- INTRODUCTION To our knowledge the first measurements of vertical distribution of radon in the troposphere were made in 1950 (1). The samples taken in Ohio showed a decrease in activity with altitude while the measurements made in California over the ocean showed an increase with altitude. The theoretical interpretation of these results indicated that in the second case the air originated in Asia and that the horizontal wind speed was greater at higher levels than near the This explained the inverted concentration profile. surface. Between 1956 and 1958 several series of measurements were made in the USSR by Kirichenko (2) who ndré used the profiles obtained in certain cases to deduce the variation of the diffusion coefficient with altitude. In the United States Wilkening in 1952 (3) carried out several measurements above New Mexico, more recently this author has used radon to study the levels above the mountains (4). A theoretical study of the vertical distribution of radon in the troposphere has been made by Jaconi (5) and more recently by Bouville and Machta (6) who considered the time variations of the diffusion profile. A knowledge of the vertical distribution of radioactivity in the tropcsphere is interesting for several reasons: 1. The natural radioactivity contributes to atmospheric ionization and therefore is of interest in atmospheric electricity. 2. Radon is the parent of long lived radionuclides (lead-210 and polonium- 210) which are used as tracers for atmospheric physics (large scale air mass movements, washout, etc.). To interpret the variation in these nuclides and to study their diffusion it is necessary to know their source, that is the distribution of radon in the atmosphere. 3. Radon is an interesting tracer, in particular for the study of diffusion. -l-