Earth Losing Air-Cleansing Ability, Study Says

The Earth’s atmosphere is beginning to lose its natural ability to remove air pollutants, a condition that could spread smog and accelerate the accumulation of greenhouse gases, according to a study published today in the journal Science.

The study documents for the first time the modest, two-decade-long worldwide decline of a key molecule that cleanses the air. Without enough of the molecules, emissions that contribute to the greenhouse effect, smog and the hole in the ozone layer do not get destroyed as fast as humans release them.

“This one molecule is very, very important. It is the critical cleaning chemical for the atmosphere,” said Ronald G. Prinn, the Massachusetts Institute of Technology scientist who led a 13-member research team responsible for the study. “If this free-radical [molecule] is decreasing, it could add to global warming.”

But the losses of the chemical, called a hydroxyl radical, are slight so far and are not currently cause for alarm, experts say.

“There’s a number of research findings that demonstrate the global atmosphere is changing, but we really do not know the effect it is going to have on us in the long term,” said Terry Keating, environmental scientist for the U.S. Environmental Protection Agency.

Indeed, some scientists say more research is necessary to understand the complex chemical reactions that occur in the atmosphere. Also, they say it is not clear whether the molecule’s decline is a temporary or cyclical event or one that portends a long-term shift.

“If this change has happened, it is a slight change. It’s significant if it’s the beginning of a trend. That would be a warning,” said F. Sherwood Rowland, a chemist at UC Irvine who won the Nobel Prize in 1995 for showing how pollutants destroy the Earth’s protective ozone shield.

The latest findings are further evidence that the planet’s fragile atmosphere is undergoing profound change and may have crossed a threshold that threatens its self-cleansing ability.

Using measurements taken from Ireland to Tasmania to the Oregon coast, the research team set out to determine changes in concentrations of the ephemeral molecule, an energized oxygen compound so reactive that it attacks and transforms many of the gases it bumps into. In the lower atmosphere, it is a catalyst for ground-hugging ozone, but it also occurs naturally up to seven miles high in the air, where it acts to destroy many man-made pollutants.

Yet over the last 22 years, concentrations of the molecule have decreased an average of 10% worldwide, the study found. Because the molecule exists for only one second before changing into something else, scientists arrived at that estimate indirectly. The research team examined changes in concentrations of methyl chloroform at five remote sites starting in 1978.

Methyl chloroform is a long-lived solvent in the air that can be neutralized only by the short-lived hydroxyl radical. Using computer models, researchers discovered that the solvent was not being removed from the atmosphere as fast as expected, a telltale sign that there are not enough of the scrubbing molecules to get rid of it.

Scientists have long been concerned that pollutants may be starting to overwhelm the atmosphere’s ability to get rid of them, but the study reveals the first documented changes in concentrations of the cleansing agents over several years.

“We are seeing for the first time that the oxidizing capacity of the atmosphere may have been reduced in recent years,” said Ray F. Weiss, a co-author of the paper and professor of geochemistry at the Scripps Institution of Oceanography.

The reduction could have enormous consequences for the health of the planet. The pollution-destroying molecules are especially efficient at neutralizing hydrocarbons, which come from paints, solvents and petroleum and cause smog and cancer. About 18 million tons of hydrocarbons are released annually in the United States, according to the EPA.

The cleansing molecules also break up methane, a powerful greenhouse gas discharged at the rate of about 181 million tons per year, as well as compounds that threaten the Earth’s protective ozone shield.

With fewer antipollutant agents in the atmosphere, smog can intensify and spread farther, greenhouse gases can accumulate faster and enough chlorofluorocarbons can reach the stratosphere to damage the ozone layer, scientists say.

Greenhouse gases, principally from the burning of fossil fuels, encircle the planet, trapping heat and affecting weather patterns.

“Eventually, the atmosphere will get to the point where it will be taxed beyond its ability to clean itself,” said Chris Cantrell, atmospheric chemist at the National Center for Atmospheric Research in Colorado.

It might take only 50 years before emissions overwhelm the atmosphere’s defense mechanisms, said Caltech scientist John H. Seinfeld, who is familiar with the study.

But Arthur M. Winer, an atmospheric chemist at UCLA who did not take part in the study, said he does not see cause for alarm. He said the atmosphere is extremely complex, with countless chemical reactions occurring that scientists have yet to fully unravel, including the fate of methyl chloroform and hydroxyl radicals.

Authors of the study acknowledge that there is a wide margin of error in their estimates. Specifically, the researchers say there are enough uncertainties that the actual loss of cleansing molecules could be 24% greater than they predicted, or much less. The range of error is due to assumptions made by the research team. They have to do with how much methyl chloroform actually reached the atmosphere, the adequacy of the five measurement stations and lack of data spanning many decades.

Indeed, scientists do not know what may be causing a loss of hydroxyl radicals, though a few theories hold promise.

As developing countries industrialize, more pollution is being added to the air, possibly overwhelming the atmosphere’s capacity to absorb the extra load.

Cleansing molecules are less abundant when El Nino conditions prevail, perhaps because greater cloud cover over the equator short-circuits formation of hydroxyl radicals. Pollution emitted in the United States and other industrialized nations of the Northern Hemisphere also could be responsible. Concentrations of the hydroxyl radical are about 14% less in the Northern Hemisphere than the Southern, the study shows.

Much of North America, Europe and Asia are shrouded in an aerosol haze, the residue of emissions from vehicles, factories and power plants. That may be just enough to block sunlight needed to produce sufficient cleansing molecules. Also, much of the haze consists of microscopic particles, and hydroxyl molecules colliding with those bits may be neutralized before they can act on other pollutants in the air, Prinn said.

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The Chemistry of Air Pollution

Scientists have discovered a decrease in levels of hydroxyl radicals, the naturally occurring chemicals that destroy air pollutants and many gases involved in ozone depletion and the greenhouse effect. Here’s how pollution forms and the role of the hydroxyl compound:

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Hydroxyl radicals (OH) are energized oxygen compounds, formed in part by nitrous oxides from lightning strikes. They last only fleetingly, cleanse the air of pollutants and transform some gases into less harmful forms.

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Water vapor is one of the elements that helps form hydroxyl radicals.

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Low-atmosphere ozone, a component of smog, forms when emissions from industry and vehicles mix with ultraviolet radiation from the sun. With fewer hydroxyls in the air, smog can intensify and spread further.

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Greenhouse gases, principally from the burning of fossil fuels, encircle the planet, trapping heat and affecting climate.

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Greenhouse gases such as carbon dioxide, methane, nitrous oxides and water vapor come from natural sources, such as cows and landfills, and some are formed by the burning of fossil fuels like coal, oil and gas.

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Sources: Prof. Ronald G. Prinn of MIT; AP; “The Weather Book”; Researched by JULIE SHEER/Los Angeles Times