It's a gas: trees emit unknown volatile substances.

The chemical reactions taking place just above a northern Michigan forest hint that trees there and elsewhere may be emitting highly reactive gases that scientists haven't yet identified or directly detected.

Many plants release large quantities of volatile organic compounds (VOCs). Some of those substances destroy ground-level ozone, says William H. Brune of Pennsylvania State University in State College. In another type of reaction, VOCs combine with hydroxyl radicals and other atmospheric constituents to produce the aerosol particles in haze (SN: 12/7/02, p. 360).

In the Michigan experiments, which took place within equipment mounted on a tower about 10 meters above 20-m-high treetops, Brune and his colleagues found that the concentration of hydroxyl radicals added to air samples during the experiments dropped much faster than the team could account for by the measured concentrations of 42 known VOCs and other atmospheric gases.

Several lines of evidence suggest that the hydroxyl radicals disappeared when they reacted with unknown VOCs, the researchers suggest in the April 30 Science. For example, concentrations of hydroxyl added to air samples during tests dropped at the same rate, regardless of whether prevailing winds were bringing clean air from Canada or polluted air from urban areas. Therefore, the phantom VOCs probably hadn't been produped by human activity, says Brune.

Also, the air samples' effect on hydroxyl during the tests varied according to the prevailing temperature. Other scientists have found that atmospheric concentrations of known tree-emitted VOCs, such as isoprene and terpenes, vary similarly. Because the anomalous effects occurred both day and night, the phantom VOCs probably aren't related to isoprene, which is emitted by vegetation only during daylight hours.

So far, chemical analyses of air samples haven't turned up any candidates for the unidentified gases. "We're not sure what these compounds are, but they're definitely there," says Brune.

The new research is "beautiful confirmation" of the notion that trees emit more types of highly reactive VOCs than are generally acknowledged, says Allen H. Goldstein, an atmospheric chemist at the University of California, Berkeley. His previous studies of the amounts of ozone absorbed by trees in a California pine forest also suggest the presence of unidentified VOCs. He proposes that they're related to terpenes.

Brune and his colleagues' arguments are "reasonably compelling," agrees Roger Atkinson of the University of California, Riverside. The work gives scientists a new group of atmospheric chemicals to look for, he says. The new data suggest that atmospheric concentrations of the purported VOCs above forests are about 1 part in 2 billion.