Extinction Measurements for Sensitive Assessment of Air Quality

Measures of horizontal extinction were initiated at the Lowell Observatory at Flagstaff, Arizona In January of 1972, and by October of 1973 an instrumental accuracy compatible with the best observing conditions was achieved. Excellent data obtained in a horizontal direction during a seven night run are compared with long term highly standardized astronomical measures of extinction obtained at Flagstaff during the period 1955–1972. The data obtained both vertically and horizontally suggest that the extinction and scattering due to aerosols were at times as low as 10% of that caused by molecular (Rayleigh) scattering alone. The vertical extinction seems to correspond (with several months lag) to rough estimates of volcanic loading; the effect of the March 17, 1963 outburst of Mount Agung, a volcano on the island of Bali, very probably caused abnormally high extinction in late summer of that year. A generally high extinction level was observed throughout 1964-1965 with a rather slow decay to a “normal” level showing up by 1966. The very high level of transparency and abnormally blue sky found in 1961 was again observed in 1972. These data strongly suggest that man-made aerosols are not yet the major contributor to the particulate matter in the upper atmosphere.