OZONE-ERODING CHEMICALS SEEN ON WANE.

Byline: Associated Press

Ozone-destroying chemicals are declining in the atmosphere for the first time, according to researchers who say that means the ozone hole high above the Earth could start closing within 10 years.

``This is the very beginning of a change,'' said Stephen A. Montzka, a National Oceanic and Atmospheric Administration scientist. ``This is the first step toward the goal of closing the ozone hole.''

The high-altitude natural ozone layer forms a barrier against ultraviolet radiation from the sun. Excessive levels of UV radiation cause skin cancer in humans and damage many plants and animals.

Montzka said ground-level measurements on three continents and on two Pacific Ocean islands detected a reduction in the concentration of the group of industrial chemicals that erode the ozone layer. A report on the study is being published today in the journal Science.

``A detectable signal for ozone recovery is expected around 2005 or 2010,'' said Montzka, a researcher in the NOAA laboratory in Boulder, Colo., and the first of eight co-authors of the study.

Thinning of the ozone layer in the stratosphere is thought to be caused by some industrial compounds containing chlorine and bromine. The most common chemicals include chlorofluorocarbons (CFCs), halons and chlorine-based solvents that have been used for more than 30 years as refrigerants, cleaning agents, spray propellants, foams and fire extinguishers.

Scientists discovered in the 1980s that the gaseous chemicals released in the ground-level atmosphere eventually migrate high above the Earth to the stratosphere, where they trigger a chemical reaction that destroys ozone molecules.

Erosion of the ozone layer has been particularly severe over the South Pole. In recent years, measurements by satellite and ground instruments found that ozone concentrations declined to a third or less of normal in the polar region, forming what has been called ``an ozone hole.''

In 1987, 23 nations meeting in Montreal signed an agreement to gradually phase out manufacture and use of ozone-destroying chemicals. Amendments later added more chemicals to the list, and Montzka said tests now show that the Montreal protocol is beginning to have an effect.

Richard Stolarski, an atmospheric scientist at NASA's Goddard Space Flight Center in Greenbelt, Md., said the NOAA study is a key finding in the worldwide monitoring of industrial chemicals in the atmosphere.

``It is particularly significant now that they have seen a peaking and a turnaround'' in the concentration of the chemicals, he said. He cautioned that the trend will have to continue for a decade or more before the ozone layer in the stratosphere starts to recover.