Journal of Quantitative Spectroscopy & Radiative Transfer 75 (2002) 239–255 www.elsevier.com/locate/jqsrt Parametrisation of Earth’s polarisation spectrum in the ultra-violet  N.A.J. Schutgens ∗ , P. Stammes Royal Netherlands Meteorological Institute (KNMI), P.O. Box 201, 3730 AE de Bilt, Netherlands Received 16 August 2001; accepted 23 November 2001 Abstract 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 prole. 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 proles, through radiative transfer modelling. We show that it is possible to parametrise the spectral shape of polarisation (as dened 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. ? 2002 Elsevier Science Ltd. All rights reserved. 1. Introduction Although sunlight itself is unpolarised (when integrated over the full solar disk), its reection o the Earth’s atmosphere is not. Air molecules, cloud droplets, aerosols and ice crystals all contribute to the polarisation of scattered light. The polarisation of the earth-shine inuences the radiometric response functions of many space-borne instruments: GOME, GOME-2 (Global Ozone Measure- ment Experiment [1]), SCIAMACHY (Scanning Imaging Absorption SpectroMeter for Atmospheric CartograpHY [2]), OSIRIS (Optical Spectrograph and Infrared Imaging System [3]) and POLDER (POLarisation and Directionality of Earth Reectances [4]). Through extensive pre-ight calibration and in-ight monitoring of polarisation, this eect can be corrected for.  Supported by the Space Research Organisation Netherlands (SRON), through grant EO-027. * Corresponding author. 0022-4073/02/$-see front matter ? 2002 Elsevier Science Ltd. All rights reserved. PII:S0022-4073(01)00248-5