentations and discussion sessions. It waspart of DYCHO, a research project within the German AFO-2000 program and of QUOBI, an EC fundedresearch project, which are both part of the EU research cluster SOLO. The workshop was supported bySPARC.〈br〉〈br〉

Description: It is well established that extensive depletion of ozone, initiated by heterogenous reactions on polar stratospheric clouds(PSCs) can occur in both the Arctic and Antarctic lower stratosphere. Moreover, it has been shown that ozone lossrates in the Arctic region in recent years reached values comparable to those over the Antarctic. But until now theaccumulated ozone losses over the Arctic have been the smaller, mainly because the period of Arctic ozone loss hasnot-unlike over the Antarctic-persisted well into springtime. Here we report the occurrence-during the unusuallycold 1995-96 Arctic winter-of the highest recorded chemical ozone loss over the Arctic region. Two new kinds ofbehaviour were observed. First, ozone loss at some altitudes was observed long after the last exposure to PSCs. Thiscontinued loss appears to be due to a removal of the nitrogen species that slow down chemical ozone depletion. Second,in another altitude range ozone loss rates decreased while PSCs were still present, apparently because of an earlytransformation of the ozone-destroying chlorine species into less active chlorinenitrate. The balance between these twocounteracting mechanisms is probably a fine one, determined by small differences in wintertime stratospherictemperatures. If the apparent cooling trend in the Arctic stratosphere is real, more dramatic ozone losses may occurin the future.

Description: The chemically induced ozone loss inside the Arctic vortex during the winter 1994/95 has beenquantified by coordinated launches of over 1000 ozonesondes from 35 stations within the Match94/95 campaign. Trajectory calculations, which allow diabatic heating or cooling, were used totrigger the balloon launches so that the ozone concentrations in a large number of air parcels areeach measured twice a few days apart. The difference in ozone concentration is calculated foreach pair and is interpreted as a change caused by chemistry. The data analysis has been carriedout far January to March between 370 K and 600 K potential temperature. Ozone loss along thesetrajectories occurred exclusively during sunlit periods, and the periods of ozone loss coincidedwith, but slightly lagged, periods where stratospheric temperatures were low enough for polarstratospheric clouds to exist. Two clearly separated periods of ozone loss show up. Ozone lossrates first peaked in late January with a maximum value of 53 ppbv per day (1.6 % per day) at475 K and faster losses higher up. Then, in mid-March ozone loss rates at 475 K reached 34 ppbvper day (1.3 % per day), faster losses were observed lower down and no ozone loss was foundabove 480 K during that period. The ozone loss in hypothetical air parcels with average diabaticdescent rates has been integrated to give an accumulated loss through the winter. The most severedepletion of 2.0 ppmv (60 %) took place in air that was at 515 K on 1 January and at 450 K on20 March. Vertical integration over the levels from 370 K to 600 K gives a column lass rate,which reached a maximum value of 2.7 Dobson Units per day in mid-March. The accumulatedcolumn loss between 1 January and 31 March was found to be 127 DU (similar to 36 %).

Description: A Lagrangian approach has been used to assess the degree of chemically induced ozone loss inthe Arctic lower stratosphere in winter 1991/1992. Trajectory calculations are used to identify airparcels probed by two ozonesondes at different points along the trajectories. A statistical analysisof the measured differences in ozone mixing ratio and the time the air parcel spent in sunlightbetween the measurements provides the chemical ozone loss. Initial results were first describedby von der Gathen et al. [1995]. Here we present a more detailed description of the technique anda more comprehensive discussion of the results. Ozone loss rates of up to 10 ppbv per sunlit hour(or 54 ppbv per day) were found inside the polar vortex on the 475 K potential temperaturesurface (about 19.5 km in altitude) at the end of January. The period of rapid ozone loss coincidesand slightly lags a period when temperatures were cold enough for type I polar stratosphericclouds to form. It is shown that the ozone loss occurs exclusively during the sunlit portions of thetrajectories. The time evolution and vertical distribution of the ozone loss rates are discussed.