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 mmol J1 (summer) with an annual mean of 1.995 0.032 mmol J1 for (QP/RS) and from 2.298 0.27 (winter) to 2.538 0.22 mmol J1 (summer) with an annual mean of 2.434 0.26 mmol J1 for (QPd/Rd). The hourly ratio (QP/RS) increased from 1.982 0.025 to 2.106 0.029 mmol J1 as sky conditions changed from clear to overcast; whereas the ratio (QPd/Rd) decreased from 2.556 0.27 to 2.422 0.24 mmol J1 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. # 2007 Elsevier B.V. All rights reserved.