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. …