Earth's atmosphere has a certain ability to clean itself, but it hasn't been doing such a great job of it during the past decade, according to a study published today in Science. Ronald Prinn from the Massachusetts Institute of Technology and colleagues from around the world base their conclusion on an analysis of the hydroxyl radical (OH) in the atmosphere. This self-cleaning agent promotes the destruction of such pollutants as nitrogen dioxide, sulfur dioxide and carbon monoxide, and also helps fortify the ozone layer.

Because the hydroxyl radical is short-lived and cannot be measured directly in the atmosphere, researchers examine long-term measurements of trace gases, such as methyl chloroform, that are primarily broken down by OH. In this case, they collected data at test stations in remote coastal areas, establishing a 21-year record of methyl chloroform levels. It revealed that that OH levels had risen between 15 and 22 percent between 1979 and 1989, and had then fallen by 10 to 24 percent until 2000. The record also indicated that OH-levels in the Southern Hemisphere were 14 to 34 percent higher than in the north. The reasons remain unclear, but the researchers suggest that human activity is at least partially at fault.