Sensitivity of aerosol retrieval to geometrical configuration of ground-based sun/sky radiometer observations

Abstract. A sensitivity study of aerosol retrievals to the geometrical configuration of the ground-based sky radiometer observations is carried out through inversion tests. Specifically, this study is focused on principal plane and almucantar observations, since these geometries are employed in AERONET (AErosol RObotic NETwork). The following effects have 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, 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 set of retrieved aerosol properties over three AERONET sites: Mongu (Zambia) for biomass burning aerosol, Goddard Space Flight Center (GSFC; 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 practical advantages of the almucantar geometry: the symmetry between its left and right branches that helps to eliminate some observational uncertainties and the constant value of optical mass during the measurements, that make almucantar observations nearly independent of the vertical variability of aerosol. Nevertheless, almucantar retrievals present instabilities at high sun elevations due to the reduction of the scattering angle range coverage, resulting in decrease of information content. It is in such conditions that principal plane retrievals show a better stability, as shown by the simulation analysis of the three different aerosol models.

The last part of the study is devoted to the 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 the same sites used in the simulation analysis: Mongu (biomass burning), GSFC (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 the uncertainties estimated for the AERONET operational 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 the imaginary part of the refractive index, the differences are typically under 0.01 and 0.003, respectively. The real part of the refractive index showed a difference of 0.01 for biomass burning and urban aerosol, and a difference of around 0.03 for desert dust. Finally, it should be noted that the whole data set includes only 200 pairs, which have been taken under very stable atmospheric conditions; therefore, in a general case, differences between principal plane (PPL) and almucantar (ALM) are expected to be higher. Though the observed differences between ALM and PPL are rather small, it should be noted that this analysis has been conducted using a limited set of 200 observation pairs selected under stable atmospheric conditions.