Scripps Scientists Pinpoint New, Major Source of Atmospheric Methyl Bromide

Research scientists at Scripps Institution of Oceanography at the University of California, San Diego, have identified salt marshes as a major natural source of the environmentally and economically important compound methyl bromide. The study, which also implicates salt marshes as a source of methyl chloride, is published in the Jan. 20 issue of the journal Nature.

Methyl bromide is produced naturally from oceans and plants on land. But it is also widely manufactured around the world because of its effectiveness as a pesticide against insects, nematodes, weeds, pathogens, and rodents. Methyl bromide also is generated as a by-product of leaded fuel combustion and vegetation burning. The U.S. Environmental Protection Agency (EPA) estimates that 72,000 tons of methyl bromide is used around the world each year. Scientists estimate that about half escapes into the atmosphere. In the U.S., the EPA estimates about 21,000 tons are used annually in agriculture, commodity and quarantine treatment, and structural fumigation.

Because it is considered a significant ozone-depleting substance, governments have developed controls that limit methyl bromide production.

Scientists have estimated that 20 percent of the methyl bromide that reaches the
atmosphere can be attributed to fumigation, about 10 percent to vegetation burning, and
roughly 30 percent to production from the oceans. But the balance of this methyl bromide "budget," a significant 40 percent, was missing. The new study uncovers about 10 percent of the absent budget.

Working in this area as environmental "accountants," Scripps geochemists Robert Rhew, Benjamin Miller, and Ray Weiss looked to salt marshes for part of the missing portion. Conducting a year-long study at Mission Bay Marsh in San Diego and San Dieguito Lagoon in Del Mar, Calif., the researchers documented significant amounts of methyl bromide and methyl chloride released to the atmosphere.

Although they make up only 0.1 percent of the earth's surface, salt marshes may be responsible for producing approximately a surprising 10 percent of the total methyl bromide and methyl chloride budget, the study indicates. As a result, salt marshes may constitute Earth's largest natural terrestrial source of methyl bromide and methyl chloride.
"Scientists suspected that there was a large natural terrestrial source, based on evidence from ocean cruises and computer models, but the source proved to be somewhat elusive," said Rhew, a graduate student in the Geosciences Research Division at Scripps. "We found that salt marshes emit methyl bromide at rates greater than any other natural environment, on a per area basis. We're getting much closer to nailing down the methyl bromide budget, and this study adds a significant piece to the puzzle."

Future studies will seek to expand these findings and correspond the data to other salt marshes and different environments, such as mangrove forests.

Research for the study was funded by the University of California Natural Reserve System, the National Science Foundation Graduate Research Fellowship Program, the Methyl Bromide Global Coalition, and NASA's Upper Atmosphere Research Program.