Jungle Sky

Long-held beliefs about the chemical makeup of the atmosphere above pristine rain forests are being called into question by new research that could inform air pollution modeling and regulation in areas that are more urban.

All forms of foliage emit vast amounts of hydrocarbons into the sky, far more than cars do. In fact, trees and plants expel about 500 billion kg of isoprene per year—which is approximately the combined weight of every human in the world. But the atmospheric lifetime of these hydrocarbons is just a few hours, much shorter than that of anthropogenic hydrocarbons, says Jonathan Williams, an atmospheric scientist at Max Planck Institute for Chemistry in Mainz, Germany.

Near urban centers, isoprene reacts with polluting nitrogen oxides to form smog. Researchers have long thought that above pristine jungles and forests, isoprene is oxidized by hydroxyl radicals, thereby depleting the overall oxidative potential of jungle skies.

But when Williams, Jos Lelieveld, and their colleagues from Max Planck Institute for Chemistry sent a Lear jet packed full of equipment over South American jungles, they found that OH radicals are not depleted at all but exist at the same concentrations as over the oceans. The authors suggest that hydroxyl radicals are, in fact, being recycled, not used up, by jungle-emitted isoprene (Nature 2008, 452, 737).

"Jungles rely on an oxidative atmosphere to create the concentration gradients required for chemical ecology to work"—so a pollinating insect can hone in on nectar in a flower, for example, Williams says. "It's amazing that the jungle can maintain levels of HO^•" required for a healthy ecosystem.

The new findings also call into question models that air pollution regulators have been using to estimate the levels of smog and ozone in forested areas downwind of urban nitric oxide emissions, comments Alex Guenther, a senior scientist with the National Center for Atmospheric Research, in Boulder, Colo. "Hydroxyl radicals are critical compounds in the air, but they are also really hard to measure," Guenther says. "When the concentration of HO^• changes, the concentration of everything in numerical models changes." Regulators should, therefore, pay close attention to these findings, he notes.