Determination of Stratospheric Polar Ozone Losses (Match)
Starting in the early 1990ties we determine the chemical ozone depletion in the polar stratosphere using ozone soundings by means of a sophisticated approach called Match.
The ozone abundance above an individual polar station changes constantly due to variable transport processes. Such dynamically caused fluctuations mask anthropogenic chemical loss and have to be separated from any chemical ozone loss signal. Instead of observing time series of ozone at fixed locations (i.e. in an Eulerian sense) the Match approach is based on Lagrangian measurements. The basic idea is to perform repeated measurements in individual air masses as they drift across the polar cap and sometimes happen to approach one of many ozonesonde stations distributed in the polar region. The principle advantage of this procedure is that the advection terms which dominate ozone changes in the Eulerian framework disappear in the Lagrangian formulation of the continuity equation. The impact of diffusion terms in the continuity equation can also be limited by careful selection of the observed air masses based on properties of the flow. Hence, changes in the concentration of ozone during the time interval between two measurements can be attributed to a chemical depletion. The statistical analysis of a very large number of such pairs of measurements it is possible to directly observe anthropogenic chemical ozone depletion and to measure chemical ozone loss rates in situ.
In the Match approach the measurements are performed using ozonesondes launched at many polar and sub-polar stations. To identify “Match events” – situations where individual air masses were probed twice usually at different stations - trajectories are calculated based on ECMWF data (cf. Figure). For 1991/92 Arctic winter the Match events have been identified after about 1200 ozonesondes were launched during the winter without real time coordination (Rex, 1993; von der Gathen et al., 1995). Since the winter 1994/95 Match events are actively produced by triggering several hundred ozone soundings per winter in real time coordination during the campaign. During a Match experiment typically 300 to 600 ozonesondes are launched at about 30 stations in the northern hemisphere or at 9 stations in the Antarctic. Data from satellites are also used. But due to large vertical shear in the flow, this is limited to satellite sensors that can provide high vertical resolution data (1-1.5 km vertical resolution or better) and data from POAM, ILAS and SAGE have been successfully used. Since 1994/95 we coordinated actively in most Arctic winters and in two Antarctic winter the ozonesoundings in the polar and subpolar regions. Between 1991/92 and 1993/94 this approach had been used passively in the Arctic.
A number of important steps in ozone research rely on the Match approach. Match found for the first time unambiguous evidence for anthropogenic chemical ozone loss in the Arctic stratosphere. Further, based on Match in-situ data it was shown that sunlight is indeed needed for the ozone loss process, which confirms an important element of our theoretical understanding of the process. It was also demonstrated that denitrification has the potential to worsen ozone loss also in the Arctic, if a winter is particularly cold (Rex et al., 1997). Previously this effect was only known from the Antarctic stratosphere. Multi-annual data from the Match campaigns allowed establishing the sensitivity of Arctic ozone loss on changes in climate – a key parameter for future projections of polar ozone loss in the changing atmosphere.
Particularly two results of the research over the past decade led us decide to continue the Match activity. First, since the conditions in the air masses that are sampled by Match are very well characterized, the Match products are particularly suited for detailed comparisons with highly constrained model calculations. This allows a close inspection of the degree of our theoretical understanding of the ozone loss process. Recently large uncertainties of our theoretical understanding of the kinetics of the relevant chemical processes became apparent. Continued Match measurements can help to provide a better observational basis for reducing these uncertainties. Second, although warm and cold Arctic stratospheric winters occur driven by internal variability of the climate system, a general tendency that the cold Arctic winters became significantly colder over the past 40 years has been observed. This change in the climate conditions in the Arctic stratosphere contributed to large Arctic ozone losses since the mid-nineties. It is unclear whether this trend will continue in the future and what the effect on Arctic ozone might be. The latest new record of Arctic ozone losses observed by the Match campaigns took place in the winter 2010/11 (Manney et al., 2011).
Special Information for Station Staff
Some insight into the coordination work of a Match campaign can be found in our Match campaign manual.