A sign of healing appears in stratosphere.

Satellite measurements indicate that the amount of harmful chlorine pollution in Earth's stratosphere has started to decline--a sign that the ozone layer will soon begin its slow recovery from 70 years of chemical assault.

The observation, reported this week at a meeting of the American Geophysical Union in San Francisco, demonstrates the success of the Montreal Protocol, a 1987 treaty that forced countries to curb their use of chlorofluorocarbons (CFCs) and other ozone-destroying compounds. "What we were able to conclude is that, yes, the protocol is working. This is significant because it brings closure," says James M. Russell of Hampton (Va.) University.

Soon after the invention of CFCs in 1928, companies started mass-producing these nontoxic, nonflammable compounds for use as refrigerants and then for myriad other purposes. It took nearly 50 years before scientists recognized that these extremely stable chemicals could survive long enough to drift up to the stratosphere, where they would then display a nasty side. At that height, above 10 kilometers, CFCs and some other gases split apart and loose their destructive chlorine or bromine in the midst of the ozone layer--the shield that protects Earth's surface by absorbing harmful ultraviolet light.

Scientists detected the first step toward global recovery from chlorine pollution in 1.996. Measurements made in the troposphere--the lowermost layer of the atmosphere--indicated that chlorine concentrations there had peaked between 1992 and 1994 and were slowly starting to decline (SN: 3/9/96, p. 151).

Yet chlorine was still increasing in the most important place, the stratospheric layer where ozone resides. The reason is that it takes several years for air from the troposphere to leak up into the stratosphere. Researchers were unclear when stratospheric chlorine would start to decline.

Russell's team monitored the situation with a satellite instrument called the Halogen Occultation Experiment, which stares at the sun's rays as they pass through the atmosphere. The sensor measures how much light gets absorbed by hydrogen chloride, which contains more than 90 percent of the chlorine at the stratosphere's top. The data showed chlorine concentrations peaking in 1997.

Confirmation of this observation comes from measurements made high in the Swiss Alps. Spectrometers there record the concentration of hydrogen chloride and chlorine nitrate, which together account for 95 percent of the total chlorine in the stratosphere. The data from that site also reveals stratospheric chlorine topping out in 1997, says Rodolphe Zander of the University of Liege in Belgium, leader of the alpine team.

The stratosphere's chlorine concentration rises and falls naturally, so scientists had to wait sufficient time to be sure that it had truly peaked. "You always need a few more years to convince yourself that you have seen the maximum," says Zander.

The main factor contributing to the chlorine turnaround was a reduction in the emissions of methyl chloroform, a cleaning solvent. This compound breaks down in the atmosphere far faster than CFCs, so the amount of chlorine in the air responds quickly to changes in the solvent's usage, which developed countries curtailed in 1995.

Many ozone-harming compounds--such as bromine-rich chemicals--continue to increase in the atmosphere, some faster than scientists had expected. Still, when scientists total up all the pollutants and account for their potency, they conclude that the tide has turned for ozone destruction, says Zander.

It may take another decade before the first signs of ozone recovery begin to surface. Convalescence will last beyond 2050, according to computer calculations.