Parametrisation of Earth's polarisation spectrum in the ultra-violet

An accurate radiometric calibration of polarisation-sensitive space-borne instruments operating in the UV is important for the correct retrieval of data products that require absolute radiances such as the ozone profile. We attempt to improve this calibration through the development of a parametrisation of ultra-violet earth-shine polarisation. We have constructed a database of UV (290– 330 nm ) top-of-atmosphere Stokes vectors for various solar and viewing geometries as well as surface albedos and atmospheric profiles, through radiative transfer modelling. We show that it is possible to parametrise the spectral shape of polarisation (as defined by two characteristic wavelengths) in the geometrical airmass, the surface albedo, and the total ozone column. This work is particularly relevant to the calibration of polarisation-sensitive instruments like GOME (launched in 1995), SCIAMACHY and GOME-2 (to be launched in 2002 and 2005, respectively). We show that the parametrisation allows a substantial improvement of the radiometric calibration of GOME UV radiances.