When former President Ronald Reagan slipped and said that trees can pollute the air, it turns out he wasn't far off the mark. New research shows that some leafy green plants churn out methyl bromide, a chemical that helps destroy Earth's protective shield of ozone. The study, reported in the 1 October Geophysical Research Letters, may fill a puzzling gap in the movement of methyl bromide between land and air.
Methyl bromide, a clear and odorless gas, is a potent soil fumigant that kills crop pests. However, once the gas drifts into the upper atmosphere, sunlight breaks off bromine atoms, which eat up about 10% as much ozone as does the chlorine from banned chlorofluorocarbon compounds. In 1995, concern about the ozone hole prompted a controversial phase-out of methyl bromide, ending in a ban on its use by the year 2005. But industrial production of methyl bromide only accounts for 20% of that released into the air. Atmospheric scientists suspected that oceans and forest fires were the main additional sources, but now it appears that plants exhale large amounts as well.
Soil scientists at the U.S. Salinity Laboratory in Riverside, California, measured methyl bromide emissions from Brassica plants, a family that includes broccoli, cabbage, and other important crops. The researchers grew the plants in a greenhouse and put them in sealed glass jars for 24 hours to let the gas accumulate. The soil degraded some of the methyl bromide, but the team found that an excess lingered. "Bromide is everywhere in the soil, so it is likely that terrestrial higher plants take it up into their leaves and act as a significant source of methyl bromide worldwide," says team leader Jianying Gan.
On balance, natural and industrial processes produce as much methyl bromide as they consume, as atmospheric concentrations of the gas are stable. But when researchers tally the known methyl bromide flux, they can't trace the origins of about 80,000 metric tons of the gas per year--twice the amount produced by crop fumigation. The new findings are preliminary but point a finger at plants, says atmospheric scientist James Butler of the National Oceanic and Atmospheric Administration in Boulder, Colorado. However, he notes, "it will be difficult to get a hard-and-fast number because of the wide diversity of plants," some of which may produce little or no methyl bromide.