This study correlates experimental values with modelled values of global (K-T) and diffuse (K-D) clearness index in Athens, Greece. The experimental values come from measurements of daily global and diffuse solar irradiation on horizontal surface, while the modelled ones are from linear regression expressions fitted to the experimental data. It is found that the correlations give excellent results for nearly all experimental periods considered (1990-2000). The study also tries to correlate K-T values with black smoke (BS) concentration. The analysis shows that there is no significant trend in K-T, which would reflect an increase or decrease in light scattering by BS. (C) 2003 Elsevier Science Ltd. All rights reserved.