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 Inter ferometer for Passive Atmospheric Sounding) i.r. spectrometer will sca n across the limb in order to record high resolution emission spectra. In the course of the definition of micro-windows for retrieval of lin e of sight, temperature and trace constituents, the spectral and altit udinal regions where CO2 Q-branch line mixing has to be considered hav e been identified. Line-by-line modelling of spectra was performed tak ing account of line mixing and resulting spectra were compared to thos e calculated within purely Lorentzian pressure broadening. The accurac y of the Rosenkranz approximation was tested and found to be sufficien t in most spectral regions. The impact of CO2 Q-branch line mixing on the retrieval was compared to typical random errors due to spectral no ise. Systematic errors due to the neglection of line mixing proves to play no important role for the temperature, pressure and trace constit uents 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 regi ons assigned for on-line processing of MIPAS measurements. (C) 1998 El sevier Science Ltd. All rights reserved.