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With new equipment which allows the continuous recording of the atmospheric thoron content down to 0.5. 10-14 C/1, the thoron content of the atmosphere, at different levels was measured at the Meteorologisches Observatorium des Deutschen Wetterdienstes in Aachen. The measurements were carried out from June until September 1964.The results show a close relation between the thoron content and the meteorological variables. The thoron concentration and its decrease with height shows a remarkable diurnal variation with a maximum at noon and a minimum during the night. The thoron at this station is influenced by wind direction. Rain diminished the thoron content by 50 yo; the recovery time to reach the initial concentration is about 10 hours.It could be shown that the turbulent diffusion coefficient K increases linearly between 1 m and 4 m. A mean value of K = 330 t 30 cmp/s at 1 m level wa,s found.

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It can be concluded that, although significant differences exist between laboratories they can be attributed mainly to the narrow distribution of the results within a single laboratory, and that the overall agreement of the results for EC and TC is fairly good. These results obtained with pure diesel engine emissions, should be complemented by field samples, but they have already achieved relevant findings in the performance of the procedures used to assess exposure to diesel soot.