Calculation of multiply-scattered radiation in clean atmospheres

Assessment of the effects of air pollution on visibility requires an understanding of atmospheric radiative transfer processes. This paper presents results from both detailed and approximate computations of multiply-scattered solar radiation in the atmosphere. These calculations are intercompared and then compared with telephotometer measurements of the sky intensity obtained during the EPA-sponsored VISTTA (Visibility Impairment by Sulfate Transport and Transformation in the Atmosphere) field programs. The paper describes computational techniques and the specification of atmospheric radiative properties. Comparisons of predicted and observed intensities are made for various cloudless sky conditions. The conditions range from typical clean tropospheric values to ultraclean conditions that approach a Rayleigh atmosphere. Comparisons made at four wavelengths (405, 450, 550 and 630 nm) show the influences of observer azimuth and zenith angles and of the solar zenith angle. The results indicate that the horizon intensity values are surprisingly insensitive to aerosol amount for certain azimuth angles.