In the Arctic, the snow that covers the land mass and the pack ice is a constant source of new surprises for researchers. One of the major players in climate change, it is also closely monitored by atmospheric chemists, who suspect it of being behind fundamental alterations in atmospheric composition in spring, when sunshine returns.
The researchers had already studied episodes of total destruction of ozone at the surface of the Arctic snow cover  as well as the role played by this cover in the dangerous mercury 'rain' that pollutes ecosystems . This time they were interested in the ability of the arctic snow cover to interact with nitrogen compounds such as nitrogen oxides and atmospheric nitrate. At temperate latitudes nitrogen oxides are produced not only by natural phenomena such as lightning and forest fires, but also by human activity, such as combustion in engines and industrial activity.
They are the cause of the peaks in ozone concentration observed on the outskirts of cities during episodes of high pollution. Nitrogen oxides are rapidly oxidized to nitrate, which, incorporated into atmospheric particulate matter, is transported by air currents, bringing surplus nitrogen to distant ecosystems.
In the Arctic, in autumn, winter and spring, the nitrate is deposited onto the snow cover. Then, when the snow is exposed to solar radiation, the nitrate turns into nitrogen oxides that are emitted to the atmosphere, causing disturbances in Arctic atmospheric chemistry. However, the extent of this phenomenon remained to be quantified.
By measuring the isotopic composition of the nitrogen and oxygen in the atmospheric nitrate collected in the Canadian Arctic (Alert station, Nunavut), the researchers have shown that the 'recycling' of nitrate deposited on the snow of the Arctic pack ice returns nitrogen oxides to the atmosphere in substantial quantities. For instance, in spring, nearly one third of the Arctic atmospheric nitrate comes from emissions of nitrogen oxides from the snow cover, while the rest comes directly from atmospheric transport from middle latitudes.
The researchers also show that there are strong chemical interactions between the nitrogen oxides emitted by the snow cover and the halogenated compounds (in particular BrO radicals) that are involved in the phenomena of ozone destruction in the lower levels of the atmosphere in spring.
This study highlights the close links between the climate system (ice surfaces, snow-covered surfaces, temperatures, and percentage of solar radiation reaching the Earth's surface) and the presence of highly reactive pollutants in the Arctic atmosphere (nitrogen oxides, ozone, and particulate matter emitted by human activity). It shows the need for a global approach to environmental problems, calling for long-term monitoring and the use of new techniques for analyzing processes.
The work was funded by CNRS's National Institute of Earth Sciences and Astronomy (INSU), the Institut Polaire - Paul Émile Victor (IPEV) and by a European Science Foundation program (EUROCORE-EuroCLIMATE).
 Laboratoire de glaciologie et de géologie de l’environnement (CNRS/Université Joseph Fourier), Laboratoire d’étude des transferts en hydrologie et environnement (CNRS/Université Joseph Fourier/Institut polytechnique de Grenoble), Service d'aéronomie (CNRS/Université Pierre et Marie Curie/Université Versailles Saint Quentin)
 by the Meteorological Service of Canada ( Global Atmospheric Watch program, coordinated by the World Meteorological Organization).
Article: Morin, S., Savarino, J., Frey, M.M., Yan, N., Bekki, S., Bottenheim, J.W., Martins, J.M.F. Tracing sources and sinks of NOx in the Arctic atmosphere using stable isotopes in nitrate, Science, 31 octobre 2008.