A comparison of experimental and theoretical spectra for photosynthetically active radiation at various atmospheric turbidities

The spectral irradiance at the earth's surface in the waveband 300–740 mm, measured on two occasions of relatively high and low atmospheric turbidity, was shown to be in good agreement with values generated by two theoretical models of absorption and scattering in the atmosphere. Values for other solar elevations from the Avaste model were then used to explore the possible effects of turbidity on the global irradiance in the photosynthetically active waveband (400–700 nm), and on the photosynthetic rates of leaves, throughout a cloudless day. The results confirmed our earlier conclusion, based on only two measured spectra, that an increase in atmospheric turbidity is unlikely to reduce the photosynthetic rate of a leaf by more than 5%.