Retrieval of aerosol properties from zenith sky radiance measurements

Abstract. This study explores the potential to retrieve aerosol properties with the GRASP algorithm (Generalized Retrieval of Atmosphere and Surface Properties) using as input measurements of zenith sky radiance (ZSR), which are sky radiances measured in the zenith direction, recorded at four wavelengths by a ZEN-R52 radiometer. To this end, the ZSR measured at 440, 500, 675 and 870 nm by a ZEN-R52 (ZSR_ZEN), installed in Valladolid (Spain), is employed. This instrument is calibrated intercomparing the signal of each channel with coincident ZSR values simulated (ZSR_SIM) at the same wavelengths with a radiative transfer model (RTM). These simulations are carried out using the GRASP forward module as RTM and the aerosol information from a collocated CE318 photometer belonging to the AERONET network (Aerosol and Robotic Network) as input. Dark signal and the signal dependence on temperature are characterized and included in the calibration process. The uncertainties on each channel are quantified by an intercomparison with a collocated CE318 photometer, obtaining lower values for shorter wavelengths; between 3 % for 440 nm and 21 % for 870 nm. The proposed inversion strategy for the aerosol retrieval using the ZSR_ZEN measurements as input, so-called GRASP-ZEN, assumes the aerosol as an external mixture of five pre calculated aerosol types. A sensitivity analysis is conducted using synthetic ZSR_ZEN measurements, pointing out that these measurements are sensitive to aerosol load and type. It also assesses that the retrieved aerosol optical depth (AOD) values in general overestimates the reference ones by 0.03, 0.02, 0.02 and 0.01 for 440, 500, 675, 870 nm, respectively. The calibrated ZSR_ZEN measurements, recorded during two and half years at Valladolid, are inverted by GRASP-ZEN strategy to retrieve some aerosol properties like AOD. The retrieved AOD shows a high correlation with respect independent values obtained from the collocated AERONET CE318 photometer, with a determination coefficient (r²) about 0.86, 0.85, 0.79 and 0.72 for 440, 500, 675 and 870 nm, respectively, and finding uncertainties between 0.02 and 0.03 with respect to the AERONET values. Finally, it is studied the goodness of other retrieved aerosol properties like aerosol volume concentration for total, fine and coarse modes.