Drop in ozone killers means global gain.

Measurements of ozone-destroying compounds in the atmosphere show these chemicals declining in abundance for the first time-a significant step toward the eventual recovery of the life-sustaining ozone layer.

The reported turnaround comes 9 years after the signing of the Montreal Protocol, an international treaty that restricts the use of ozone-harming chemicals containing chlorine and bromine. The protocol and its subsequent amendments are gradually weaning the world from a half century of reliance on chlorofluorocarbons (CFCs) and other halocarbons.

According to measurements made at seven sites around the globe, total concentrations of chlorine and bromine compounds in the lower atmosphere peaked recently and have now started to drop, reports Stephen Montzka of the National Oceanic and Atmospheric Administration (NOAA) in Boulder, Colo. He described these results at the annual meeting this week of NOAA's Climate Monitoring and Diagnostics Laboratory.

Chlorine pollution from industrial compounds topped out at the start of 1994 and has decreased at 1 percent per year since then. That decline is strong enough to outweigh ongoing increases in bromine concentrations, even though each bromine atom has 40 times the destructive potential of a chlorine atom, says Montzka. Bromine in the atmosphere comes principally from halons and methyl bromide.

The observations come as welcome news to governments, environmental groups, and chemical manufacturers and users, all of whom have worked toward implementing the protocol and its amendments. Despite fears that certain countries might renege on their pledges or illicitly produce banned chemicals, the atmospheric concentrations reflect general compliance with the treaty. "We'd expect this [turnaround] to occur as long as everything went according to the book," says Montzka.

Another group of scientists, however, questions whether the reversal has occurred. Ronald G. Prinn of the Massachusetts Institute of Technology and his colleagues monitor chlorine and bromine compounds using a separate set of global stations. Their work shows that the atmospheric abundance of chlorine has reached a plateau, but they cannot say with confidence that concentrations have started to drop significantly.

Even if the destructive potential of the lower atmosphere has started to decline, ozone recovery will lag behind. It will take 2 or 3 years for recent trends to filter up to the stratosphere, home of the ozone layer. If ozone healing begins near the end of the century, convincing signs of improvement may not appear for another decade, says Charles H. Jackman of NASA's Goddard Space Flight Center in Greenbelt, Md. Because globally averaged ozone concentrations swing up and down naturally from year to year, scientists must wait until they see those concentrations climb by several percent before they can identify dividends attributable to the Montreal Protocol. "If everything goes well, by 2010 we should see that ozone has recovered enough" to be confident about the progress, says Jackman. The Antarctic ozone hole, however, will continue to reappear each southern springtime until around the year 2050, he adds.