The daily fluctuation of the concentration of fallout particles in the ground-level air whose diameters ranged from 1 to 30 μ was observed for a period of about 3 months following the fifth Chinese nuclear explosion of December 28, 1966. It appeared that fallout particles larger than about 10 μ in diameter fell on the ground mainly by gravitational settling shortly after the explosion, and some smaller particles circulated with the air motion around the globe more than once. It was also found that about 20 days after the explosion the fluctuation of the particle concentration was rather well related to the variation of meteorological situations; the particle concentration decreased during periods of precipitation and increased during periods of high atmospheric pressure.
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