ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-8-9591-2008 Nitric acid in the stratosphere based on Odin observations from 2001 to 2007 – Part 2: High-altitude polar enhancements Orsolini Y. J. 1 Urban J. 2 Murtagh D. P. 2 Norwegian Institute for Air Research, Kjeller, Norway Chalmers University of Technology, Department of Radio and Space Science, Göteborg, Sweden 26 05 2008 8 3 9591 9605 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys-discuss.net/8/9591/2008/acpd-8-9591-2008.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/8/9591/2008/acpd-8-9591-2008.pdf

The wintertime abundance of nitric acid (HNO<sub>3</sub>) in the polar upper stratosphere displays a strong inter-annual variability, and is known to be strongly influenced by energetic particle precipitation, primarily during solar proton events, but also by precipitating electrons in the auroral zone. While wintertime HNO<sub>3</sub> enhancements in the polar upper stratosphere had been occasionally observed before, from the ground or from satellite, we present here measurements by the Sub-Millimeter Radiometer instrument aboard the Odin satellite through 6 full annual cycles (2001 to 2007). Major solar proton events, e.g. during November 2001 or the Halloween solar storms of autumn 2003, lead to a two-stage HNO<sub>3</sub> enhancement, likely involving different chemical reactions: a fast (about 1 week) in-situ enhancement from the mid to the upper stratosphere is followed by a slower, longer-lasting one, whereby anomalies originating in the upper stratosphere can descend within the polar vortex into the lower stratosphere. We highlight the fact that the actual chemical coupling between the upper and lower atmosphere involves a complex interplay of chemistry, dynamics and energetic particle precipitation.

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