Abstract
In the present analysis broad-band turbidity parameters, which depend upon the attenuation over the entire solar spectrum and which can be determined from unfiltered pyrheliometric measurements of direct beam solar irradiance, are presented. Unsworth–Monteith attenuation coefficient TUM, Linke factor TL and Angstrom turbidity coefficient β (in parameterized form), were calculated from mid-day observations at the National Observatory of Athens, Greece for the period 1954–1991. Values of Angstrom turbidity coefficient β compare well with those obtained directly through the so-called pyrheliometric formula β0. Summertime turbidity levels were found to be higher than winter values. The long-term variation of the turbidity parameters in conjunction with their frequency distribution depicts the deterioration of air quality in the Athens basin during the period under study. The dependence on the aerosol amount of the fraction of the various spectral bands to direct total irradiance as well as the ratio of diffuse to total global irradiation, are also discussed.
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