Validation of GOME total ozone by means of the Norwegian ozone monitoring network

The Global Ozone Monitoring Experiment (COME) onboard the ERS-2 satellite h as been in operation since July 1995. The Norwegian ground based total ozon e network has played an important role both in the main validation during t he commissioning phase and in the validation of upgraded versions of the an alysis algorithms of the instrument. The ground-based network consists of v arious spectrometer types (Dobson, Brewer, UV filter instruments). The vali dation of the second algorithm version used until January 1998 reveals a ve ry good agreement between GOME and ground-based data at solar zenith angles <60 degrees and deviations of GOME total ozone data from groundbased data of up to +/-60 DU (similar to 20%) at zenith angles >60 degrees. The deviat ions strongly depend on the season of the year, being negative in summer an d positive in winter/spring, The deviations furthermore show a considerable scattering (up to +/-25 DU in monthly average values of 5 degrees SZA inte rvals), even in close spatial and temporal coincidence with ground-based me asurements, especially in the high Arctic. The deviations are also dependen t on the viewing geometry/ground pixel size with an additional negative off set for the large pixels used in the backswath mode and at solar zenith ang les >85 degrees, compared to forward-swath pixels.