Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 5 5 2005 10.5194/acpd-5-7929-2005 http://www.atmos-chem-phys-discuss.net/5/7929/2005/ http://www.atmos-chem-phys-discuss.net/5/7929/2005/acpd-5-7929-2005.html http://www.atmos-chem-phys-discuss.net/5/7929/2005/acpd-5-7929-2005.pdf 7929 7964 2005-09-02 Intercomparison exercise between different radiative transfer models used for the interpretation of ground-based zenith-sky and multi-axis DOAS observations F. Hendrick M. Van Roozendael A. Kylling A. Petritoli A. Rozanov S. Sanghavi R. Schofield C. von Friedeburg T. Wagner F. Wittrock D. Fonteyn M. De Mazière Institut d’Aéronomie Spatiale de Belgique, Brussels, Belgium Norwegian Institute for Air Research, Kjeller, Norway Institute of Atmospheric Science and Climate, Bologna, Italy Institute of Environmental Physics, University of Bremen, Bremen, Germany Institute of Environmental Physics, University of Heidelberg, Heidelberg, Germany National Institute of Water and Atmospheric Research, Omakau, Central Otago, New Zealand now at: St. Olavs University Hospital, Trondheim, Norway now at: NOAA Aeronomy Laboratory, Boulder, Colorado, USA We present the results of an intercomparison exercise between six different radiative transfer (RT) models carried out in the framework of QUILT, an EU funded project based on the exploitation of the Network for the Detection of Stratospheric Change (NDSC). RT modeling is an important step in the interpretation of Differential Optical Absorption Spectroscopy (DOAS) observations. It allows the conversion of the slant column densities (SCDs) into vertical column densities (VCDs) using calculated air mass factors (AMFs). The originality of our study resides in comparing SCD simulations in multi-axis (MAX) geometry (trace gases: NO₂ and HCHO) and in taking into account the photochemical enhancement for calculating SCDs of rapidly photolysing species (BrO, NO₂, and OClO) in zenith-sky geometry. Concerning the MAX simulations, good agreement is observed between the different models with the calculated NO₂ and HCHO SCDs differing by no more than 5% in the elevation and solar zenith angles (SZA) ranges investigated (5°–20° and 35°–85°, respectively). The impacts of aerosol scattering, ground albedo, and relative azimuth on MAX simulations have also been tested. Large discrepancies appear for the aerosol effect, suggesting differences between models in the treatment of the aerosol scattering. A better agreement is obtained in the case of the ground albedo and relative azimuth effects. In zenith-sky geometry, the different models agree generally well, especially below 90° SZA. At higher SZA, larger discrepancies are observed with relative difference values between 2% and 14% in some cases. All the initialization data and results have been made publicly available through the QUILT project web site (http://nadir.nilu.no/quilt/), enabling the testing of other RT codes designed for the calculation of SCDs/AMFs.