CO2 line mixing in MIPAS limb emission spectra and its influence on retrieval of atmospheric parameters

Aboard the European ENVISAT polar platform, the MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) i.r. spectrometer will scan across the limb in order to record high resolution emission spectra. In the course of the definition of micro-windows for retrieval of line of sight, temperature and trace constituents, the spectral and altitudinal regions where CO2 Q-branch line mixing has to be considered have been identified. Line-by-line modelling of spectra was performed taking account of line mixing and resulting spectra were compared to those calculated within purely Lorentzian pressure broadening. The accuracy of the Rosenkranz approximation was tested and found to be sufficient in most spectral regions. The impact of CO2 Q-branch line mixing on the retrieval was compared to typical random errors due to spectral noise. Systematic errors due to the neglection of line mixing proves to play no important role for the temperature, pressure and trace constituents retrieval in spectral regions of more than 2 cm(-1) distance to CO2 Q-branch centres. Apart from a few exceptions retrieval errors due to the neglection of line mixing are negligible for the spectral regions assigned for on-line processing of MIPAS measurements. (C) 1998 Elsevier Science Ltd. All rights reserved.