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A better forecast. Levels of ozone-eating bromine in the lower atmosphere are declining faster than predicted.

Ozone Killer Tamed

There's good news for the ozone layer--levels of ozone-destroying bromine have been dropping in the lower atmosphere since 1998, thanks to tighter regulation. The drop is larger than scientists had predicted, meaning that either natural sources of bromine also declined or that industry was responsible for a larger fraction of atmospheric bromine than experts thought.

Bromine destroys ozone 50 times more efficiently than chlorine, the most infamous player in ozone destruction. Luckily, there is far less bromine in the atmosphere. The gas originates from natural sources such as oceans and wetlands, as well as from human activity. Methyl bromide, used in agriculture, and halons, which are used to fight fires, are the principal humanmade sources of the chemical, and their use helped double the amount of bromine in the atmosphere from the mid to the late 20th century. Twenty-three countries began regulating methyl bromide in the late 1980s and since then more than 100 other countries have ratified the legislation, called the Montreal Protocol, and strengthened restrictions on manufacture and use.

To see if these regulations affected bromine concentrations, atmospheric chemist Stephen Montzka of the National Oceanic and Atmospheric Administration in Boulder, Colorado, and colleagues analyzed air samples taken several times each month from 10 land-based sites across the globe between 1995 and 2002. They report in the 15 August issue of Geophysical Research Letters that the total amount of bromine-containing compounds peaked in 1998 and has since declined by 5%. The measures don't reveal exactly how much bromine is in the upper atmosphere, where the ozone layer sits, but they can be used to predict how much of the ozone-destroying chemical will eventually make its way there. The timing of the 1998 peak suggests that curbs on industrial production were responsible, at least in part, for the decline, but the magnitude of the drop has baffled scientists because the best models had predicted a smaller decline.

"It's a great paper because it surprises us," says atmospheric chemist Michael Prather at the University of California, Irvine. "It's encouraging to see environmental [progress] through regulations." Now, he says, scientists must figure out whether natural or industrial changes are responsible for unexpectedly large gains.

Related sites
Montzka's site
FAQs about ozone depletion (PDF)
A 2002 assessment of ozone depletion by the World Meteorological Organization