Trends in Atmospheric Transmission at Three Locations in the United States from 1940 to 1977

Abstract

The pyrheliometric measurements at three locations in the United States axe examined to determine if there are trends in atmospheric transmission. Although the radiation values at Madison are depressed in the 1940's because of local pollution, there is no strong evidence of other man-made aerosol pollution at Madison, Albuquerque or Blue Hill. Collectively, there is a suggestion of increased aerosol pollution for the three stations, but the decrease in atmospheric transmission is rather small and confined to the urban areas where the measurements are made. The detected trends, if real, are too small to be important to climate on a regional or global scale because the estimated growth rate is ∼0.05% per year. There is evidence for the eruptions of Agung, Awu and Fuego in the radiation records. The eruption of Agung increases the optical depth by ∼0.035 several months after its eruption. By 1965 the atmospheric transmission returned to normal. The Lamb dust veil index in the Northern Hemisphere for these three eruptions is estimated to be 250. 85 and 165, respectively.