NASA identifies cause of ozone depletion.

Some people call it a doomsday myth. Others say it's a real global threat. The debate about the destruction of Earth's protective ozone layer centers on whether man-made chemicals called chlorofluorocarbons (CFCs), used mainly in refrigeration and air conditioning, are eating it away.

According to the ozone-thinning theory, CFCs release chlorine into the stratosphere, the upper region of Earth's atmosphere, leading to ozone destruction and exposing the planet to harmful ultraviolet rays. Critics maintain that the chlorine in the atmosphere comes from natural sources such as volcanic eruptions and causes no permnent damage.

Now, NASA researchers say they have collected evidence that points a definitive finger at CFCs. On the basis of data gathered by the agency's Upper Atmospheric Research Satellite (UARS), they find hydrogen fluoride -- a CFC by-product that has no natural source -- present with chlorine in the stratosphere. NASA presented its findings at a Dec. 19 press conference.

"The detection of stratospheric fluorine gases, which are not natural, eliminates the possibility that chlorine from volcanic eruptions or some other natural source is responsible for the ozone hole over the Antarctic," says Mark Schoeberl, a NASA physicist on the UARS project.

Natural chlorine accounts for only about 20 percent of the chlorine found in the stratosphere, Schoeberl explains, because most of it dissolves in water and falls out as rain before reaching the ozone layer. In contrast, CFCs, which are insoluble, eventually rise to the ozone layer, where they can survive for decades. Once in the stratosphere, the CFC molecules break apart. Chlorine atoms attach to oxygen atoms and form chlorine monoxide, which erodes the ozone.

Global ozone loss reaches about 3 percent per decade, considered fairly significant by most scientists. The ozone hole that forms over Antarctica from May to October each year remains the clearest example of ozone depletion. Schoeberl and his colleagues attribute this phenomenon both to the CFCs and to the extreme cold of the region.

Schoeberl says NASA will continue to analyze the UARS data for indications of ozone depletion in other areas.

"UARS has seen high levels of chlorine monoxide in the Northern Hemisphere," says Schoeberl, "but with the warmer temperatures, the high levels do not last as long over the Arctic." Low temperatures and sunlight are key parts of the chemical reaction that allows CFCs to attack the ozone, he explains, making the dramatic ozone loss seen over the South Pole unlikely elsewhere."

NASA launched UARS in 1991 to compile a database about Earth's atmospheric chemistry, processes controlling ozone in the stratosphere, the effect of the sun's radiation on the atmosphere, and other atmospheric phenomena.