SPECTRAL DISTRIBUTION OF SOLAR-RADIATION IN ATHENS

A 25-year (1966-1990) record of measurements of the broad-band direct solar irradiances performed in Athens, has been utilized to determine the radiant energy distribution in several spectral bands. Using these data the year-to-year trend of the time sequences of the mean values of the irradiation ratios in the various spectral intervals, viz. blue, green/orange, red, and photosynthetically active radiation (PAR) and the total direct irradiance for the whole spectrum, are evaluated. From this trend the following can be concluded: the blue spectral band (0.380-0.525 mum) decreased from the beginning of the examined period until the mid-seventies and then increased gradually; the red band (0.630-0.710 mum) shows similar trend but opposite in sense. The PAR band (0.380-0.710 mum) decreased slightly until 1985 and then increases. In the green/orange band (0.525-0.630 mum) the trend can be considered as nearly constant throughout the examined period. This trend of the irradiation ratios is related to the aerosol and gaseous pollutant content of the atmosphere, which probably reflects the urbanization, industrialization and heavy traffic conditions of the Athens area, during the last three decades.