Intercomparison exercise between different radiative transfer models used for the interpretation of ground-based zenith-sky and multi-axis DOAS observations

We present the results of an intercomparison exer- cise between six different radiative transfer (RT) models car- ried 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 modelling is an important step in the interpretation of Differential Optical Absorption Spectroscopy (DOAS) observations. It allows the conversion of slant column densities (SCDs) into vertical column densi- ties (VCDs) using calculated air mass factors (AMFs). The originality of our study resides in comparing SCD simula- tions in multi-axis (MAX) geometry (trace gases: NO2 and HCHO) and in taking into account photochemical enhance- ment for calculating SCDs of rapidly photolysing species (BrO, NO2, and OClO) in zenith-sky geometry. Concerning the zenith-sky simulations, the different models agree gener- ally well, especially below 90 SZA. At higher SZA, larger discrepancies are obtained with relative differences ranging between 2% and 14% in some cases. In MAX geometry, good agreement is found between the models with the calcu- lated NO2 and HCHO SCDs differing by no more than 5% in the elevation and solar zenith angle (SZA) ranges inves- tigated (5 -20 and 35 -85 , respectively). The impacts of aerosol scattering, ground albedo, and relative azimuth on MAX simulations have also been tested. Significant discrep- ancies appear for the aerosol effect, suggesting differences between models in the treatment of aerosol scattering. A bet-

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