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Severe stratospheric ozone depletion in the Arctic
Severe stratospheric ozone depletion in the Arctic
Keywords: European Commission, Ozone layer, Environment
The European Commission reports that ozone losses of over 60 per cent have occurred in the Arctic stratosphere near 18km altitude during one of the coldest stratospheric winters on record. This is one of the most substantial ozone losses at this altitude in the Arctic. Measurements from the largest international campaign ever investigating stratospheric ozone depletion have provided more insight into the processes that control stratospheric ozone. They have also reinforced concerns that the Arctic ozone may continue to decline despite the benefits of reductions in stratospheric chlorine levels (a result of the Montreal Protocol), due to the global climate change. Research Commissioner Philippe Busquin points out that "European cooperation within an international team has made it possible to achieve these research results. They will provide the best possible scientific advice to the regulatory process concerning ozone depleting substances in the framework of the Montreal Protocol and, equally important, to the citizens".
During the 1999/2000 winter, the 3rd European Stratospheric Experiment on Ozone (THESEO 2000) sponsored by European Union and the NASA sponsored SAGE III Ozone Loss and Validation Experiment (SOLVE) obtained measurements of ozone and other atmospheric gases and particles using satellites and aircrafts, large, small and long duration balloons, and ground-based instruments.
Scientists from Europe, the USA, Canada, Russia and Japan joined forces in mounting the biggest field measurement campaign yet to measure ozone amounts and changes in the Arctic stratosphere. The total amount of information collected by the THESEO 2000/SOLVE campaign is believed to be more extensive than any information collected by past polar measurement campaigns. Most of the measurements were made near Kiruna, Sweden, with additional measurements being made from satellites and through a network of stations at mid- and high northern latitudes.
During the winter of 1999-2000 large ozone losses were observed inside the Arctic stratospheric polar vortex. These ozone losses in the lower stratosphere have been observed by a number of European techniques based on ozonesondes and ground-based measurements developed in the past decade. At altitudes around 18km cumulative losses of over 60 per cent have occurred between January and March. These are among the largest chemical losses at this altitude observed during the 1990s. The effect on column ozone was slightly mitigated by the fact that ozone loss was less dramatic above 20km altitude. Satellite observations (e.g. by the ESA Global Ozone Monitoring Experiment (GOME)) showed a clear ozone minimum over the polar region during February and March. The average polar column amount of ozone for the first two weeks of March was 16 per cent lower than that observed in the 1980s.
The mixing of polar air into middle latitudes, both during the winter and as the polar vortex broke down in late March, influences ozone levels over the populated middle latitudes. Dilution of ozone-depleted air into middle latitudes is a major contributor to the long-term mid-latitude ozone decline, along with other chemical and dynamical processes. In March 2000, the World Meteorological Organisation Mapping Centre at the University of Thessaloniki reported that the mean column ozone amounts over Europe were 15 per cent below the pre-1976 average.
Results from THESEO 2000/SOLVE have reinforced the scientific concern that the recovery of the ozone layer may be delayed. Cooling of the stratosphere could be caused by increasing concentrations of greenhouse gases, by reduced concentrations of ozone in the stratosphere, or by changes in the dynamics of the stratosphere. Even as the stratospheric concentrations of chlorine and bromine decline, such a cooling and, possibly, a more stable Arctic vortex are currently predicted to prolong Arctic ozone depletion.
The ozone loss is directly related to the high concentrations of chlorine compounds that exist in the stratosphere.The principal chlorine compound involved in ozone loss is the chlorine monoxide molecule (ClO). Complementary measurements of ClO were made by instruments on aircraft, balloons, a satellite, and from the ground so that good spatial and temporal coverage was obtained. These instruments showed high ClO concentrations from January to March consistent with the large ozone loss measurements, as did measurements of other important chemical species such as bromine monoxide (BrO).
Polar stratospheric clouds (PSCs) are directly involved in the conversion of chlorine from benign molecular forms into the CIO molecule. PSCs were observed in very extensive portions over the Arctic region from early December to early March. The last PSCs were observed around 10 March.
The temperatures in the polar stratosphere were extremely low over the course of this last winter, which increases ozone losses. Polar stratospheric clouds can only form in these low temperature regions. At 20km on 28 January, the area covered by temperatures low enough to form these clouds was 14.8 million km2 - compared with the total surface area of Europe: just over 10 million km2. This is the largest areal coverage recorded in over 40 years of stratospheric analyses. The low temperatures first appeared in mid-November at about 24km, and persisted at lower altitudes into mid-March. The low temperatures generally resulted from a stratosphere that has been relatively undisturbed by large-scale tropospheric weather systems over the course of the winter.
Measurements in THESEO 2000/SOLVE were made using a large suite of instruments aboard several European aircraft - the German DLR Falcon, the French ARAT and Myste(c)re 20 and a Swiss Air Force Lear Jet - and on NASA's DC-8 and ER-2. Most of these planes were based in Kiruna during the campaign. Nearly 30 research balloons, carrying payloads weighing up to several hundred kilogrammes were launched from Esrange (The Swedish Space Research Centre), Kiruna, by teams from CNES, Esrange and NASA. Atmospheric readings made at the European network of over 30 stations of ground-based instruments have shown how the Arctic stratosphere evolved through the winter and measurements of ozone have been made at the WMO Global Atmospheric Watch network. In addition, ozone losses have been derived from over 600 ozonesondes launched from an international experiment coordinated by the Alfred Wegener Institute in Germany. All these measurements were complemented by observations from a number of satellite instruments including GOME.
In all, more than 500 international scientists, technicians and support workers were involved in the THESEO 2000/SOLVE experiment. The THESEO 2000/SOLVE campaign represents a new level of active cooperation between European, US and other national research scientists. This cooperation will be continued during the analysis of the measurements and there will be a joint science meeting held in Palermo, Sicily, in September 2000. Such scientific collaboration has been encouraged under the 1998 European Union/United States Science and Technology Cooperation Agreement.
THESEO 2000 consists of a core of 12 major EU funded projects within the Environment programmes of both Fourth and Fifth Framework programmes for Research and Technological Development. The EU has a major research programme on stratospheric ozone and UV-B which includes laboratory-based research into the fundamental principles of stratospheric chemistry, the ozone-climate interactions, the development of new devices to measure the atmosphere's composition, research into improving atmospheric chemical models and UV-B radiation field measurements. The research funded by the EC Research DG in THESEO 2000 is closely coordinated with, and substantially increased by, the national research programmes. European research on stratospheric ozone and UV-B makes a valuable contribution to the international research which underpins the Montreal Protocol.
Details available from THESEO 2000 and SOLVE Web sites: