Air Cleaning by Convective Rains

Abstract

The rain cleansing patterns observed in five rain situations, all characterized by convective structures, are presented. The data comprise radioactivity and pollen concentrations measured in sequential samples of rain water from each rain. The rain systems are categorized, three being classed as ?well-organized,? and the remaining two as ?diffuse? convective systems. The temporal patterns of rain water contamination are relatively consistent in the well-organized showers, but much less so in the diffuse ones. Because the scavenging patterns of the pollens are quite parallel with those of the long-lived radioactive debris, it is inferred that both classes of contaminant enter the rain-producing parts of the showers by the same route, namely, in the low-level air that forms the convective updraft. It is therefore proposed that contamination that enters the troposphere from above (the stratospheric nuclear test debris reservoir) is not importantly scavenged until it passes through a low level tropospheric trajectory. The cleansing accomplished by the diffuse showers shows a tendency to be quite spotty, leaving pockets of contaminated air relatively unaffected between showers.