Alternative polarisation retrieval for SCIAMACHY in the ultraviolet

We introduce an alternative method for the retrieval of polarisation in the ultraviolet by the satellite spectrometer SCIAMACHY. Unlike the operational polarisation retrieval algorithm, this method does not use the Polarisation Measurement Devices (PMDs) onboard SCIAMACHY, but only requires the reflectance signal. This makes the algorithm more robust and less sensitive to calibration errors caused by either improper characterisation of the instrument’s response functions (key data) or degradation of the optical components. The alternative polarisation retrieval is able to retrieve the full state of atmospheric polarisation in the wavelength range between 330 and 400 nm, which is essentially the wavelength region covered by SCIAMACHY’s PMD 1. This allows a direct comparison with the current operational product. When we compare the alternative polarisation algorithm with the operational algorithm, we find in some cases agreement, but not in other cases. The alternative algorithm compares well with an analytical model of the polarisation of a cloud-free scene. Using the alternative algorithm the polarisation-sensitive feature in the SCIAMACHY reflectance around 350 nm is automatically corrected for.