A radical mechanism for methane buildup.

A radical mechanism for methane buildup

Two scientists suggest a link between the atmospheric buildup of methane--an important greenhouse gas--and a shortage of a highly reactive molecule called the hydroxyl radical. Their surprising finding indicates that hydroxyl depletion in the Northern Hemisphere is about twice as severe as previously believed.

Hydroxyl acts as an atmospheric cleanser, breaking down a variety of pollutants. One of these is methane, which ranks second only to carbon dioxide as a contributor to the greenhouse effect. When methane reacts with hydroxyl, it forms a much less stable compound that dissipates quickly.

Theoretically, hydroxyl should help limit the greenhouse effect by shortening methane's lifespan in the atmosphere, notes Jim Kao, an atmospheric scientist at Los Alamos (N.M.) National Laboratory. Yet global methane levels are currently increasing by about 1 percent a year. Scientists have yet to fully account for that rise, but Kao asserts that a shortage of hydroxyl may help explain it.

In the new study, Kao and Xuexi Tie used a three-dimensional theoretical model, verified with measurements of atmospheric chemicals from stations scattered worldwide. The results, initially discussed at a climate symposium in December, suggest that the Northern Hemisphere has about one-fourth as much hydroxyl as the Southern Hemisphere. Kao says the northern depletion could be tied to the preponderance of industrial polluters there.

Kao and Tie announced their finding earlier this month in a press release. Several other atmospheric scientists maintain that the Los Alamos researchers should have delayed publicizing their unusual results until the study passed peer review and appeared in a scientific journal. For one thing, notes Clarisa M. Spivakovsky of Harvard University, a similar study using a different 3-D model -- reported in the Oct. 20 JOURNAL OF GEOPHYSICAL RESEARCH--did not support severe hydroxyl depletion.

Spivakovsky, who led the earlier study, remains skeptical of Kao's findings. Nonetheless, she says, if levels of hydroxyl have dwindled, this would help explain the rise in methane.

The widely varying estimates may stem in part from hydroxyl's unstable nature. With a lifetime of only a few seconds or less, the compound is extremely difficult to measure, so scientists must make indirect estimates using complex theoretical models and relying on supercomputers to make sense of the vast amount of information. Kao says his results may reflect his model's higher resolution, which subdivided data into 20 atmospheric layers instead of nine.

Researchers need better hydroxyl estimates because the compound could have important effects on greenhouse gases and other pollutants, says Jose Rodriguez of Atmospheric and Environmental Research, Inc., in Cambridge, Mass. While he questions the early release of the Los Alamos results, he adds that a depletion "could well be happening."

"You would expect hydroxyl to be decreasing," Rodriguez says. "The question is how much."