WASHINGTON, D.C. -- Ozone depletion in the Arctic has been less extensive than the corresponding phenomenon in the Antarctic, but it has been observed during the late 1990's and reached record high levels during the winters of 1995-1996 and 1996-1997. The latter winter was also marked by an extremely long-lived Arctic polar stratospheric vortex, a low pressure system occurring between 14 and 35 kilometers [8-21 miles] altitude. The vortex is formed during the polar winter, when the lack of sunlight lowers the temperature and produces a circular wind system. Atmospheric scientists believe that the formation of the vortex is a key step in providing the chemical conditions for ozone depletion.
Generally, the Antarctic is colder than the Arctic, causing more frequent stratospheric clouds of the type that initiate ozone depletion. In 1996-1997, however, the Arctic vortex produced low temperatures, and ozone depletion as great as that recorded in the Antarctic during the early 1980s. Researchers Georg Hansen of the Norwegian Institute for Air Research in Tromsoe, Norway, and Martyn P. Chipperfield of the University of Cambridge in the United Kingdom studied this vortex intensively in an effort to confirm the exact causes of ozone depletion and to evaluate the effectiveness of existing models. Their research will be reported in the January 20 issue of the Journal of Geophysical Research, published by the American Geophysical Union.
Hansen and Chipperfield took advantage of the fact that the 1996-1997 vortex was often centered on the North Pole, with its edge over northern Scandinavia, allowing intensive observation from the Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR) on the Norwegian island of Andoeya, located at 69 degrees north latitude. In 1995-1996 and 1996-1997, overall ozone depletion reached a maximum of 48 percent below the long term average and up to 60 percent at the most affected altitude, 20 kilometers [12 miles].
Furthermore, the 1996-1997 vortex lasted exceptionally long, into early May, extending the period of reduced ozone levels. This led in turn to increased ultraviolet radiation in northern Europe. Hansen and Chipperfield report that until the end of March, the established wintertime chemical action involving chlorine activation in polar stratospheric clouds was the principal cause of ozone depletion. After that and into early May, the further depletion was caused by chemical interactions of nitric oxide [NO] and nitrogen dioxide [NO2] chemistry, which was made possible by the unusual persistence of the vortex.
The researchers say their studies have helped them to evaluate and refine the models they use for studying and predicting ozone depletion. Further research and observations are also important, they say, and this winter (1998/99) the European Union is funding THESEO, an extensive measurement campaign that will include further observations at Andoeya. The mechanisms detected there could be increasingly important if prolonged winter polar vortices become the norm due to changing climatic conditions. This would in turn create the potential for greater air masses with low ozone to persist, the depletion caused either by early spring halogen chemistry or by summertime nitrogen oxide [NOx] chemistry. It would result in increased levels of ultraviolet radiation penetrating into the mid-high latitudes of the Eurasian land mass and North America, the researchers say.
Materials provided by American Geophysical Union. Note: Content may be edited for style and length.
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