Aerosol features retrieved from measurements of sky irradiance at ground

Measurements of the direct solar and diffuse sky irradiances are used for deriving optical and physical features of the atmospheric aerosols: optical depth, size distribution, and complex refractive index. Sky irradiance is measured by means of a sky-radiometer (aureolemeter), which consists essentially of a photometer with annexed interferential filters for the selection of the wavelengths at which to operate, and a pointing system. The device used in the authors´ research program, manufactured by the Japanese Company Prede (Tokyo), operates at the following wavelengths: 0.315, 0.400, 0.500, 0.870, 0.940 and 1.040 μm. Measurements are undertaken in the two following different ways. Almucantar: pointing along a conical surface with the same zenith angle of the Sun. Principal plane, the instrument aims the sky along a plane with the same azimuth angle of the Sun. The two quantities considered in their scheme of data elaboration are the direct solar irradiance V and the ratio R between the diffuse and the direct irradiance