The dependence of global and diffuse PAR radiation components on sky conditions at Athens, Greece

The interrelationship between broadband solar components, global (RS) and diffuse (Rd) and the photosynthetic photon flux density components, global (QP) and diffuse (QPd), is investigated using a 2-year data archive obtained at Athens, Greece. These data are used to determine temporal variability of the ratios (QP/RS) and (QPd/Rd) and their dependence on sky conditions. The seasonal variation of the ratios obtained from hourly data ranges from 2.006 ± 0.032 (winter) to 1.989 ± 0.028 μmol J−1 (summer) with an annual mean of 1.995 ± 0.032 μmol J−1 for (QP/RS) and from 2.298 ± 0.27 (winter) to 2.538 ± 0.22 μmol J−1 (summer) with an annual mean of 2.434 ± 0.26 μmol J−1 for (QPd/Rd). The hourly ratio (QP/RS) increased from 1.982 ± 0.025 to 2.106 ± 0.029 μmol J−1 as sky conditions changed from clear to overcast; whereas the ratio (QPd/Rd) decreased from 2.556 ± 0.27 to 2.422 ± 0.24 μmol J−1 as sky conditions changed from clear to overcast skies. The seasonal daily ratio patterns are presented and discussed through clearness index kt (the ratio of the global-to-extraterrestrial solar radiation). Finally, the relationship between the spectral diffuse fraction kdP (the ratio of diffuse PAR-to-global PAR radiation) and the clearness index ktP (the ratio of global PAR-to-extraterrestrial solar PAR) is established for the Athens atmosphere.