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

Abstract

We present the first global observations of stratospheric NOx(= NO + NO2) from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on Envisat during 24 July, 18 to 27 September, and 11 to 13 October 2002. Volume mixing ratio profiles of both NOx 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 NOx 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 NOx 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 NOx abundances are also consistent with Halogen Occultation Experiment (HALOE) NOx observations.