The total beta activity of dry atmospheric particles collected in Chicago, Illinois, ranged from 8973 pc per 100,000 ft 3 of air in December 1958 to 177 pc per 100,000 ft 3 in November 1959. From July to November 1959 the total beta activity decreased from 6990 to 399 pc per g of particles, and the strontium-90 activity from 229 to 16 pc per g. The activity of 1-to 10-micron particles was related to the cube of the particle diameter in the summer and to the fourth power in the winter. It is postulated that the activity resulted from rainsoaking during the particles' residence on the earth's surface. It was found that nonradioactive particles produced by urban activity dilute various size fractions of radioactive debris to different degrees.
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