Analysis is made of atmospheric transmission measurements taken during the first half of the twentieth century at thirteen widely separated sites by the Astrophysical Observatory of the Smithsonian Institution. Long and short period variations in the atmospheric transmission have been examined for the individual sites. Significant variations from one day to the next are not unusual. Seasonal variations are found for all sites, with characteristic summer maxima. The variations appear to be due almost entirely to variations in the aerosol component above the observing sites. The observed phases of these variations, the derived characteristic size distributions of the particles which cause them, and the correlations between atmospheric moisture and transmission lead to the conclusion that differences in the nature and sources of aerosols above arid and vegetated regions are detectable. Including the results of more recent studies, our best estimate is that, except for sporadic perturbations due to volcanic activity, there has been no detectable change in the global atmospheric transmission measured from remote, high altitude sites in the last half century.

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