Retrieval of stratospheric NO_x from 5.3 and 6.2 μm nonlocal thermodynamic equilibrium emissions measured by Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on Envisat

We present the first global observations of stratospheric NO_x(= NO + NO₂ ) from the Michelson Interferometer for Passive Atmosphenic Sounding (MIPAS) on Envisat during 24 July, 18 to 27 September, and 11 to 13 October 2002. Volume mixing ratio profiles of both NO_x species were derived from MIPAS limb emission spectra by means of an innovative retrieval scheme under consideration of nonlocal thermodynamic equilibrium (non-LTE) effects. In the quiescent atmosphere, the estimated accuracy of retrieved NO_x at the altitude of its stratospheric mixing ratio maximum at 35-40 km is around 1-2 ppbv, and the vertical resolution is around 3.5-6.5 km at altitudes between 20 and 50 km. In order to correctly consider NO₂ non-LTE effects in the retrievals, the photochemical excitation rate of NO₂(v₃ > 0) vibrational states was derived from NO₂(002→001) emissions and was found to be about 50 times smaller than previously estimated from Limb Infrared Monitor of the Stratosphere (LIMS) measurements. The NO_x partitioning of the retrieved data is in excellent agreement with steady state photochemistry, which confirms predicted stratospheric NO(v > 0) non-LTE population enhancements. The retrieved NO_x abundances are also consistent with Halogen Occultation Experiment (HALOE) NO_x observations.