Airborne measurements have been made of aerosol particle size distributions (>0.01 micrometer) in aged air masses, in the plumes from several coal power plants and a large Kraft paper mill, and in the emissions from a volcano, before and after rain or snow showers. These measurements have been used to deduce the precipitation scavenging collection efficiencies of aerosol particles ranging in size from approximately equal to 0.01 to 10 micrometers diameter. Despite large variations in the nature of the aerosol particles and the precipitation, the scavenging collection efficiencies as a function of particle size showed marked similarities, with some well-defined maxima and minima values. The measurements agree well with theoretical calculations for aerosol particles use than 1 micrometer, but for the submicron aerosol particles the scavenging collection efficiencies are generally much higher, and the region of very low scavenging efficiencies (the 'scavenging gap') much narrower, than current theories predict. Some possible explanations for these discrepancies are suggested.