Ozone-destroying forms of chlorine existed for much longer in the Arctic stratosphere this winter than last, according to scientists at NASA's Jet Propulsion Laboratory.

Northern Hemisphere ozone abundance was also observed to be about 10 percent below that measured during the same period last year, with some regions 20 percent lower.

Using their microwave limb sounder aboard NASA's Upper Atmosphere Research Satellite (UARS) launched in September 1991, Dr. Joe Waters and his colleagues at JPL and Edinburgh University have collected daily maps of ozone and other gases and of temperature in different layers of the stratosphere. One of their most critical measurements is chlorine monoxide, a form of chlorine that destroys ozone. They reported results in the international scientific journal Nature.

"Ozone in the Arctic in a layer about 20 kilometers (12 miles or 66,000 feet) high, where most chlorine monoxide was located, decreased by 0.7 percent per day from mid-February through early March 1993," Waters said. Ozone normally increases in this area at this time of year, he added.

Chlorine already in the stratosphere, from industrial chlorofluorocarbons (CFCs), is converted to ozone-destroying forms by chemistry occurring on clouds which form at low temperature.

Last year the scientists measured large abundances of chlorine monoxide in the Arctic, but the abundance decreased after the stratosphere warmed in late January. This winter the stratosphere remained cold through February, and chlorine monoxide remained abundant through early March. About as much chlorine monoxide was seen in the north in February 1993 as was measured in the south before the 1992 Antarctic ozone hole formed.

"We do not see an obvious Arctic ozone hole," Waters said, "but the smaller abundances seen throughout the Northern Hemisphere this winter raise the question of whether the chlorine destruction of ozone has been diluted over a wide area." Record low values of ozone have also been reported recently by the World Meteorological Organization and Environment Canada.

The microwave limb sounder was developed and is operated by a team at JPL, led by Waters and sponsored by NASA, with additional members at Edinburgh University, Heriot-Watt University and the Rutherford-Appleton Laboratory, in the United Kingdom. The Upper Atmosphere Research Satellite, launched Sept. 12, 1991, is managed by NASA's Goddard Space Flight Center.

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