Ozone-killing bromine is on the decline.

Chemical analyses of Earth's lower atmosphere show that the overall concentration of bromine, a component of many potent ozone-destroying chemicals, has dropped about 5 percent since reaching a peak in 1998. Besides being a promising sign for Earth's beleaguered ozone layer, the decline validates the decision by many countries to regulate the production and use of the predominant bromine-releasing compound, some scientists say.

Half the bromine that makes its way into the upper layers of the atmosphere comes from methyl bromide, says Stephen A. Montzka of the National Oceanic and Atmospheric Administration's Climate Monitoring and Diagnostics Laboratory in Boulder, Colo. Much production and use of that chemical, often as an agricultural fumigant, is being phased out under the Montreal Protocol on Substances that Deplete the Ozone Layer as amended in 1997.

The researchers estimated the recent slump in lower-atmospheric bromine using data gathered up to four times each month at 10 remote sites worldwide. They also found that a 13 percent decline in methyl bromide concentrations in the lower atmosphere since 1998 was responsible for much of the bromine drop.

Unlike other ozone-destroying chemicals, methyl bromide has natural sources, including fungi that surround microscopic plant roots (SN: 12/22&29/01, p. 389), wetlands, and burning vegetation.

Nevertheless, says Montzka, the new research suggests that the bromine decline stems largely from reductions in manufactured methyl bromide. For one thing, lower-atmosphere bromine concentrations measured in the Northern Hemisphere are decreasing at twice the rate that they are at Southern Hemisphere sites. Also, concentrations of the gas began to drop soon after industrial production of methyl bromide decreased in the late 1990s.

Finally, the concentration of methyl chloride--a similar gas that has only natural sources--showed no long-term decrease over the same period. Montzka and his colleagues report their findings in the Aug. 15 Geophysical Research Letters.

Although scientists were already convinced that methyl bromide is the biggest contributor to bromine concentrations in Earth's upper atmosphere, their uncertainty about the magnitude and the variability of natural sources of methyl bromide made it "a bit of a gamble" to restrict the production and use of the gas, says Steven C. Wofsy of Harvard University. However, he notes, that gamble seems to have paid off.

Methyl bromide lasts just a few months in the atmosphere before it breaks down, but bromine and other byproducts continue to waft upward and, after a few years, reach the stratospheric ozone layer, says Mike Newchurch of the University of Alabama in Huntsville. If bromine concentrations peaked in the lower atmosphere in 1998, then they should soon be declining up in the ozone layer, if they aren't already.

"This [decrease in atmospheric bromine] is nothing but good news for the ozone layer," notes Newchurch. "It's healing the atmosphere where it matters."