ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-10-24595-2010 Measurement from sun-synchronous orbit of a reaction rate controlling the diurnal NO<sub>x</sub> cycle in the stratosphere Walker J. C. 1 Dudhia A. 1 Atmospheric, Oceanic and Planetary Physics, Clarendon Laboratory, University of Oxford, Oxford, OX1 3PU, UK 21 10 2010 10 10 24595 24620 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/10/24595/2010/acpd-10-24595-2010.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/10/24595/2010/acpd-10-24595-2010.pdf

A reaction rate associated with the nighttime formation of an important diurnally varying species, N<sub>2</sub>O<sub>5</sub>, is determined from MIPAS-ENVISAT. During the day, photolysis of N<sub>2</sub>O<sub>5</sub> in the stratosphere contributes to nitrogen-catalysed ozone destruction. However, at night concentrations of N<sub>2</sub>O<sub>5</sub> increase, temporarily sequestering reactive NO<sub>x</sub> (NO + NO<sub>2</sub>), in a natural cycle which regulates the majority of stratospheric ozone. In this paper, the reaction rate controlling the formation of N<sub>2</sub>O<sub>5</sub> is determined for the first time from an instrument aboard a sun-synchronous platform. The observed reaction rate is compared to the currently accepted rate determined from laboratory measurements. Good agreement is obtained between the observed and experimental reaction rates down to temperatures as low as 205 K for the first time.

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