Abstract
Natural radioactivity in the atmosphere is used as a ``tracer'' in the study of aerosol particle‐size distribution in the submicroscopic range. Particulate matter is collected by a device which first ionizes the particles and then separates them according to their mobilities. The results indicate that most of the natural radioactivity in the atmosphere attaches itself to particles having diameters in the range 0.001 to 0.04 micron. A predominant grouping of the activity occurs in the vicinity of two particle diameters: 0.009 and 0.018 micron. The relationship between the distribution of radioactivity and the abundance of particles in different size ranges is discussed.

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