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Mercury Rising: Natural wildfires release pollutant

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1:24pm, August 23, 2006
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Fires in high-latitude forests and peaty soils of the Northern Hemisphere may loft hundreds of tons of mercury into the atmosphere each year, much more than scientists had expected, a new analysis suggests.

Much of the world's industrial emissions of this toxic pollutant originates from the burning of coal contaminated with the element. "When it comes back to the ground, mercury forms strong chemical bonds with organic material, so it often gets locked away in rich forest soils and in peat," says Merritt R. Turetsky, an ecologist at Michigan State University in East Lansing.

Scientists estimate that industrial sources together with natural ones such as volcanoes annually send between 4,400 and 7,500 tons of mercury into the atmosphere. Previous studies suggested that wildfires in upland forests of the Northern Hemisphere release about 23 tons of the pollutant each year. However, soil data and new computer models now indicate that wildfire emissions of mercury could be much higher, Turetsky and her colleagues report in the Aug. 28 Geophysical Research Letters.

In fires in Alaskan and Canadian forests, much of the material that burns is twigs, moss, and other organic material on the ground and in the soil, says Turetsky. Each square meter of forest soil contains about 3.4 milligrams of mercury. Concentrations are even higher in peaty soils, where the dry surface layers hold about 11.5 mg/m2 of mercury, the team finds.

Scientists had generally considered peat lands not susceptible to fire, says Turetsky. However, by examining the carbon content of peat, she and her colleagues recently found that those areas burn, on average, once every century or so—a rate that's similar to that in northern forests. The scientists estimate that more than 10,000 square kilometers of forest and peat lands burn worldwide each year, sending more than 340 tons of mercury into the atmosphere.

"This is a sizable pool of mercury," says Richard Bindler, an environmental scientist at Umeå University in Sweden. The amount of the pollutant returned to the atmosphere by fires is much larger than the quantity that leaves the forests and peat lands via runoff, he notes.

The acreage burned each year in North American boreal forests has approximately doubled in the past few decades, says Eric S. Kasischke, a fire ecologist at the University of Maryland at College Park. Moreover, increased droughts anticipated in arctic regions could cause water tables to drop, leaving more mercury-tainted peat vulnerable to fire. "When water levels are low enough, a substantial amount of peat can be consumed [by fire]," thereby boosting mercury emissions, he notes.

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