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Polar spring arrives, mercury rises — in sky

Wednesday, April 4, 2001

By Koren Capozza

 

An aerial view of Barrow Observatory in Barrow, Alaska, where researchers are charting mercury in the sky. With the arrival of spring, atmospheric researchers in the American and Canadian Arctic await a dramatic rise in mercury — but not in their thermometer gauges where temperatures still register an icy -30 degrees Fahrenheit. The spike in certain forms of mercury occurs annually in the lower atmosphere, where scientists are recording the highest levels ever found on Earth. The puzzling phenomenon was unexpectedly discovered by Bill Schroeder, a Canadian atmospheric researcher. Schroeder was the first to observe these mercury showers in 1995, when he conducted air quality research on Ellesmere Island in Canada's high Arctic. Schroeder recorded stable atmospheric mercury levels until the arrival of the first period of sustained sun in mid-March, when signals on his monitoring system went haywire. Background levels of elemental mercury vapor dropped dramatically while another type of mercury, a reactive gaseous form, increased just as sharply. At the time, Schroeder suspected his equipment was malfunctioning and so ignored the measurements, waiting until the following year to bring in new instruments. The next spring, he observed the same wild fluctuation in mercury vapor concentrations. A few months later, once the surface snow began to melt, the phenomenon subsided. Scientists are just beginning to understand why this spring cycle, known as the "mercury sunrise," occurs in the area. One theory is that long-range atmospheric transport brings mercury-laden, polluted air to the Arctic, where it accumulates during the dark winter months. A chemical chain reaction is set in motion with the first sunlight of spring, causing chemicals found in sea salt aerosols — bromine and chlorine — to convert into highly reactive species. These airborne chemicals interact with elemental mercury vapor and the ozone layer, ultimately causing the mercury to oxidize into soluble forms that easily deposit onto snow and ice surfaces. At the National Oceanic and Atmospheric Administration research station in Barrow, Alaska, scientists are monitoring these showers of reactive gaseous mercury and have recently recorded the highest concentrations measured anywhere on the globe.

Steve Lindberg of ORNL sets up a system to measure atmospheric mercury speciation at the CMDL Point Barrow Observatory in Alaska.

"We know that reactive gaseous mercury can't be transported long distances from where it is emitted because its residence time is too short. It reacts and deposits too quickly," explains Steve Lindberg, a researcher with Oak Ridge National Laboratories in Tennessee. Arctic snowpack in regions where the mercury depletion phenomenon occurs have alarming levels of mercury pollution — much higher than levels found near industrialized cities of the eastern United States. The degree of contamination found in the polar region is all the more surprising because there are no local sources of pollution. Scientists are still unsure how much environmental damage results from these depletion events, but they suspect that the deposited mercury is accumulated through the local food chain and affects more adversely wildlife at the top of the chain. These mercury storms occur at a particularly damaging time for Arctic wildlife. March and April are key months for breeding. "Just as the ecosystem is waking up from the winter, it's being bathed in runoff from mercury in the snowpack," says Lindberg. Indeed, research on polar bears in the high Arctic indicates that the level of mercury in the livers of polar bears is 10 times higher than the level found in populations of bears farther south. "Mercury (also) accumulates in Arctic peoples as a result of consuming large quantities of meat in the subsistence hunting and fishing environment," says Steve Brooks, an atmospheric scientist at the NOAA laboratory in Barrow.