ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-9-7045-2009 Nitric acid in the stratosphere based on Odin observations from 2001 to 2009 – 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 23 09 2009 9 18 7045 7052 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.net/9/7045/2009/acp-9-7045-2009.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/9/7045/2009/acp-9-7045-2009.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 (EPP), primarily by protons during solar proton events (SPEs), but also by precipitating auroral or relativistic electrons. We analyse a multi-year record (August 2001 to April 2009) of middle atmospheric HNO<sub>3</sub> measurements by the Sub-Millimeter Radiometer instrument aboard the Odin satellite, with a focus on the polar upper stratosphere. SMR observations show clear evidence of two different types of polar high-altitude HNO<sub>3</sub> enhancements linked to EPP. In the first type, referred to as direct enhancements by analogy with the EPP/NO<sub>x</sub> direct effect, enhanced HNO<sub>3</sub> mixing ratios are observed for a short period (1 week) after a SPE, upwards of a level typically in the mid-stratosphere. In a second type, referred to as indirect enhancements by analogy with the EPP/NO<sub>x</sub> indirect effect, the descent of mesospheric air triggers a stronger and longer-lasting enhancement. Each of the three major SPEs that occurred during the Northern Hemisphere autumn or winter, in November 2001, October–November 2003 and January 2005, are observed to lead to both direct and indirect HNO<sub>3</sub> enhancements. On the other hand, indirect enhancements occur recurrently in winter, are stronger in the Southern Hemisphere, and are influenced by EPP at higher altitudes.

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