In the course of a lengthy series of observations since 1975, a large, continuous decrease of the brightness of the solar aureole has been found west of Boston, at Hanscom Air Force Base and at Lexington, Mass. This points to the virtual disappearance from the lower atmosphere of giant particles larger than ~ 10 µm. in size while total suspended particulates in Boston and other U.S. cities have barely decreased. Results of calculations to better understand the relation between forward scattering and aerosol mass distribution [coarse fraction (CF)] are presented. In addition, a method to modify steep scattering functions calculated for a plane-wave source (Sun treated as a star) to those of the actual (and limb-darkened) Sun is presented. The calculated wavelength dependence of extinction, which is lower than that observed, is found to be little affected by the CF, but seems, like forward scattering, to be sensitive to mass distribution of sizes of < 0.4 and ~ 0.6 µm because of the anomal scattering behavior of spheres.

© 1993 Optical Society of America

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