Call it the mystery of the missing radicals.
For nearly a decade, their ranks grew. Nine years ago, however, they gradually began to disappear. While a majority of these radicals remain active, their thinning ranks puzzle a small band of people dedicated to tracking them.
Is the disappearance natural? Or do more-troubling circumstances lie behind it?
These are some of the questions atmospheric scientists are asking in the wake of a recent study documenting the worldwide decline of a powerful chemical ally in the battle against air pollution.
Simple molecules - one part oxygen and one part hydrogen (OH) - are vanishing from the atmosphere. Known as hydroxyl radicals, these molecules play a key role in forming and breaking down smog. They also neutralize heat-trapping greenhouse gases and chemicals that destroy the protective layer of ozone in the upper atmosphere.
From 1978 to 1992, the researchers report, global average concentrations of hydroxyl radicals in the atmosphere grew by about 15 percent. Then they began falling rapidly. By 2000, concentrations had dropped to about 10 percent below 1979 levels. Within the averages, the research team found another puzzle: Hydroxyl radical concentrations in the Southern Hemisphere typically run about 14 percent higher than in the Northern Hemisphere.
"The results are a surprise," notes Ronald Prinn, lead author of the study and head of the department of earth, atmospheric, and planetary sciences at the Massachusetts Institute of Technology in Cambridge.
Current theories describing the behavior of hydroxyl radicals in the atmosphere, he explains, fail to account for the North-South differences as well as for the long-term rise and fall of concentrations.
"This is a tremendous piece of work," says Michael Trainer, an atmospheric chemist at the National Oceanic and Atmospheric Administration Aeronomy Laboratory in Boulder, Colo. "It's crucial to document what OH does on a global scale and how it changes."
The reason: The molecule "is the principal atmospheric scrubber," says Fred Fehsenfeld, also with NOAA's Aeronomy Lab.
Moreover, a debate rages over how human activities may be affecting it.
Researchers say that some computer-based studies show OH declining steadily since the dawn of the Industrial Age, as rising pollution began to swamp the rate at which OH forms. Other studies suggest that man-made pollution - laced with OH-forming chemicals such as ozone and nitric oxide - should increase OH levels.
If the atmosphere's cleanser is vanishing, for example, then heat-trapping greenhouse gases such as methane could build faster and last longer than current forecasts suggest.
Naturally occurring OH forms when ozone and water molecules react in the presence of sunlight, Dr. Fehsenfeld explains. Emissions from cars and power plants also can stimulate its formation. But the molecule is so reactive that it pops up for no more than a second before it recombines with other molecules in the atmosphere. This makes it fiendishly difficult to measure directly on a regional or global scale. Faced with this problem, Dr. Prinn and colleagues from the Scripps Institution of Oceanography, Georgia Tech, and research institutes in Britain and Australia, used methyl chloroform as a surrogate.
Methyl chloroform has been measured since 1978 as part of a global effort to track trace gases in the atmosphere. OH does a good job of scrubbing it. Using the measurements and computer models, the team compared the measured amounts of methyl chloroform with estimates of the amount emitted by human activities each year, as well as the small amount that vanishes through reactions that don't involve OH. The amount of methyl chloroform destroyed yields an estimate of the amount of OH available for clean-up. From year to year, OH levels varied widely. But the rise-and-fall trend over the long term was clear, the researchers report, despite uncertainties inherent in their method.
Less clear in this chemical "whodunit" is the answer to the question: Whodunit?
Rising levels of short-lived pollutants in the Southern Hemisphere could be a culprit, the team speculates. So could man-made aerosols and soot, which reflect sunlight back into space, depriving OH precursors of the energy to form the radical.
Yet those potential culprits also have natural counterparts - volcanic activity generates aerosols that can persist in the atmosphere, for example. Moreover, the team notes, the climate phenomenon known as the El Nino Southern Oscillation in the Pacific can increase cloudiness, cutting down on the sunlight needed to stimulate OH-forming reactions.
"Deciding the cause will take quite a bit of work," NOAA's Dr. Trainer says.
(c) Copyright 2001. The Christian Science Monitor