Scientists say they have detected wide swings and, most recently, a sharp drop in atmospheric concentrations of chemicals that naturally purge the air of many kinds of contaminants and methane, a powerful heat-trapping greenhouse gas.
The scientists say they suspect that the decline is related to human activity, because the biggest drop was measured in the northern hemisphere, where most industry and other human activity is concentrated.
The researchers, who described their work in today's issue of the journal Science, said there were still many uncertainties involved in calculating amounts of the molecules.
"It's a surprise as well as cause for deep concern," said Dr. Ronald G. Prinn, the study's lead author and chairman of the department of earth, atmospheric and planetary sciences at the Massachusetts Institute of Technology. "If we don't understand it and it's going down, we'd better find out what's going on."
The chemicals are hydroxyl radicals, which are created as ultraviolet light knocks hydrogen atoms from water molecules in air in the presence of ozone, a highly reactive form of oxygen.
The radicals vanish almost as quickly as they are created, usually in less than a second, chemically reacting with an array of air pollutants, including such undesirables as carbon monoxide, methane and sulfur dioxide. They are also a major ingredient in smog.
The puzzle is particularly complicated because the amount of radicals can be affected by the rates at which they are created and destroyed. One of many possible influences, atmospheric scientists say, is an increase in haze, which could block ultraviolet light and impede the reaction that creates the molecules.
It is important to clarify what is going on, Dr. Prinn said, because the potent molecules attack some things that are almost indestructible, most notably methane, which many scientists have identified as a significant contributor to global warming.
Another target of the radicals is sulfur dioxide, which is emitted by smokestacks, volcanoes and other sources. The hydroxyl radicals are thought to purge more than half the sulfur dioxide added to the air.
Experts in atmospheric chemistry who were not involved in the study said it offered important hints about hydroxyl radicals, but they emphasized the difficulties in measuring something that comes and goes so quickly and varies mile by mile.
Dr. Ralph J. Cicerone, an atmospheric chemist and chancellor of the University of California at Irvine, said he doubted there was a way to confirm that the hydroxyl radicals were exhibiting wide swings.
Nonetheless, Dr. Cicerone said, the study sharpened a fuzzy picture of an essential atmospheric ingredient.
"This is a terrifically important question because hydroxyl radicals are the central chemical in the lower atmosphere for processing everything," he said. "For 25 years, people have been struggling to measure it."
Indeed, the study, like several other recent efforts, did not rely on direct measurements of the radicals but of a synthetic gas, methyl chloroform, which the radicals destroy.
Companies stopped manufacturing and using methyl chloroform, a solvent, in the mid-1990's under agreements aimed at restoring the ozone layer high in the atmosphere.
The amount remaining in the air is declining, mainly as it is destroyed by hydroxyl radicals, so the rate of destruction can be an indirect measure of hydroxyl radicals.
Using this method, researchers estimated with a substantial margin of error that the average amount of hydroxyl radicals in the atmosphere rose 15 percent from 1979, when methyl chloroform measurements began, to 1989. After 1990, the amount of radicals appears to have dropped sharply. The concentration in 2000 was 10 percent below that of 1979.
But the technique adds another level of uncertainty, said Dr. Stephen A. Montzka, a research chemist for the National Oceanic and Atmospheric Administration. It is still possible that the changes in methyl chloroform levels are not coming from reactions with hydroxyl radicals, but are a result of continuing but undetected releases of these chemicals.
"It's the best barometer of hydroxyl radicals that we have," he said. "But there are still big potential sources of error."
We publish four FREE monthly email newsletters: Click Here to Subscribe to One or More Newsletters