Abstract

Because of their strong forward scattering, large airborne particles of as much as 100 μm in diameter, which originate from industry and transportation (especially from roads and highways), can nearly always be detected at the edge of the sun’s disk. During May and June, airborne pollen sometimes makes these aureoles significantly brighter (and colored). In the balance between their production, dispersal by turbulence, and fallout, their brightness generally increases significantly during the day. By measurably suppressing the glare, it was found that in observations since 1974, the average brightness (and, therefore, the column load) of giant aerosols, gradually decreased to approximately one-tenth of that in 1979. This is approximately eight times faster than reduction of pollution measures in the central and eastern United States (~4% per year), which is related to small-sized haze and smoke. A corresponding, yet small, effect can be deduced from high-volume aerosol collections from urban areas in two size ranges. Had those 24-hour and all-weather collections been made in daytime and had they been for fair weather only, the results might have confirmed our conclusions more quantitatively.

© 1990 Optical Society of America