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Tree Leaves Fight Pollution

Environmental organizations keep reminding us that we need to reduce pollution and greenhouse gases. But plants are already doing their part. Research published online today in Science shows that deciduous tree leaves, such as those from the maple, aspen, and poplar, suck up far more atmospheric pollutants than previously thought.

The study concerns the most abundant class of carbon-based particles in the atmosphere, so-called volatile organic compounds (VOCs). Plants produce VOCs when they decay, but a major source comes from automobile exhaust, coal burning, and other human activities. Some atmospheric VOCs combine with oxygen to form tiny airborne particles called oxygenated VOCs (oVOCs), which insulate the atmosphere and lead to warming. These oVOCs have been "poorly represented or partly even neglected in [climate] models," says atmospheric chemist Jos Lelieveld of the Max Planck Institute for Chemistry in Mainz, Germany, who was not involved in the study. And even though plants take up a major greenhouse gas—carbon dioxide—when they photosynthesize, researchers weren't sure if they also consumed oVOCs in large amounts.

So scientists led by Thomas Karl of the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, decided to re-examine how deciduous plants interacted with oVOCs. Via a combination of computer modeling, laboratory experiments, and field studies, the researchers looked at how many oVOCs were taken up by a poplar tree leaf sample. Plants exposed to oVOCs increased their normal uptake of the compounds, absorbing 40% more than expected. Although scientists suspected that plants consume small amounts of oVOCs, this study provides the "first concrete data," says chemist Roger Atkinson of the University of California, Riverside, who was not involved with the research.

In another set of experiments, the team showed that stressing out the plants—by exposing them to ozone pollution, for example—also increased their uptake of oVOCs. This suggests, says Karl, that deciduous trees provide good negative feedback for atmospheric pollution: The more polluted the atmosphere becomes, the more oVOCs the plants take up. However, he says, there is a limit to how much pollution plants can handle. Karl and his team now plan to look at pines and other coniferous trees to see if they also suck up oVOCs and other pollutants.

Those data, along with the new work, will help scientists devise more accurate climate models, says atmospheric chemist Mattias Hallquist of the University of Gothenburg in Sweden.

Karl suggests another possible benefit of understanding how plants process oVOCs. "This is looking very far into the future," he says, "but perhaps these new models could even lead to genetically engineering plants to uptake more air pollution."