Solar Spectral Irradiance under Clear Skies around a Major Metropolitan Area

Abstract

This paper investigates the influence of gaseous pollutants and aerosol on the spectral composition of various segments of the solar spectrum in cloudless conditions. This investigation is done by using data of the spectral energy distribution of global and diffuse solar irradiances collected during a field experiment in Athens. The authors found that the Ångström turbidity coefficient ? always shows a temporal pattern with high values in the morning and the afternoon and low values at midday, and the wavelength exponent α widely varies over 1.02?1.4. Atmospheric turbidity produced a measurable but variable effect on spectral solar irradiances. The authors found that the relative attenuations caused by high urban aerosol can exceed 36% ± 7.5%, 30% ± 5.8%, and 26% ± 4.1% in the ultraviolet, visible, and near-infrared portions of the solar spectrum, respectively, as compared with ?background? values. On the other hand, the relative increase in scattered irradiance was greater in the near-infrared band (40% ± 4.8%), and in visible and ultraviolet bands the relative increase reached 31% ± 5.5% and 18% ± 6.5%, respectively. Spectrally reduced (Rayleigh corrected) and aerosol (Ångström) optical depths were retrieved, representing different aerosol loadings over the Athens atmosphere. The effects of altitude and the temporal and spatial variability of spectral optical depth values were analyzed. The overall results suggest that the shortest wavelengths are very sensitive to aerosol loading.