RAIN SCAVENGING OF RADIOACTIVE PARTICULATE MATTER FROM THE ATMOSPHERE

Abstract

On the basis of Langmuir's theory of collection efficiencies, a model is derived to explain the removal of radioactive particulate matter from the atmosphere by rain. In this connection, Langmuir's collection efficiencies are modified to take into account the density of the solid particles, and are adjusted to express the fraction removed from a vertical cylinder of constant diameter. It is assumed that electrostatic effects can be neglected in the presence of ionizing radiation. It is found that direct interaction of raindrops and particles does not account for the efficient removal of material whose diameter is below approximately 1 micron. To explain the removal of these smaller particles, a further mechanism is investigated. This consists of allowing the particles to mix with the water cloud before the rain starts. Small particles that are scavenged by cloud droplets by a process of coagulation are then placed in a position to be more efficiently removed by the rain. Fractions of a given particle size removed as a result of both these mechanisms operating are presented as a function of rainfall rate and time, and application is made to an assumed size distribution of radioactive solid particles.