Determination of aerosol optical thickness from measurements of spectral sky radiance

Simultaneous measurements of spectral optical thickness and path radiance are used to test the relationship between path radiance and aerosol optical thickness for a given geometry of illumination and observation. The selected geometry is representative of satellite remote sensing conditions. The measurements have been accomplished in an urban location with a slightly polluted atmosphere. Ground-based measurements were made with a device that combines sun-photometer and sky radiometer functions. The measurement sequence includes measurements of transmission of the direct sunlight in order to retrieve the aerosol optical thickness in nine spectral bands between 440 and 1030 nm, followed by measurements of the sky radiance in the solar almucantar on both sides of the sun in the same spectral bands. For this purpose we have used a set of radiances acquired around 60° solar zenith angle in the antisolar direction of the solar almucantar. The results obtained show that aerosol optical thickness estimates are possible from path radiance measurements. On the other hand, it seems feasible to derive the atmospheric correction in a given spectral band from information acquired at another wavelength. The aerosol optical thickness (radiance) in one spectral band allows appropriate estimates of aerosol optical thickness (radiance) in another spectral band. In this sense, we also obtain a relationship between the spectral dependencies of aerosol optical thickness and path radiance.