August 24, 2001
As thousands of acres burned across the western United States this summer, scientists from the National Center for Atmospheric Research and the University of Washington were flying over wildfires in the Pacific Northwest to measure mercury emissions in the smoke.
According to NCAR scientist Hans Friedli, during a wildfire, mercury stored in the foliage and ground litter is released and carried into the atmosphere. Friedli and colleague Lawrence Radke are trying to understand the global sources of atmospheric mercury and determine how much mercury ends up in the food chain after deposition on land and water.
Gaseous elemental mercury in the atmosphere travels the globe for about a year before being deposited on land or water. About 6,500 tons, all well mixed, are circulating at any one time. About half the atmospheric mercury entered the atmosphere from natural sources in soil, oceans and volcanoes. The remainder is thought to enter the atmosphere through human activity. The U.S. Environmental Protection Agency estimates that 41 tons are contributed annually from U.S. coal-fired plants. Mercury is transformed in the atmosphere through chemical processes and then rains or falls out as wet or dry deposition to the surface. For trees, “wet deposition is most important,” said Friedli. “Mercury is picked up by the surfaces — the leaves or needles — and it stays there,” he said. Until, that is, those trees burn.
Friedli and Radke conducted laboratory tests to find out how much mercury a fire could release. For the experiment, forest samples from across the continental United States were set alight at the U.S. Forest Service Fire Science Laboratory’s burn facility in Missoula, Montana. The team’s sensors immediately detected mercury. All samples released nearly all the mercury they had stored — from 94 percent to 99 percent. All the coniferous and deciduous samples contained mercury at levels ranging from 14 to 71 nanograms per gram of fuel.
The team extrapolated their findings to global biomass burning from wildfires and from human activities, such as clearing land for agriculture. They estimated the contribution at up to 800 tons per year, or 25 percent of all anthropogenic sources of airborne mercury. Their work with Julia Lu of the Meteorological Service of Canada is described in a forthcoming paper in Geophysical Research Letters. The lab experiment and this summer’s flights were funded by the Electric Power Research Institute.
The mercury studies grew out of Friedli and Radke’s National Science Foundation-sponsored research with colleagues at NCAR to understand and predict the behavior of wildfires. To develop better forecasts of wildfire behavior for firefighters, the researchers are combining computer models with observations from infrared cameras.
Friedli and Radke planned to aim ground-based sensors at a prescribed burn in Prince Albert National Park in Saskatchewan, Canada this September. Last summer, when the team flew over a wildfire in Quebec, the mercury emissions were higher than in the lab experiment, “presumably because mercury in real fires is also emitted from heated soil, a source not yet considered in our experiments,” said Friedli.