The radiative scattering by clouds leads to errors in the retrieval of column densities and concentration profiles of atmospheric trace gas species from satellites. Moreover, the presence of clouds changes the UV actinic flux and the photo-dissociation rates of various species significantly. The Global Ozone Monitoring Experiment (GOME) 5 instrument on the ERS-2 satellite, principally designed to retrieve trace gases in the atmosphere is also capable of detecting clouds. Four cloud fraction

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... ieval methods for GOME data that have been developed are discussed in this paper (the Initial Cloud Fitting Algorithm, the PMD Cloud Retrieval Algorithm, the Optical Cloud Recognition Algorithm and the Fast Retrieval Scheme for Cloud Observables). Their results of cloud 10 fraction retrieval are compared to each-other and also to synoptic surface observations. It is shown that all studied retrieval methods calculate an effective cloud fraction that is related to a cloud with a high optical thickness. Generally, we found ICFA to produce the lowest cloud fractions, followed by OCRA, then FRESCO and PC2K along four processed tracks (+2%, +10% and +15% compared to ICFA respectively). Synoptical 15 surface observations gave the highest absolute cloud fraction when compared with individual PMD sub-pixels of roughly the same size. 20 From this data, it is possible to retrieve information about atmospheric constituents. The retrieval of density columns of trace gases such as O 3 , NO 2 , BrO, SO 2 , OClO and HCHO and profiles of ozone, especially in the troposphere, are to some extent dependent on a correct description of the partially cloudy scenes in the field of view (Burrows et al., 1999; Hoogen et al., 1999; van der A et al., 1998; Munro et al., 1998; Koelemeijer 25 and Stammes, 1999a). In radiative transfer calculations for retrieval, clouds are often 624 ACPD 2, 2002