Wildfires and mercury pollution: A smoking gun?

Wildfires and mercury pollution: A smokinggun?

July 2001

published by www.ucar.edu

What do forest fires have to do with mercury in fish? Hans Friedli and Larry Radke (senior research associates in ASP and ATD, respectively) are trying to find out.

In the 1950s, photographs from Japan showed the neurological damage inflicted on children whose mothers had eaten fish contaminated by mercury from an industrial spill into Minimata Bay. In the United States, concerns about elevated mercury levels over the last several decades have led the Environmental Protection Agency (EPA) and the Food and Drug Administration (FDA) to issue warnings to vulnerable populations to limit consumption of fish, especially from the Great Lakes. This past December, the EPA recommended that mercury emissions from coal-fired power plants be curbed, and the Bush administration has included mercury on the list of pollutants it is considering for increased regulation. Most industrial sources of mercury have been phased out in the United States, but a few continue, including electrical switches and thermometers.

But, as Hans and Larry have learned, the classic model of point- source pollution affecting people downstream or downwind may be only part of the story.

Gaseous elemental mercury in the atmosphere travels the globe for about ayear before being deposited on land or water. About 6,500 tons (5,900 metrictons), all well mixed, are circulating at any one time. “So a concentrationof mercury downwind of a coal-fired power plant may not be much different fromthat upwind,” says Larry. About half the atmospheric mercury got there fromnatural sources (in soil, oceans, and volcanoes) and the other half throughhuman activity. The EPA estimates that 41 tons (37 metric tons) is contributedannually from U.S. coal-fired plants. Mercury is removed from the atmospherethrough oxidation, which creates ionic mercury in the free troposphere and inclouds; it then rains or falls out as wet or dry deposition to the surface.

Mercury in fish

The livers of mammals, including humans, can process moderateamounts of mercury, but it becomes toxic at high doses. Victorian hat makers,like Lewis Carroll’s fictional Mad Hatter, suffered from mercury poisoning, andHans points out that indigenous gold miners in Amazonia are exposed to toxiclevels of mercury today.

Mercury becomes most dangerous when ionic mercury is deposited onto soils orwater bodies and becomes methylated by microbial or abiotic processes.Methylmercury, the neurotoxic form, enters the food chain and becomesconcentrated at higher and higher levels as larger fish and mammals eat smallerfish and biota.

The highest levels of gaseous ionic mercury ever observed were reported thisspring from NOAA’s Point Barrow Observatory in Alaska; indigenous Arcticpopulations consuming the region’s game and fish are thought to be at severerisk. Long-range atmospheric transport brings the mercury to the Arctic, wherechemical reactions tied to springtime ozone depletion lead to elevated levels ofionic species and deposition to the surface.

Whether in the Arctic or elsewhere, it’s the reactive ionic species that,through precipitation or dry deposition, fall on land, water, and vegetation.For trees, “Wet deposition is most important,” says Hans.”[Mercury is] picked up by the surfaces—the leaves or needles—and itstays there.”

Mercury in smoke

Hans and Larry became interested in smoke from biomass burning as a potentialsource of atmospheric mercury when their wildfire research called theirattention to the high levels of mercury threatening Arctic Inuit communities.(For more on wildfire research at NCAR, see the August2000 issue of Staff Notes Monthly; URL is below.) The elevatedlevels of sulfur dioxide coming off forest fires in the Los Angeles basinsuggested that other pollutants absorbed by trees might also be liberated in awildfire’s intense heat.

To conduct laboratory tests, they enlisted financial support from EPRI andcooperation from the Meteorological Service of Canada, which provided astate-of-the-art Tekran mercury vapor analyzer and the methodology to measuremercury contained in smoke particles. Their work with Julia Lu (MSC) will bereported in a forthcoming paper in Geophysical Research Letters.

Since most of what burns in a wildfire is foliage and ground litter, not treetrunks, volunteers collected samples from seven forests across the continentalUnited States. Those samples were set alight at the U.S. Forest Service FireScience Laboratory’s burn facility in Missoula, Montana. All the coniferous anddeciduous samples contained mercury at levels ranging from 14 to 71 nanogramsper gram of fuel. Ground litter had the highest concentrations, reflectingaccumulation during annual or biannual cycles before the leaves or needles wereshed.

When the team ignited the samples, their sensors immediately detected mercury,and plenty of it. All samples released nearly all the mercury they had stored—from94 to 99 percent. Most was released as gaseous elemental mercury, although up to10% was attached to aerosol particles.

The authors extrapolated their findings to global biomass burning andestimated the contribution at up to 800 tons (730 metric tons) per year, or 25%of all anthropogenic sources.

A complicated budget

Mercury from naturally occurring sources in geothermal mineralized zones isanother factor contributing to the atmospheric budget. According to Hans,researchers are now finding that in zones with high concentrations of mercury,such as those in the U.S. Southwest, “the fluxes are much higher thanexpected—about three times higher.”

Last summer, Hans and Larry were back in the air, flying over a wildfire inQuebec. “The emissions were higher than in the lab experiment,” Hanssays, “presumably because mercury in real fires is also emitted from heatedsoil, a source not yet considered in our experiments.”

Hans also flew with the Tekran analyzer aboard the NSF/NCAR C-130 duringACE-Asia flights this spring (see the Spring2001 issue of UCAR Quarterly). Based on source data published byChinese researchers, the team expected to find mercury distributed at “somelevel above background,” perhaps around 0.6 nanograms per cubic meterincrease above a global background estimated at 1.4–1.6 ng/m3.Instead, “there was tremendous variety in the observations.” For someplumes, “we went up to 5 nanograms.” For Hans, “this was thebiggest surprise” of the project. By working with the ACE-Asiameteorologists, modelers, and scientists making other tracer measurements, Hansand Larry identified plume sources, including the Shanghai industrial area. OverJapan “we saw mercury emissions from areas with high concentrations ofindustry and also from the Miyake Jima volcano.”

This summer the team plans to measure mercury emissions from a large wildfirein cooperation with Peter Hobbs (University of Washington) and also willparticipate in a prescribed burn in Saskatchewan’s Prince Albert National Park.They’ll be gathering fuel and ash and will use the Thermacam deployed in earlierwildfire experiments to track fire intensity and behavior.

All this suggests that the mercury budget is far from solved, and it won’t bea simple matter to keep the toxin out of the world’s fisheries. But, Hans muses,perhaps this work is helping put the global picture in better perspective.


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