Ground-based evaluation of aerosol transmittance for cloudless and scattered cloudy skies

摘要

Abstract: An evaluation of atmospheric turbidity parameters related with aerosols from ground-based broadband measurements of global and diffuse radiation is presented. First, an inversion of a direct-diffuse broadband radiation splitting model is applied to cloudless sky radiation data. The inversion requires diffuse fraction as the main input parameter. Subsequently we perform an empirical correction to the diffuse fraction in order to extended the previous method to scattered cloud conditions. The correction is based on temporal variations of global atmospheric transmissivity normalized with respect to the air mass. In order to test the applicability of the broadband method and our extension, concurrent measurements of spectral radiation were also taken. Spectra were taken by a spectroradiometer equipped with an integrating sphere and an occulting tube. Under scatted cloudy conditions, spectra were taken during direct shining episodes, that is between occultation intervals. Values of aerosol extinction and of angstrom's $beta coefficient have been obtained. The latter were obtained through two procedures: a single wavelength evaluation, and a fit to the aerosol spectral extinction coefficients. Values of $beta obtained from the cloudiness correcte diffuse fraction show a smooth daily evolution that would correspond to an equally turbid but cloudless atmosphere. Results suggest that cloudiness correction to the diffuse fraction can extend the inversion of cloudless broadband radiation models to scattered cloudy skies, in order to retrieve aerosol parameters. !11