Signs of success with CFCs.

Monitoring stations around the globe have detected a substantial slowdown in the atmopheric buildup of the two prime ozone-destroying compounds. The measurements suggest that atmospheric concentrations of these chlorofluorocarbons (CFCs) will peak before the turn of the century and then start to decline, allowing the stratospheric ozone layer to begin the slow process of repairing itself.

The CFC measurements come from a network of sampling stations spread from Point Barrow, Alaska, to the South Pole. The data show that during the late 1980s the two most abundant CFC's, called 11 and 12, accumulated at a rate of 11 and 19.5 parts per trillion (ppt) per year, respectively. Since 1988, however, that rate has declined dramatically, says James W. Elkins of the National Oceanic and Atmospheric Administration in Boulder, Colo. In the Aug. 26 NATURE, Elkins and his colleagues report that the annual accumulation rate has dropped to 2.7 ppt for CFC-11 and 10.5 ppt for CFC-12.

The two compounds are used mostly in refrigeration, air conditioning, and foam insulation. Some manufacturers outside the United States use CFC-11 in aerosol spray cans, a practice banned in the United States in the late 1970s. Through a treaty called the Montreal Protocol, most nations have agreed to cease production and use of CFCs of CFCs by 1993 and eliminate some other ozone-destroying chemicals at later dates.

Elkins says he was surprised to find a slowdown in CFC accumulation during the late 1980s, even before the limits under the Montreal Protocol took effect. However, companies that manufacture CFCs have reported that they did reduce production of such chemicals in the late 1980s, in response to decreased demand.

Past studies have indicated that it would take 50 to 100 years for reactions in the atmosphere to reduce the concewntrations of ozone-destroying chlorine and bromine back to natural levels. Such chemicals will continue to erode the global ozone layer and open up an ozone hole over Antarctica for decades, says Elkins.