URBAN AEROSOL AND CLEAR SKIES SPECTRA FOR GLOBAL AND DIFFUSE PHOTOSYNTHETICALLY ACTIVE RADIATION

From a data archive of spectral energy distribution of global and diff use solar irradiances measured in Athens during a field experiment, th e influence of atmospheric turbidity caused by urban aerosol on spectr al composition of visible radiation and transparency of urban atmosphe re has been determined for cloudless conditions. The effect of urban a erosol is significant in the PAR part of solar spectrum, where attenua tion of global PAR irradiance surpasses 18% on very polluted atmospher ic conditions. The increase in diffuse PAR irradiance caused by urban aerosol ranges in the interval 7%-51%. The ratio of global PAR to tota l global irradiance was found to depend slightly on air pollution wher eas ratio's dependencies on site elevation were not identified clearly . Nevertheless, this ratio was found to be in good agreement with thos e reported in the literature. The ratio of diffuse-to-global PAR irrad iance was found to depend on both, air mass and turbidity. For very tu rbid atmosphere and large air masses the 40% of PAR irradiance results from diffuse radiation. Finally, some biological implications of the results are briefly discussed. (C) 1997 Elsevier Science B.V.