Oct. 9, 2003 Fires in the Canadian boreal forest may be contributing significant amounts of mercury to the atmosphere above the northeastern United States. New research from scientists at Yale and Harvard shows that a huge fire in northern Quebec in July 2002 sent a pulse of mercury to a site in rural Massachusetts, providing clear evidence that mercury was transported over long distances in the resulting plume of smoke.
The researchers also used this data to estimate annual emissions from Canadian forests and the worldwide boreal forest. The findings are in the Sept. 29 issue of Environmental Science & Technology, a peer-reviewed journal of the American Chemical Society, the world's largest scientific society.
The boreal forest, also known as the taiga, nearly encircles the globe just below the Arctic Circle. It stretches across North America and Eurasia and constitutes about a third of Earth's total forest area.
In July 2002, a series of widespread fires swept the Canadian boreal forest north of Montreal, Quebec. The plume of smoke blanketed the entire region, enveloping the city of Montreal and affecting visibility as far south as Virginia.
"Simultaneously, our research group monitored atmospheric mercury levels at a rural site in Massachusetts," says Jeff Sigler, a Ph.D. candidate at Yale University and lead author of the study. "We detected a large increase in mercury at this site coincident with the smoke plume." The vast plume allowed the researchers to clearly link mercury from the fire to mercury at the test site, more than 500 miles away at Harvard Forest in Petersham, Mass.
The measurements provide the first clear evidence that Canadian forest fires may significantly increase levels of atmospheric mercury in the northeastern United States, which is under more mercury stress than any other region in the country, according to the U.S. Environmental Protection Agency.
Although mercury is a natural element that is abundant throughout the environment, roughly half of it is anthropogenic — from human sources — mainly from burning coal for electricity.
While mercury, both naturally occurring and anthropogenic, is not typically a health concern in soil and vegetation, a fire releases mercury from the trees and forest floor to the atmosphere, where it can be transported to other locations. Atmospheric mercury can stay in the air for up to two years, circling the globe and eventually depositing in remote bodies of water, where it transforms to a highly toxic form that builds up in fish.
"Its primary health risk to humans is from ingestion of fish," Sigler says, "but much of the mercury contained in fish is ultimately linked to removal from the atmosphere."
The subarctic boreal forest is rife with evergreen trees, such as black spruce and jack pine. The trees' needle-like foliage is filled with a resinous "antifreeze" to prevent frost damage. The resin burns well, and when the thick litter layer that forms on the forest floor ignites, substantial forest fires can occur. These fires are highly sporadic in nature, which implies that boreal fires can cause occasional large pulses in atmospheric mercury levels, according to Sigler.
"We used existing data to determine a ballpark estimate of mercury emissions, not only from this fire event, but also from the Canadian and global boreal zone on an average, annual basis," Sigler says. The researchers estimate that Canadian forest fires may emit about 3.5 tons of mercury each year, and boreal forest fires worldwide about 22.5 tons.
In Canada, industrial sources of mercury are much fewer, so the effects of mercury from forest fires may be even more important. "Our results imply that typically, mercury emissions from Canadian boreal fires may be equivalent to as much as 30 percent of the total Canadian anthropogenic emissions, and during a year characterized by very intense burning, may be as much as 100 percent," Sigler says.
"Canadian forest fires are known to cause enhancements in pollutants such as carbon monoxide and nitrogen oxides in the United States," according to Sigler. "Modeling the trajectories of air masses during the event and correlating the mercury increase with dramatic enhancements in carbon monoxide levels allowed us to clearly link the mercury enhancement to the fire plume."
Other studies of mercury from forest fires have been done on a laboratory scale or by taking spot measurements in small portions of a forest, but this is the first study to focus on a large-scale fire.
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