USA — A study by researchers at the University of Michigan has revealed that forest fires release more mercury into the atmosphere than previously thought.
Doctoral student Abir Biswas, the paper’s lead author, came up with the idea for the project when he was a student at U-M’s Camp Davis Rocky Mountain Field Station near Jackson Hole, Wyoming.
Wildfires were burning all around the station that summer, and smoke had blanketed the camp. Around that time, Biswas happened to read a new scientific paper suggesting the possible role of fires in global mercury emissions.
“There I was, watching forest fires around our field camp, and it seemed like the ideal place to study the problem,” he said.
For his study, Biswas collected core samples of forest soil from burned and unburned areas using sections of PVC pipe sharpened at one end to obtain the cylindrical samples.
Together with Professor Joel Blum, Prof. Gerald Keeler and former research scientist Bjorn Klauke, he took air samples at Camp Davis over two summers.
This, as Biswas said, provided them with a picture of the atmospheric background on which the fires were superimposed.
According to him, forests act as mercury traps because mercury in the atmosphere-which comes from both natural and human-generated sources such as coal-fired power plants and municipal waste incinerators-collects on foliage.
“When the foliage dies, it falls to the forest floor and decomposes, and the mercury enters the soil. Since it binds strongly to organic molecules, mercury is most prevalent in the top several inches of soil, where organic matter is concentrated,” he said.
As such, by comparing the mercury content of burned soil with that of unburned soil, it was possible to estimate how much mercury was released when forests burned.
The team further found that the type of trees in the forest and the severity of the fire affected the amount of mercury released.
Evergreens took up more mercury from the atmosphere on their needles compared to broad-leafed trees, leading to more mercury accumulation in the soil prior to the fire.
Based on their analysis and estimates of the area of forest and shrub land burned annually in the US, the team calculated that wildfires and prescribed burns account for approximately 25 percent of human-generated mercury emissions in this country.
Of the study, Blum, a John D. MacArthur Professor of Geological Sciences and director of Camp Davis, said that understanding the role of in mercury emissions was particularly important in light of predictions that forest fires would increase as global warming made some parts of the world hotter and drier”.
“The findings also have implications for forest fire management. When you let fires run free in an area where they have been suppressed for a long time, as happened in the Yellowstone fire of 1988, the fires end up burning a huge area that has been accumulating mercury for a long time, so a lot of mercury is released,” said Biswas.
“By contrast, when you allow fires to occur in natural 50- to 100-year cycles, you end up with more frequent, less severe fires, which release less of the mercury in the soil. So the current shift in management practices from suppressing fires to letting some of them burn suggests that in the immediate future there may be a lot of high mercury release fires, but that down the road the amount of mercury released from these fires should drop,” hesaid.
The study is published online in the journal Global Biogeochemical Cycles.