Hydroxyl holiday

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The day the planet's cleaner didn't show up for work

It could be the doomsday that creeps up on us unawares: the day the atmosphere's cleaning service fails to show up for work. For one of the most disturbing secrets of our planet's metabolism is that just one chemical is responsible for cleaning most of the pollution out of the atmosphere. If it took a day off, we would be in serious trouble, with smog spreading unchecked across the planet.

The chemical in question is called hydroxyl. Its molecules are made up of one atom of oxygen and one atom of hydrogen. They are created when ultraviolet radiation bombards common gases such as ozone and water vapor. But it is the most ephemeral of chemicals. Almost as soon as it is created, it reacts with some other molecule, mostly some polluting substance, and is gone again. It has an average lifetime of about a second. Because it comes and goes so fast, it is also rather rare, with an average concentration in the atmosphere of less than one part per trillion. You could pack every last molecule of the stuff into the Great Pyramid of Egypt and still have room for two more atmospheres' worth.

Yet it is crucial to life on Earth. For hydroxyl is, more or less literally, the atmosphere's detergent. It transforms all manner of gaseous pollutants so that they become soluble in water and wash away in the rain. The process is called oxidation. To take one example, hydroxyl converts sulfur dioxide, which would otherwise clog up the air for months, to acid rain, which soon falls to the ground. Much the same happens to carbon monoxide and methane (both of which are oxidized to carbon dioxide), nitrogen oxide, and many others. The one major pollutant it doesn't neutralize is carbon dioxide, which, partly as a result, has a much longer lifetime in the atmosphere than most other pollutants.

Concentrations of hydroxyl are generally much higher in the warm air over the tropics, where ultraviolet radiation is most intense, but are close to nonexistent in the Arctic, where, despite ozone holes, there is usually little ultraviolet around to make more hydroxyl. As a result, "toxic chemicals that might survive for only a few days in the tropics will last for a year or more in Arctic air," says Frank Wania, of the University of Toronto. That is one reason, he says, why pollutants like acid hazes and pesticides accumulate in the Arctic, poisoning polar bears and much else.

Hydroxyl has a hard life keeping up with our polluting gases, especially since it is destroyed in the process of oxidizing them. Fears that the atmosphere's janitor could be overworked and in trouble go back a few years. But because the chemical is so transient and rare, it is virtually impossible to measure hydroxyl concentrations directly. All the estimates are indirect, based on measuring chemicals with which it reacts. So when Joel Levine, a NASA chemist, suggested back in the 1980s that hydroxyl in the air could have declined by 25 percent over the previous thirty years, his argument didn't make much headway, because he couldn't prove it. There was no chance of his producing something definitive like the Keeling curve on carbon dioxide.

In 2001, a brief forecast in the IPCC report of a possible 20 percent decline in hydroxyl by 2 100, because of excess demands placed on it by a rising tide of pollution, met much the same fate. So did a report the same year by Ronald Prinn, a leading atmospheric chemist from the Massachusetts Institute of Technology, of a possible decline in global hydroxyl levels during the 1990s.

But we should be concerned. Hydroxyl spends more energy oxidizing one chemical than any other. That chemical is carbon monoxide. Emitted mostly from forest fires, fossil fuel burning, and small domestic stoves, it has for many years been the Cinderella pollutant. Dangerous to humans in confined spaces, it has been largely ignored as an environmental pollutant threat. The biggest concern has been that it oxidizes to carbon dioxide. But its concentration in the air tripled worldwide during the twentieth century. That suggests a bottleneck that could be the prelude to a wider breakdown of the cleaning service.

In the absence of good data on hydroxyl and its works, probably the best hope of finding a problem ahead of time is through modeling. Sasha Madronich, of the National Center for Atmospheric Research, in Boulder, Colorado is one of the few researchers who have attempted to model how hydroxyl might respond to changing pollution levels. He says that the atmospheric cleaning service could have a breaking point: "Under high pollution, the chemistry of the atmosphere becomes chaotic and extremely unpredictable. Beyond certain threshold values, hydroxyl can decrease catastrophically." Many urban areas, he says, "are already sufficiently polluted that hydroxyl levels are locally suppressed." This is partly because the sheer volume of pollution consumes all the available hydroxyl, but also because the smog itself prevents ultraviolet radiation from penetrating into the air to create more.

"The oxidation processes that should clean the air virtually shut down in smog-bound cities like Athens and Mexico City," he says. It takes a breath of fresh air from the countryside to revive them. "If, in future, large parts of the atmosphere are as polluted as these cities are today, then we could anticipate the collapse of hydroxyl on a global scale." With large areas of Asia becoming submerged beneath a cloud of brown haze every year, it may be that the atmosphere is approaching just such a crisis. Nobody knows.

But the doomsday scenario may require another element. If the cleanup chemical is under pressure from too much dirt, the worst thing to happen would be a decline in supply of the chemical. So the critical question may be: What might reduce the amount of hydroxyl produced by the atmosphere? Clearly smog is a problem, because it reduces ultraviolet radiation in the lower atmosphere. But a thicker ozone layer, nature's protective filter against ultraviolet, could have the same effect. And the world is currently working quite hard to repair the damaged ozone layer and make it thicker. Our efforts to solve one environmental problem could exacerbate another.

The worry is that over the past thirty years or so, we have been living on borrowed time with hydroxyl. Pollutants like CFCs have thinned the ozone layer, and so let more ultraviolet radiation into the lower atmosphere. And while that is bad for marine ecosystems, and probably causes more skin cancers, it has ensured a beefed-up supply of hydroxyl to cleanse the air of many other pollutants. Arguably, it has helped the planetary cleaning service keep on top of a rising tide of pollution. Over the next half century, we should succeed in healing the ozone layer once again.

There are good ecological, human-health, and even climatic reasons for doing this. But it could have a downside for hydroxyl.

So here is the doomsday scenario. If we repair the ozone layer, we will reduce hydroxyl production to the levels of the mid-twentieth century. But we will be doing it at a time when the demands on hydroxyl's services are considerably higher than they were then. That could be the moment when Madronich's threshold is crossed, and oxidation processes in the atmosphere go into sharp decline. I have no data, no models, and no peer-reviewed papers to justify this scenario. It is just that: a scenario and not a prediction. But it is plausible speculation. It could conceivably happen.

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