Below-cloud scavenging of aerosol particles: Particle-bound radionuclides—Experimental

To calculate the wet deposition of aerosol-bound radionuclides on ground and vegetation the scavenging coefficients must be well known. In case of rainfall there are, however, great differences between some of the experimental results published and theory. In the case of snowfall there is a large scatter in the experimental data published. Too little is known about the snow-out coefficient vs particle diameter as well as precipitation intensity. In the course of investigating snow-out, an outdoor experiment was set up to determine both the overall snow-out and wash-out coefficients (below cloud) of monodisperse test aerosols by natural hydrometeors. All spurious effects are eliminated as far as possible. The experimental set-up is described in detail. The results are shown and discussed. The collection efficiency of simple snowflake models was determined in laboratory experiments. The snow-out coefficient was calculated using empirical snowflake size spectra data. In the case of wash-out by rain, the results of the outdoor experiments are compared to results theoretically derived by an earlier study. In the case of snow-out, the results of the outdoor experiments are compared with those calculated from the collection efficiency data. The snow-out coefficients measured outdoors are about five times larger than the wash-out coefficients.