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

Abstract

Abstract. The results of a comparison exercise of radiative transfer models (RTM) of various international research groups for Multiple AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) viewing geometry are presented. In contrast to previous comparison exercises, box-air-mass-factors (box-AMFs) for various atmospheric height layers were modelled, which describe the sensitivity of the measurements as a function of altitude. In addition, radiances were calculated allowing the identification of potential errors, which might be overlooked if only AMFs are compared. Accurate modelling of radiances is also a prerequisite for the correct interpretation of satellite observations, for which the received radiance can strongly vary across the large ground pixels, and might be also important for the retrieval of aerosol properties as a future application of MAX-DOAS. The comparison exercises included different wavelengths and atmospheric scenarios (with and without aerosols). The results were systematically investigated with respect to their dependence on the telescope's elevation angle and the azimuth angle. For both dependencies, a strong and systematic influence of aerosol scattering was found indicating that from MAX-DOAS observations also information on atmospheric aerosols can be retrieved. During the various iterations of the exercises, the results from all models showed a substantial convergence, and the final data sets agreed for most cases within about 5%. Larger deviations were found for cases with low atmospheric optical depth, for which the photon path lengths along the line of sight of the instrument can become very large. The differences occurred between models including full spherical geometry and those using only plane parallel approximation indicating that the correct treatment of the Earth's sphericity becomes indispensable. The modelled box-AMFs constitute an universal data base for the calculation of arbitrary (total) AMFs by simple convolution with a given trace gas concentration profile. Together with the modelled radiances and the specified settings for the various exercises, they can serve as test cases for future RTM developments.

MAX-DOAS measurements in southern China: retrieval of aerosol extinctions and validation using ground-based in-situ data

“…RTMs were reviewed by Hendrick et al (2006) and Wagner et al (2007). Different RTMs differ in the way of simulating photon transverse process in the atmosphere, the treatment of the Earth's sphericity, the way of considering aerosol scattering, the inclusion of the photo-enhancement of short lifetime species, etc.…”

“…In this study we concentrate on the effect of aerosols and investigate only measurements under mostly cloud free conditions. Over the last years, different radiative transfer models have been developed (see Wagner et al, 2007, and references therein). We used McArtim (Deutschmann, 2009), which is a backward Monte-Carlo model.…”

Determination of aerosol properties from MAX-DOAS observations of the Ring effect

“…In this study we use the full spherical Monte-Carlo atmospheric radiative transfer model McArtim (Monte Carlo Atmospheric Radiative Transfer and Inversion Model) (Deutschmann, 2008); it was compared to various other radiative transfer models (Wagner et al, 2007a) and excellent agreement was found. McArtim allows the simulation of ensembles of individual photon trajectories for a given Table 2.…”

Ship-based MAX-DOAS measurements of tropospheric NO<sub>2</sub>, SO<sub>2</sub>, and HCHO distribution along the Yangtze River

“…This method is so-called "geometric approximation". However, geometric approximation of AMF could result in large errors under high aerosol load conditions (Wagner et al, 2007). In addition, relative azimuth angle, defined as the angle between the viewing direction and the sun, also plays an important role in the AMF calculation.…”