Mercury Rising

Snow isn't the only thing falling north of Canada's Sixty this time of year. Recent atmospheric data show that mercury is continuing to build up in the Arctic as a result of toxic downpours that occur each spring--and that these mercurial showers are expected to arrive again in the coming weeks.

Scientists from Canada, the United States and Norway are working to discover the geographic extent of this unusual phenomenon--the result of complex chemical and physical processes that are also responsible for a depletion in ground-level ozone over the Arctic each spring. Although mercury is released into the atmosphere from a variety of natural sources, rising global mercury levels began with the Industrial Revolution in the mid-18^th century. Still released through industrial use, mercury remains of considerable concern because of its environmental persistence, toxicity and ability to bioaccumulate in humans and other species high on the food chain.

These mercurial storms were discovered in early 1995 by Environment Canada researchers who had begun measuring concentrations of gaseous mercury at an air chemistry observatory near Alert, at the northern tip of Ellesmere Island. Although their readings remained relatively constant through the winter, in late March they began to fluctuate wildly--plummeting rapidly and then shooting up again, the process repeating itself numerous times before finally quieting down about eight weeks later.

Initially, researchers thought their equipment was malfunctioning, but new equipment received the following year recorded the same phenomenon. A separate set of tests and subsequent measurements showed that mercury was being transformed from a gas into a solid in the frigid polar atmosphere after sunrise--a development that has intrigued scientists. This aerosol mercury falls to the earth's surface much faster than gaseous mercury, and accumulates in the ice, snow and spring meltwater in an oxidized form that is much more easily taken up into the food chain.

Average values for concentrations of surface total gaseous mercury (TGM) and ozone at Alert, 1995. During the three-month period after polar sunrise in mid-March, there is a dramatic increase in fluctuation.

This phenomenon is active primarily in the earth's boundary layer, the area within a kilometre of its surface. Levels of gaseous mercury stay fairly constant during the six weeks of winter darkness and begin to fluctuate wildly with the return of the sun. Scientists believe the conversion of mercury into aerosol form is closely related to ground-level ozone destruction, because these changes occur only in areas where tropospheric ozone has been depleted over the frozen surface of the Arctic Ocean. Sunlight causes chemical reactions in sea salt found on Arctic pack ice, generating bromine atoms that attack and destroy ozone near ground level. When this occurs, bromine oxide is created--which likely oxidizes the mercury vapours and converts them into particles. A number of scientists worldwide are currently working to better understand the chemistry involved in this fascinating natural phenomenon.

Mercury's newly revealed behaviour, at least in the polar regions, is scientifically significant. Until now, it was widely believed that mercury vapour in the atmosphere was inert. However, this Canadian discovery indicates a need to re-examine the traditional understanding of mercury's atmospheric characteristics. Furthermore, because these mercury showers take place in the spring as the ecosystem is coming to life, they expose the area's humans, plants, animals and fish to yet another form of insidious toxic pollution.

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