Science News - November 3, 2004

Controversial results downplay power plant mercury emissions

Mercury emissions from power plants may have minimal regional impacts, according to data and analysis presented by scientists funded by the electricity generating industry and the U.S. Department of Energy (DOE).

If confirmed, these unexpected observations could have a dramatic impact on the intense debate over mercury regulations in the United States, because they suggest that local power plant emissions may have a trivial impact on local mercury deposition and hence on mercury inputs to lakes, which ultimately bioaccumulate in fish. However, many nonindustry mercury scientists who have seen presentations of the work at recent meetings say that large uncertainties in the data and the lack of a plausible mechanism make it difficult to draw conclusions from the unpublished studies.

Coal-fired power plants are the largest source of mercury emissions in the United States, according to U.S. EPA estimates. Previously, researchers had thought that roughly half of this mercury came out of plant chimneys in the form of reactive gaseous mercury (RGM), which deposits rapidly after being emitted and should have regional impacts. Most of the remaining mercury is emitted as elemental mercury, which stays in the atmosphere long enough to undergo long-range transport around the globe (http://pubs.acs.org/subscribe/journals/esthag-w/2004/oct/science/rr_rethinking.html).

Industry scientists believe they have data to support the hypothesis that RGM is very rapidly reduced to elemental mercury as the plumes mix with ambient air. If the hypothesis is correct, then U.S. power plant emissions are only a small part of the problem in any particular area, because they constitute only about 1–5% of total global emissions. Ambient-air monitoring, airplane monitoring, and experimental data that could support the rapid reduction hypothesis were presented at the Seventh International Conference on Mercury as a Global Pollutant in Ljubljana, Slovenia, in July and at recent meetings in the United States.

“This is an extremely important question with huge consequences,” says National Oceanographic and Atmospheric Administration mercury modeler Mark Cohen. If the reduction hypothesis is true, then the scientific rationale for regulating power plants for mercury may be significantly weakened. “However, there are many uncertainties, and it is far from clear that the hypothesis is correct—there may be other explanations for the evidence that the industry has presented,” he adds.

In the ambient-air modeling studies, atmospheric chemists Eric Edgerton with Atmospheric Research and Analysis, a consulting firm in Cary, N.C., and his colleagues collected three years’ worth of ground-based ambient-air data downwind of power plants at two sites in Georgia and one in Florida. Edgerton used sulfur dioxide to track atmospheric dilution of the plume and estimated mercury speciation out of the stack using coal analyses and factors related to the combustion conditions. Mercury out of the stack should be about 67% RGM, he says, but by the time it reached his monitoring stations some 10–25 kilometers away, the levels of RGM were only about 20%. Wet deposition of RGM can’t account for the loss, because no rain fell when Edgerton measured. Dry deposition of RGM is also unlikely to explain the loss, because he monitored the plume in the early morning. Modeling indicates that when the nighttime air is calm, the plume remains aloft because the RGM requires contact with the earth’s surface to deposit.

But the lack of actual stack measurements leaves a major uncertainty in this analysis, according to Steve Lindberg at Oak Ridge National Laboratory. Edgerton acknowledges this, noting that the stack estimates have large uncertainties of ±30%. Lindberg also notes that very large uncertainties exist for dry deposition, which may be more significant than Edgerton estimated because the plume was at ground level when he made the measurements.

However, a study that started with direct stack measurements and then monitored speciation in the plume by airplane has produced similar results. A study of the Pleasant Prairie Power Plant near Kenosha, Wis., found a 66% reduction in RGM, from the stack concentration of 2.9 picograms per cubic meter to 1.7 at 5 miles downwind. But the mass balances in these measurements are poor and suggest large uncertainties, according to some of the scientists who have seen presentations of these data. These results were presented at a DOE Research and Development review in Pittsburgh, Pa., in July. Experiments using static and dynamic dilution chambers also indicate that rapid reduction is occurring, says Eric Prestbo, an atmospheric chemist with Frontier GeoSciences in Seattle, Wash., and a participant in the Pleasant Prairie project.

But EPA and Florida Department of Environmental Protection scientist Robert Stevens speaks for many nonindustry scientists who remain skeptical of the hypothesis when he says, “We as scientist can’t make broad-brush statements about the transformation of mercury. I think that folks who do are speaking prematurely.”

Stevens and EPA scientist Matthew Landis are now collecting detailed speciation data downwind of coal-fired power plants at Steubenville, Ohio, and at an oil-fired plant in Tampa Bay, Fla., that should provide a test to the rapid reduction hypothesis. “In principle, if the hypothesis is robust, we should see something similar, ” says Stevens. —REBECCA RENNER