Comparison of box-air-mass-factors and radiances for Multiple-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) geometries calculated from different UV/visible radiative transfer models

摘要

The results of a comparison exercise of radiative transfer models (RTM) ofvarious international research groups for Multiple AXis Differential OpticalAbsorption Spectroscopy (MAX-DOAS) viewing geometry are presented. Besidesthe assessment of the agreement between the different models, a second focusof the comparison was the systematic investigation of the sensitivity of theMAX-DOAS technique under various viewing geometries and aerosol conditions.In contrast to previous comparison exercises, box-air-mass-factors(box-AMFs) for different atmospheric height layers were modelled, whichdescribe the sensitivity of the measurements as a function of altitude. Inaddition, radiances were calculated allowing the identification of potentialerrors, which might be overlooked if only AMFs are compared. Accuratemodelling of radiances is also a prerequisite for the correct interpretationof satellite observations, for which the received radiance can strongly varyacross the large ground pixels, and might be also important for theretrieval of aerosol properties as a future application of MAX-DOAS. Thecomparison exercises included different wavelengths and atmosphericscenarios (with and without aerosols). The strong and systematic influenceof aerosol scattering indicates that from MAX-DOAS observations alsoinformation on atmospheric aerosols can be retrieved. During the variousiterations of the exercises, the results from all models showed asubstantial convergence, and the final data sets agreed for most caseswithin about 5%. Larger deviations were found for cases with lowatmospheric optical depth, for which the photon path lengths along the lineof sight of the instrument can become very large. The differences occurredbetween models including full spherical geometry and those using only planeparallel approximation indicating that the correct treatment of the Earth'ssphericity becomes indispensable. The modelled box-AMFs constitute anuniversal data base for the calculation of arbitrary (total) AMFs by simpleconvolution with a given trace gas concentration profile. Together with themodelled radiances and the specified settings for the various exercises,they can serve as test cases for future RTM developments.