After the long, dark polar winter, the sun returns to shine on the high-latitude snows. Simultaneously, the concentration of ozone at low altitudes begins to drop. In research conducted on the Arctic snowpack last spring, scientists used sunlamps to illuminate some of the chemistry of this phenomenon.
The experiment showed that pollutants trapped in Arctic snow, when energized by sunlight, can escape the snowpack and deplete ozone in the lower atmosphere, says Jan W. Bottenheim, a senior scientist at the Meteorological Service of Canada in Toronto, Ontario.
The artificial light stimulated production of nitrous acid and organic chemicals such as formaldehyde in the upper layers of the snowpack. These chemicals, as well as snowbound chlorine and bromine derived from sea salt, then took part in reactions that consumed ozone within the snow, adds Bottenheim.
Chlorine and bromine driven from the snow by the spring sun also remove industrial mercury emissions from the atmosphere, says Leonard A. Barrie, an atmospheric scientist at the Pacific Northwest National Laboratory in Richland, Wash. Measurements taken last spring by aircraft flying over the Canadian Arctic showed there was almost no mercury vapor in the lowest 1,500 feet of altitude.
This news isn’t as good as it seems because the reactions are depositing the mercury on the snowpack. From there, the toxin wends its way into the food chain. About half the samples of some types of fish caught in Canadian lakes exceed that nation’s recommended levels of mercury, Barrie points out. Much of the contamination probably stems from the 50 tons of mercury removed annually from the atmosphere by the sunlight-driven reactions.