In order to specifically account for the effects of pyrogenic aerosols on the PAR/K↓ ratio, two time intervals were considered: the period prior to biomass burning and the period of peak burning. Fig. 3 shows the relationship between the PAR/K↓ ratio and solar zenith angle for 2 days prior to the main burning period, and 2 days during the peak of the burning period. For each period, 1-min averages of the PAR/K↓ ratio for a cloudless and near-cloudless day are presented
(data for solar zenith angles >75◦ are excluded). The PAR/K↓ ratio remained relatively constant at all zenith angles on days which occurred during the preburning period, when atmospheric turbidity values were low. During the peak biomass burning period, when turbidity values increased considerably, the PAR/K↓ ratios were lower and also exhibited an inverse relationship with the zenith angle. At low zenith angles, the optical path through the aerosol layer is minimal and therefore,
there would be less waveband specific attenuation of PAR by pyrogenic aerosols. At large zenith angles, the optical path is greater and the attenuation of PAR by these aerosols is enhanced