Critical evaluation of the role of the volcanic dust introduced by three major eruptions--Krakatoa (1883), Katmai (1912), and Agung (1963)--in increasing atmospheric particulate turbidity. Typical turbidity anomalies, expressed as absolute increments in optical thickness in the middle of the visual spectrum, are found to be 0.55 for Krakatoa, 0.35 for Katmai, and 0.25 for Agung. The last represents a fivefold increase of normal turbidity away from cities over a period of 2 to 3 years. No evidence of climatic effects directly related to the volcanic dust incursions is found. The possible contribution by the operation of 500 commercial supersonic transport vehicles to particulate turbidity is estimated to be small and climatologically not significant. An initial "black cloud" experiment, consisting of a simple reduction by 10 to 20 percent of the incoming short-wave radiation, is suggested for use with numerical models of the general circulation of the atmosphere to simulate volcanic dust effects. 73 pp. Ref. (Author)
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