Magazine article Science News

A Sign of Healing Appears in Stratosphere

Magazine article Science News

A Sign of Healing Appears in Stratosphere

Article excerpt

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. …

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