Calculations of diffusely reflected radiance emerging at the top of the atmosphere for a known aerosol size distribution have been made using the radiative transfer equation. The computed radiance was compared with the CZCS radiance collected within 1-2 hours after the aerosol measurements from the same area. An optical depth of the Earth's atmosphere is inferred from the comparison. The procedure was carried out for wavelengths corresponding to the CZCS channels (443, 520, 550, 670 and 750nm). Values of optical depth are plotted against the wavelengthsto provide an optical depth spectrum. The results indicate that there isa strong dependenceof optical depth on the wavelength. Thus, the ratio of optical depths at different wavelengths throughout the whole spectrum is not constant. Also, at various aerosol concentrations of large particles, the ratio is found to be variable. This suggests that a precise value of the optical depth ratio cannot be easily estimated although the use of the ratio in an atmospheric correction algorithm can provide satisfactory accuracy. The computed radiance is found to be linearly related to the optical depth and, therefore, a simple approximation for the aerosol radiance based on a lineardependenceon the aerosol optical depth should be accurate. The study also shows that the effect due to misclassification between the contaminated and uncontaminated pixels by thin haze or cloud, is to cause a significant reduction in the optical depth ratio.(443)/.(550). The resultingvaluesof the optical depth ratio cannot beestimated by the approximation suggestedin Gordon and Clark (1981)and Gordon et al. (1983).