1Group of Atmospheric Optics, Valladolid University, Valladolid, Spain
2Laboratoire d'Optique Amosphérique, UMR8518, Université des Sciences et Technologies de Lille, Villeneuve d'Ascq, France
3Izana Atmospheric Research Center, Spanish Meteorological Agency, Tenerife, Spain
Abstract. A sensitivity study of the aerosol optical properties retrieval to the geometrical configuration of the ground-based sky radiometer observations is carried out through the inversion tests. Specifically, the study is focused on the principal plane and almucantar observation, since these geometries are employed in Aeronet (AErosol RObotic NETwork). The following effects has been analyzed with simulated data for both geometries: sensitivity of the retrieval to variability of the observed scattering angle range, uncertainties in the assumptions of the aerosol vertical distribution and surface reflectance, possible instrument pointing errors and the effects of the finite field of view. The synthetic observations of radiometer in the tests were calculated using a previous climatology data of retrieved aerosol over three Aeronet sites: Mongu (Zambia) for biomass burning aerosol, Goddard Space Flight Center (Maryland-USA) for urban aerosol and Solar Village (Saudi Arabia) for desert dust aerosol. The results show that almucantar retrievals, in general, are more reliable than principal plane retrievals in presence of the analyzed error sources. This fact partially can be explained by to practical advantages of almucantar geometry: the symmetry between its left and right branches that helps to eliminate some observational uncertainties and the constant value of optical mass constant during the measurements that makes almucantar observations nearly independent on vertical variability of aerosol. Nevertheless, almucantar retrievals present instabilities at high sun observations due to the reduction of the scattering angle range coverage resulting in decrease of information content.
The last part of the study is devoted to identification of possible differences between the aerosol retrieval results obtained from real Aeronet data using both geometries. In particular, we have compared Aeronet retrievals at three different key sites: Mongu (biomass burning), Beijing (urban) and Solar Village (desert dust). Overall this analysis shows robust consistency between the retrievals from simultaneous observations in principle plane and almucantar. All identified differences are within uncertainties estimated for Aeronet aerosol retrieval. The differences in the size distribution are generally under 10% for radii between 0.1 μm and 5 μm and outside this size range, the differences can be as large as 50%. For the absorption parameters, i.e. single scattering albedo and imaginary part of refractive index, the differences are typically under 0.01 and 0.002 respectively. The real part of the refractive index showed an error of 0.01 for biomass burning and urban aerosol and around 0.03 for desert dust.