Aug. 28, 2001 BOULDER -- As thousands of acres continue to burn across the western United States, scientists are flying over wildfires in the Pacific Northwest to measure mercury emissions in their smoke. The National Center for Atmospheric Research and the University of Washington are conducting the flights. NCAR's primary sponsor is the National Science Foundation (NSF).
During a wildfire, mercury stored in the foliage and ground litter is released and carried into the atmosphere, says NCAR scientist Hans Friedli. He and colleague Lawrence Radke are conducting experiments in the laboratory as well as in research flights over wildfires and prescribed burns. Scientists are trying to understand the global sources of atmospheric mercury, as well as how much ends up in the food chain after deposition on land and water. Friedli and Radke's research provides one more piece in the global inventory puzzle.
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 got there from natural sources (in soil, oceans, and volcanoes) and the other half 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," says Friedli. "Mercury is picked up by the surfaces--the leaves or needles--and it stays there." At least until 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, and plenty of it. All samples released nearly all the mercury they had stored-- from 94% to 99%. All the coniferous and deciduous samples contained mercury at levels ranging from 14 to 71 nanograms per gram of fuel (a nanogram is one trillionth of a gram; about 28 grams make an ounce).
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% of all anthropogenic sources of airborne mercury. Their work with Julia Lu (Meteorological Service of Canada) is described in a forthcoming paper in Geophysical Research Letters. The lab experiment and this summer's flights are funded by EPRI (Electric Power Research Institute).
The mercury studies grew out of Friedli and Radke's NSF-sponsored research with colleagues at NCAR to understand and predict the erratic, deadly 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 will 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," says Friedli, "a source not yet considered in our experiments."
NCAR is managed by the University Corporation for Atmospheric Research, a consortium of 66 universities offering Ph.D.s in atmospheric and related sciences.
On the Web: Mercury background: http://www.ucar.edu/communications/staffnotes/0107/mercury.html
Wildfire research at NCAR: http://box.mmm.ucar.edu/science/fire/firehome.html
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