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SCIAMACHY is a passive imaging spectrometer onboard ENVISAT designed to obtain trace gas  abundances from measured radiances and irradiances in the UV to SWIR range in nadir-, limb- and  occultation-viewing modes. Its grating spectrometer introduces a substantial sensitivity to the  polarization of the incoming light with nonnegligible effects on the radiometric calibration. To  be able to correct for the polarization sensitivity, SCIAMACHY utilizes broadband Polarization  Measurement Devices (PMDs). While for the nadir-viewing mode the measured atmospheric polarization  has been validated against POLDER data (Tilstra and Stammes, 2007, 2010), a similar validation study  regarding the limb-viewing mode has not yet been performed. This paper aims at an assessment of  the quality of the SCIAMACHY limb polarization data. Since limb polarization measurements by other  air/spaceborne instruments in the spectral range of SCIAMACHY are not available, a comparison  with radiative transfer simulations by SCIATRAN V3.1 (Rozanov et al., 2013) using a wide range of  atmospheric parameters is performed. SCIATRAN is a vector radiative transfer model (VRTM) capable  of performing calculations of the multiply scattered radiance in a spherically symmetric  atmosphere.  The study shows that the limb polarization data exhibit a large time-dependent biasthat decreases with wavelength. Possible reasons for this bias are a still unknown combination ofinsufficient accuracy or inconsistencies of the on-ground calibration data, scan mirrordegradation and stress induced changes of the polarization response of components inside theoptical bench of the instrument. It is shown that it should in principle be feasible torecalibrate the effective polarization sensitivity of the instrument using the in-flight dataand VRTM simulations

Polarization data from SCIAMACHY limb backscatter observations compared to vector radiative transfer model simulations