Scavenging of aerosol-fixed radionuclides with rain and snow

In this report results are presented of field measurements to the process of below-cloud scavenging of aerosol-fixed radionuclides. The experiments were performed to quantify the washout process in dependence of the relevant parameters. Using a trace method, monodisperse particles of a wide size range (0.4-7.7 μm) were emitted. The focus of the parameterisation is on the characterization of the hydrometeors: raindrops by their size, snow crystals by their shape and size. It was found that the scavenging coefficient depends strongly on the kind and size of the hydrometeors. An increase of the scavenging coefficient with increasing drop size is measured for raindrops, up to 3 orders of magnitude depending on the particle size. The increase of the scavenging coefficient with particle size, however, is not as large as expected, and the minimum of this relation is shifted to larger particles. For snow scavenging, differences between the crystal types are found, e.g. the scavenging by column crystals is about 3 times more effective than the scavenging by graupel. Comparing the means of the rain and snow scavenging experiments, snow scavenging is more than 10 times more effective than rain scavenging. (orig.)