New measurements show that the atmosphere?s ability to keep itself clean is decreasing. In the past 10 years, levels of an important cleansing chemical have declined markedly in the Northern Hemisphere.

Chemicals are generated in the air by forces such as ultraviolet light and lightening. These chemicals are highly reactive and tend to remove polluting gases. One of the most important of these is the hydroxl radical, OH, which is the water molecule minus one hydrogen atom. The missing hydrogen atom makes it highly reactive so that it easily sucks up pollutants. OH changes pollutants into gases that are easily dissolved in water and thus are removed from the atmosphere during rainfall.

?If the trend we?ve seen in the past 10 years continues,? says atmospheric chemist Ronald Prinn, ?it is something to be deeply concerned about. We may be decreasing the atmosphere?s ability to clean itself.? According to Prinn, OH levels in 2000 were 10 percent below 1979 levels.

The decline in OH may be due to the burning of coal, gas and oil, the clearing of forests (since trees use up carbon dioxide) and the release of chemicals into the air. The decrease is greater in the Northern Hemisphere, where most of the world?s manufacturing takes place.

?It is certainly true that the atmosphere has a limited ability for how much stuff you can dump into the air before it starts choking,? says Chris Cantrell of the National Center for Atmospheric Research. ?It would be my impression that the atmosphere is starting to show signs of being overtaxed.?

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