Sunrise Sheds Light on Polar Chemistry

When the sun rises over the Arctic in March after nearly six months of darkness, its rays stir unexpected chemical reactions at the snow's surface. Driven by the release of compounds that have somehow been mobilized by the snow and ice, these reactions have likely been taking place since time began. However, it is only recently that scientists have begun studying them and the effects similar reactions in other snow-covered regions of the world could be having on global atmosphere and climate.

More than 30 scientists and university students from Canada, the United States, France, Italy, Germany and Japan were in Alert, Nunavut, from February to May 2000 to take part in the largest international study ever conducted of the photochemical effects of the polar sunrise. Some 15 scientists from Environment Canada's Meteorological Service of Canada were involved in the field campaign, which focused on ground-level ozone depletion and the potential role being played by chemical reactions in the snow pack and ice surface.

The long period of darkness leading up to the sunrise enabled scientists to monitor changes in the gaseous components of the air and snow before and after the event, using sophisticated methods such as mass spectrometry and laser-induced fluorescence. Thousands of other readings were taken of temperature and snow physics—including crystal shape, surface area, and air volume—and ice core samples were collected to further study changes in the chemical content of the snow over time. It was found that the use of an artificial light source to irradiate the snow in the pre-dawn darkness induced many of the same processes that occurred after the sun came up.

The central phenomenon scientists were studying was the depletion of ground-level ozone at the surface during the Arctic spring. Previous polar sunrise experiments indicated that this is likely due to chemical reactions involving bromine. The fact that bromine is not normally reactive in the atmosphere points to the likelihood that it must somehow be transforming into an active state in the snow. Furthermore, recent satellite observations made over the region at the same time of year found increased levels of bromine oxide—a reactive form of bromine that is produced when bromine destroys ozone.

Scientists also found that mercury depletion in the atmosphere in the Arctic closely follows ozone depletion. This is of particular importance with regard to human and environmental health, as this toxic chemical may be falling back to earth and entering the food chain.

The expedition observed increased levels of nitrogen oxide (NO_x) being emitted from the snow after sunrise. NO_x is produced by the burning of fossil fuels, converted into other chemicals in the atmosphere, and eventually washed back down to earth. The fact that it is being emitted in its original form from the snow is further evidence of the snow's active role in changing and activating chemicals, since NO_x is not easily reformed once it has been converted.

Photochemistry also seems to activate the formation of formaldehyde and nitrous acid in the snow, which leads to the production of hydroxyl (OH) in sunlight. OH is central to atmospheric chemistry, reacting with almost everything. If these chemicals are being produced in snow, then the same could be happening in glaciers, ice caps and clouds—a phenomenon that could have wide-ranging implications.

The scientists involved in the experiment are spending this summer and fall analyzing and checking the quality of their data, and plan to present the results of their research at the American Geophysical Union conference in San Francisco, California, this December. In the meantime, plans are in the works for a follow-up expedition that would take place at least 100 kilometres north of Alert to minimize the impacts of the camp itself, and to more easily study the air over the Arctic ocean. Knowledge gained through these studies will add important dimensions to existing atmospheric models, and improve our ability to accurately predict the future state of our climate.

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