The chemical element bromine, whose compounds contribute significantly to the depletion of ozone in the lower atmosphere, is also released in polar regions to a great extent from snow on land. This is the result reached by an international research team of scientists from the Institute of Environmental Physics of Heidelberg University and colleagues from the USA, who performed measurements and sampling together in Alaska. Until now, science has assumed that sea ice was the sole source of bromine emissions. A novel spectroscopic measurement device, developed in Heidelberg, was used aboard an American research aircraft for this study.
The results of this research have now been published in Nature Geoscience.
Ozone plays a key role not only in the stratosphere, but also on the ground. While at ground level it is not particularly relevant for the protection from UV radiation, it is for the self-cleaning of the atmosphere and removal of contaminants. In the 1990s Heidelberg researchers working with Prof. Dr. Ulrich Platt had already discovered that the extensive ozone depletion in the atmosphere close to the ground in the Arctic and Antarctic was due to a reaction of bromine with ozone, producing bromine oxide. This bromine is released in autocatalytic processes. During the polar spring, the resulting bromine oxide clouds can spread over several thousand square kilometres. "It is by far the largest release of bromine on our planet," says Prof. Platt of the Institute of Environmental Physics at Heidelberg University. The precise processes involved are quite complex and are still a topic of current research.
Now the investigations in Alaska by the international team of scientists have yielded new information on the release of bromine from ice and snow. The researchers studied a variety of samples taken on site. It appears that the bromine-release processes are correlated to daylight, and thus involve photochemical reactions. Most importantly, however, the team was able to prove that the bromine emissions depended heavily on the pH value of the snow or ice sample. "The more acidic the sample, the more bromine was released. This led to the surprising result that snow on land, which is typically acidic, releases more bromine than alkaline sea ice, even though sea ice clearly contains more bromine," explains Dr. Denis Pöhler, a member of Prof. Platt's team.
The scientists from Heidelberg University confirmed these findings particularly through simultaneous observations from the aircraft. The instrument, which was developed as part of a project funded by the German Research Foundation at the Institute of Environmental Physics, measures the sunlight reflected and scattered on the surface of the snow and in the atmosphere. Bromine oxide absorbs some of the sunlight. Based on the amount of absorption, the Heidelberg scientists were able to determine the bromine concentration and its vertical distribution up to several kilometres altitude. They also obtained data on its horizontal distribution. "This kind of comprehensive data allows us to precisely find the sources of bromine release in the Arctic," stresses Dr. Pöhler.
The studies were conducted as part of the "Bromine, Ozone and Mercury Experiment" (BROMEX). Collaborators included researchers from Purdue University in West Lafayette/Indiana, the Cold Regions Research and Engineering Laboratory in Fort Wainwright/Alaska, the University of Alaska Fairbanks and the Georgia Institute of Technology in Atlanta. The instrument developed in Heidelberg will be used aboard the new German HALO research aircraft to measure not only bromine oxide, but also other compounds of significance for the atmosphere, such as nitrogen dioxide and sulphur dioxide.
- Kerri A. Pratt, Kyle D. Custard, Paul B. Shepson, Thomas A. Douglas, Denis Pöhler, Stephan General, Johannes Zielcke, William R. Simpson, Ulrich Platt, David J. Tanner, L. Gregory Huey, Mark Carlsen, Brian H. Stirm. Photochemical production of molecular bromine in Arctic surface snowpacks. Nature Geoscience, 2013; DOI: 10.1038/ngeo1779
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