Ozone losses of over 60 percent have occurred in the Arctic stratosphere near 60,000 feet (18 km) in one of the coldest winters on record. This is one of the worst ozone losses at this altitude in the Arctic.

Investigations into the Arctic stratosphere have provided better insights into the processes that control polar ozone. These insights considerably add to our ability to predict ozone levels in the future as chlorine levels decline as a result of the Montreal Protocol, and as greenhouse gases increase. Climate change in the stratosphere will likely enhance ozone losses in the Arctic winter in the coming decades as chlorine levels decrease.

During the 1999/2000 winter, the NASA sponsored SAGE III Ozone Loss and Validation Experiment (SOLVE) and European Union sponsored Third European Stratospheric Experiment on Ozone (THESEO-2000) obtained measurements of ozone, other atmospheric gases, and particles using satellites, airplanes, large, small and long duration balloons, and ground-based instruments.

Scientists from the United States joined with scientists from Europe, Canada, Russia and Japan in mounting the biggest field measurement campaign yet to measure ozone amounts and changes in the Arctic stratosphere. The activities were conducted from November 1999 through March 2000. The total amount of information collected by the SOLVE/THESEO 2000 campaign is greater than the information collected in any past polar measurement campaign. Most of the measurements were made near Kiruna, Sweden with additional measurements being made from satellites and a network of stations at mid and high northern latitudes.

During the winter of 1999-2000 large ozone losses were observed in the Arctic stratosphere. These lower stratospheric ozone losses were observed by a number of instruments and techniques, including a National Oceanic and Atmospheric Administration ozone instrument aboard the high altitude NASA ER-2 aircraft. "Measurements from the NASA ER-2 show ozone in the Arctic region decreasing by about 60 percent between January and mid-March," said ER-2 co-project scientist Dr. Paul A. Newman of NASA's Goddard Space Flight Center, Greenbelt, Md.

These measurements are comparable to the large chemical losses at this altitude observed in several winters in the mid-1990s. The effect on total column ozone was slightly mitigated by the fact that reductions in ozone were smaller above 66,000 feet (20 kilometers). Spacecraft observations by NASA's Total Ozone Mapping Spectrometer-Earth Probe showed a clear ozone minimum over the polar region during February and March. The average polar column amounts of ozone for the first two weeks of March were 16 percent lower than observed in the early 1980's.

Polar stratospheric clouds (PSCs) are necessary for the conversion of chlorine from benign molecular forms into the chlorine monoxide molecule which directly destroy ozone. PSCs were observed over very extensive portions of the Arctic region from early December to early-March. "We were somewhat surprised to see PSCs so early in December," said Dr. Mark Schoeberl, who was the SOLVE co-project scientist for observations made from NASA's DC-8 aircraft. "Some of the PSC types and their locations which we observed in December did not fit within our current understanding." The last PSCs were observed on March 8 by instruments aboard the DC-8, and on March 15 by satellite.

The polar stratosphere temperatures were extremely low over the course of this last winter. PSCs can only form in these low temperature regions. At 66,000 feet on Jan. 28, the area covered by temperatures low enough to form PSCs was 5.7 million square miles (14.8 million square kilometers), which is larger than the United States. This is the largest area coverage recorded in over 40 years of Northern Hemisphere stratospheric analyses.

"The polar stratospheric clouds covered a larger area, and persisted for a longer period of time, than for any other Arctic winter during the past 20 years. These conditions heighten our concern regarding possible couplings between climate change and stratospheric ozone depletion," said ozone researcher Dr. Ross Salawitch of NASA's Jet Propulsion Laboratory, Pasadena, Calif.

The mixing of polar air into middle latitudes, both during the winter and as the polar circulation broke down in late March, influences ozone levels over the populated middle latitudes. Dilution of ozone depleted air into latitudes is a major contributor to the long-term mid-latitude decline. These mixing processes have been studied during SOLVE/THESEO-2000 and detailed analysis of these processes continues.

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