WASHINGTON -- Scientists at NASA's Goddard Space Flight Center say they have the first conclusive evidence that the Earth's protective ozone layer is being eroded by manmade chemical products and not by natural events like volcanic eruptions.


Data gathered by the National Aeronautics and Space Administration's Upper Atmosphere Research Satellite, they said, point to ozone-destroying chlorine in the stratosphere over Antarctica, along with fluorine. It is a coincidence, the scientists said at a news briefing Monday, that can be explained only by the breakdown of chlorofluorocarbons -- a compound that contains both elements, and the chief suspect in ozone destruction.


"We don't see any alternative (explanation)," said Dr. Mark Schoeberl, project scientist at Goddard's Laboratory for Atmospheres. "The data confirm that chlorofluorocarbons are the major source of chlorine in the stratosphere and are the cause of the ozone loss we are seeing (there)."


"This is nailed," said his deputy, Dr. Anne Douglass. "There is no other possibility."


John Passacantando, executive director for Ozone Action, a Washington-based environmental group, welcomed the Goddard findings and urged accelerated international action to stop the manufacture and use of CFCs and their replacement by products less damaging to the ozone layer.


Schoeberl, however, said such measures are too costly to warrant moving faster. "I think if we stick to what we planned, that's pretty good, and the CFCs are going to come down," he said.


Chlorofluorocarbons are compounds invented in the 1930s as a safer alternative to the ammonia that was at the time used in refrigerators. The compounds have been used since as refrigerants, propellants in spray cans, and as cleaning agents.


Ozone is a three-atom form of oxygen. When formed naturally in the upper atmosphere, it acts as a barrier to harmful ultraviolet radiation from the sun.


When scientists noticed in the 1970s that the ozone layer seemed to be thinning each spring over the Antarctic, they began to suspect it was being broken down in a seasonal chemical reaction with chlorine faster than it could be replaced by nature.


As scientific support for the theory grew, governments responded. Since a 1987 treaty called the Montreal Protocol, 87 nations have agreed to halt production of CFCs by Jan. 1, 1996.