ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-8-5787-2008 Mesospheric N<sub>2</sub>O enhancements as observed by MIPAS on Envisat during the polar winters in 2002–2004 Funke B. 1 López-Puertas M. 1 Garcia-Comas M. 1 Stiller G. P. 2 von Clarmann T. 2 Glatthor N. 2 Instituto de Astrofísica de Andalucía, CSIC, Granada, Spain Forschungszentrum and University of Karlsruhe, Institut für Meteorologie und Klimaforschung (IMK), Karlsruhe, Germany 07 10 2008 8 19 5787 5800 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/8/5787/2008/acp-8-5787-2008.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/8/5787/2008/acp-8-5787-2008.pdf

N<sub>2</sub>O abundances ranging from 0.5 to 6 ppbv were observed in the polar upper stratosphere/lower mesosphere by the MIPAS instrument on the Envisat satellite during the Arctic and Antarctic winters in the period July 2002 to March 2004. A detailed study of the observed N<sub>2</sub>O-CH<sub>4</sub> correlations shows that such enhancements cannot be explained by dynamics without invoking an upper atmospheric chemical source of N<sub>2</sub>O. The N<sub>2</sub>O enhancements observed at 58 km occurred in the presence of NO<sub>x</sub> intrusions from the upper atmosphere which were related to energetic particle precipitation. Further, the inter-annual variability of mesospheric N<sub>2</sub>O correlates well with observed precipitating electron fluxes. The analysis of possible chemical production mechanisms shows that the major part of the observed N<sub>2</sub>O enhancements is most likely generated under dark conditions by the reaction of NO<sub>2</sub> with atomic nitrogen at altitudes around 70–75 km in the presence of energetic particle precipitation (EPP). A possible additional source of N<sub>2</sub>O in the middle and upper polar atmosphere is the reaction of N<sub>2</sub>(A<sup>3</sup>Σ<sub>u</sub><sup>+</sup>), generated by precipitating electrons, with O<sub>2</sub>, which would lead to N<sub>2</sub>O production peaking at altitudes around 90–100 km. N<sub>2</sub>O produced by the latter mechanism could then descend to the mesosphere and upper stratosphere during polar winter. The estimated fraction of EPP-generated N<sub>2</sub>O to the total stratospheric N<sub>2</sub>O inside the polar vortex above 20 km (30 km) never exceeds 1% (10%) during the 2002–2004 winters. Compared to the global amount of stratospheric N<sub>2</sub>O, the EPP-generated contribution is negligible.

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