Solar radiation data collected over the last 27 years at the University of Arizona have been analyzed to determine the major causes of time variations in the local turbidity of the atmosphere. The most extreme perturbations have been associated with significant volcanic eruptions, Agung on Bali in 1963 and, especially, El Chichón in Mexico in 1982. There is also evidence for a correlation of turbidity with local climatic factors and local smelter activity, but no detectable long-term upward or downward trend in turbidity has been observed. Routine pyrheliometric measurements were used to calculate monthly Linke turbidity factors for the period from June 1956 to June 1983 and related monthly aerosol values for the period from January 1977 to February 1983.