THE USE OF THE NATURAL RADIOACTIVE NOBLE-GASES RADON AND THORON AS TRACERS FOR THE STUDY OF TURBULENT EXCHANGE IN THE ATMOSPHERIC BOUNDARY-LAYER - CASE-STUDY IN AND ABOVE A WHEAT FIELD

The application of the radioactive noble gases radon (Rn-222) and thoron (Rn-220) as naturally occurring tracer elements for the measurement of turbulent transport in the atmosphere has been studied in a flat river valley in the vicinity of Gottingen. The measuring site was covered with a crop of wheat. The vertical distribution of radon and thoron was measured continuously over the complete vegetation cycle of the crop. A strong dependence of activity concentrations on meteorological conditions could be observed, yielding during up to 50-times higher night maxima of activity concentrations compared to the noon mimima. Eddy diffusivities were calculated for two height intervals inside and above the crop using a combination of thoron and radon data. A comparison with eddy diffusivities obtained above the crop from meteorological data yielded good agreement between both approaches. The tracer method showed, especially inside the crop, where meteorological methods fall short, the best results, caused by the large concentration gradient due to low vertical exchange.