GREENBELT -- It lasts a second or two and makes up only a tiny fraction of the Earth's vast ocean of air, but during its fleeting life it gorges on pollutants.
This voracious scavenger is hydroxyl, a fragmentary molecule that is born when ultraviolet light collides with a mixture of water and ozone. It quickly vanishes after reacting with and destroying such noxious chemicals as gasoline fumes, smog-producing carbon monoxide or globe-warming methane.
When nature was the only source of airborne poisons, hydroxyl kept the air we breathe in balance. But for more than a century, modern industry and agriculture have been pouring more health-threatening ingredients into the atmospheric stew.
As a result, some scientists think, the beleaguered hydroxyl -- which consists of one atom each of oxygen and hydrogen -- may have too much on its plate, shortening its already brief life.
Some scientists think its concentration in the atmosphere may have been reduced by as much as 20 percent since 1850.
That could leave the fragile bubble of air surrounding the planet a bit more vulnerable to assault, like a patient whose disease-fighting immune system is out of whack.
"To the extent you have a decline in the hydroxyl radical, it would be an extremely important environmental issue," said John Hoffman, director of the U.S. Environmental Protection Agency's Stratosphere Protection Program.
"As pollutants increase, the net concentrations of OH [hydroxyl] decrease," warned a recent NASA publication. "The ability of the atmosphere to cleanse itself is reduced in what could become a destructive feedback mechanism, worsening global pollution and enhancing the greenhouse effect."
But the hydroxyl story isn't as simple as it first sounds. In fact, Anne Thompson, an atmospheric scientist at NASA's Goddard Space Flight Center, said that the saga of the chemical is a good example of why the atmosphere's mounting problems -- global warming, ozone depletion and smog -- cannot be attacked piecemeal.
"When I see how complicated and interconnected things are, I'm skeptical that any one action will solve all our problems or won't produce some unanticipated new ones," said the chemist, a pioneer in hydroxyl research.
Some hydroxyl-enhancing gases, she pointed out, contribute to acid rain, which scars forests and kills fish. Others contribute to the so-called greenhouse effect, the blanket of heat-trapping chemicals in the air that may be causing global temperatures to rise.
At the same time, hydroxyl plays a vital role in stemming depletion of the earth's ozone layer, which protects us from harmful ultraviolet radiation. World governments have agreed to pTC phase out chlorofluorocarbons, the chemicals blamed for ozone
destruction, and replace them with similar chemicals designed to be destroyed by hydroxyl.
Without enough hydroxyl, those substitutes would be just as destructive as the chemicals they replaced.
There are subtler problems: While historic levels of hydroxyl may have fallen, increasingly sophisticat
ed computer models of the atmosphere's complex chemical mechanisms predict a surprising switch.
As pollution rises further, these models suggest, the earth's lower atmosphere -- called the troposphere -- could shift gears and actually begin manufacturing more poison-gobbling hydroxyl.
That's because certain industrial gases -- primarily nitrogen oxides, generated by gasoline engines and oil-fired power plants -- can help revive hydroxyl after it reacts with methane or carbon monoxide molecules.
Good news, right? To fine-tune the atmosphere, governments could set higher limits on the production of hydroxyl-friendly gases produced by autos and industry while clamping down on other chemicals.
But Dr. Thompson warns that it is not that simple. While adding a certain critical amount of nitrogen oxides to the atmosphere begins creating hydroxyl, adding a certain amount more probably begins destroying it again.
She said that it's not at all clear "where the magic control point is."
These aren't just academic issues.
The EPA, which now regulates emissions of hydrocarbons and carbon monoxide from vehicles, is trying to decide on similar
Federal policy-makers, Dr. Thompson said, are asking scientists how much nitrogen oxide they should let new cars emit.
"They want a number to give Detroit. Millions of dollars ride on that answer," she said. "But we can't give them a magic number."
Then there is the sheer uncertainty.
No one, it seems, can be certain about the atmospheric levels of hydroxyl, either historically or today. "That's the $64,000 question," said Donald Blake, a research chemist with the University of California at Irvine.
That's because the amount in the atmosphere is so small -- about one molecule of hydroxyl for every 50 trillion molecules of other gases -- that until the past few years scientists didn't have the instruments to measure it accurately.