A simple, physically clear method for assessing the amount of atmospheric turbidity on a cloudless day by observing the circumsolar aureole--the ring of brightness around the sun--and comparing its brightness with that of the equivalent Rayleigh scattering (particle-free) atmosphere. The aureole is known to be caused by aerosols, sub-micron and micron sized particles derived from natural sources as well as from human activity. This report applies exact Mie theory phase functions for idealized polydispersions of spherical particles to account for the aureole. It shows that the brightness gradient within a disc of 10-deg radius around the sun indicates the size distribution of the aerosol particles, and the overall extent of the aureole within 40 deg around the sun indicates the degree of turbidity. With some modifications to account for the earth's sphericity, the method may be adequate for initial interpretation of certain low-orbit satellite observations with the sun close to the horizon. (See also R-456.) 48 pp. Ref.
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