Retrieval of abundances of atmospheric species from limb infrared emission spectra requires accurate knowledge of the pointing of the instrument in terms of elevation, as well as temperature and pressure profiles. An optimal estimation‐based method is presented to infer these quantities from measured spectra. The successful and efficient joint retrieval of these largely correlated quantities depends strongly on the proper selection of the retrieval space, the selection of spectral microwindows, and the choice of reasonable constraints which force the solution to be stable. The proposed strategy was applied to limb emission spectra recorded with the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on board the Envisat research satellite in order to validate the instrument pointing information based on the satellite's orbit and attitude control system which uses star tracker information as a reference. Both systematic and periodic pointing calibration errors were detected, which meanwhile have been corrected to a major part. Furthermore, occasional pitch jumps were detected, which could be assigned to parameter uploads to the satellite's orbit and attitude control system. It has been shown that in general, it is justified to assume local thermodynamic equilibrium below 60 km for these purposes. The retrieval method presented has been proven to be suitable for independent monitoring of MIPAS line‐of‐sight pointing.

Pronounced upper stratospheric and mesospheric NO_x enhancements were measured by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) in the Southern Hemisphere (SH) polar vortex from May to August 2003, reaching average abundances of 60 ppbv at 50–60 km in July. Peak mixing ratios of around 200 ppbv were measured in the polar night, representing the highest values ever recorded in the SH. The observed NO_x enhancements are attributed to production by electron precipitation in the upper mesosphere and lower thermosphere and subsequent descent with the meridional circulation. Using measured CH₄ and CO distributions as dynamic tracers, the downward transport of NO_x‐rich air masses into the lower and middle stratosphere has been investigated. Upper atmospheric air with average NO_x abundances of 15 ppbv reached the 800–1000 K potential temperature region (around 30 km) by the end of July, where it remained until the final warming in late October. The NO_x descent was confined to the polar vortex, although significant mixing of tropical and NO_x‐rich vortex air masses began already in August above 40 km. The amount of upper atmospheric NO_y measured inside of the SH vortex in late spring was 1.1 Gigamoles (GM) which is in good agreement with previous estimates from HALOE data. The global coverage of MIPAS data further allows to quantify the upper atmospheric NO_x dispersed into the stratosphere during August‐September, estimated in 1.3 GM. The net deposition of NO_x into the stratosphere during the 2003 Antarctic winter (2.4 GM) makes up 9% of the N₂O oxidation source in the SH, twice as much as estimated in previous studies. NO_x and tracer distributions observed on several days during the NH winter 2002/2003 have been analyzed for comparison. We found that high planetary wave activity, resulting in the major midwinter warming had led to a rather inefficient NO_x downward transport with negligible deposition of NO_x into the lower and middle stratosphere.

We report on the dependence of ozone volume mixing ratio profiles, retrieved from limb emission infrared spectra of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), on different retrieval setups such as the treatment of the background continuum, cloud filtering, spectral regions used for analysis and a series of further more technical parameter choices. The purpose of this investigation is to better understand the error sources of the ozone retrieval, to optimize the current retrieval setup and to document changes in the data versions. It was shown that the cloud clearing technique used so far (cloud index 1.8) does not reliably exclude all cloud-contaminated spectra from analysis. Through analysis of spectra calculated for cloudy atmospheres we found that the cloud index should be increased to a value of 3.0 or higher. Further, it was found that assignment of a common background continuum to adjacent microwindows within 5 cm⁻¹ is advantageous, because it sufficiently represents the continuum emission by aerosols, clouds and gases as reported in the literature, and is computationally more efficient. For ozone retrieval we use ozone lines from MIPAS band A (685–970 cm⁻¹) and band AB (1020–1170 cm⁻¹) as well. Therefore we checked ozone retrievals with lines from bands A or AB only for a systematic difference. Such a difference was indeed found and could, to a major part, be attributed to the spectroscopic data used in these two bands, and to a minor part to neglection of modelling of non-local thermodynamic (non-LTE) emissions. Another potential explanation, a bias in the radiance calibration of level-1B spectra of bands A and AB, could largely be ruled out by correlation analysis and inspection of broadband spectra. Further upgrades in the ozone retrieval consist of application of an all-zero a-priori profile and a weaker regularization. Finally, the ozone distribution obtained with the new retrieval setup (data versions V3o_O3_7) was compared to the data version used before (V2_O3_2). Differences are smaller than $\pm$0.4 ppmv in the altitude region 15–50 km. Further, differences to ozone measured by the HALogen Occultation Experiment (HALOE) on the Upper Atmospheric Research Satellite (UARS) are partly reduced with the new MIPAS data version.

The large solar storm in October–November 2003 produced enormous amounts of high‐energy protons which reached the Earth and penetrated into the middle atmosphere in the polar regions. At this time, the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on board the Environmental Satellite (ENVISAT) was observing the atmosphere in the 6–68 km altitude range. MIPAS observed significant enhancements of the NO_y components HNO₃, N₂O₅, and ClONO₂ in the northern polar stratosphere after the intense solar proton events. Two distinct HNO₃ enhancements were observed. An instantaneous increase of 1–2 ppbv was observed immediately after the SPEs and is attributed to gas‐phase chemistry: NO₂ + OH + M → HNO₃ + M, accelerated by SPE‐produced excess OH. A very large second increase of 1–5 ppbv started around 10 November and lasted until the end of December. It is attributed to NO_x (NO + NO₂) produced in the mesosphere during the major SPEs in late October/early November and then transported downward during November and December, partially converted to N₂O₅ in the upper stratosphere, which finally formed HNO₃ via ion cluster reactions. N₂O₅ was observed to increase by 0.1–0.4 ppbv 1–3 days after the major SPEs and reached down to 30 km altitude. A second, more pronounced N₂O₅ enhancement of up to 1.2 ppbv at 40 km appeared about 12–13 days after the major SPEs. With a delay of 1–2 days after the major SPEs ClONO₂ increased by up to 0.4 ppbv (40%) at 32 km altitude. NO_y enhancements in the Southern Hemisphere were generally less pronounced.