St Dialaatalea Re: sage eb ee ce i INTERNATIONAL SYMPOSIUM ON TRACE GASES AND quilibrium is reached approx imately stratosphere, ‘oor RADIOACTIV ITY 3811 y shows the important, result that itios must be expected in the tropo- ¢ without any removal of Pb*™ and lout or rainout. In this case Pb**/ if about 10” and Po*™’/ Pb” ratios | are obtained in ground-le vel air. n be understood by consideri ng the ihe steady-state distribu tions of pp@!9 - concentration (10° dpm/kg) * atoms. The vertical dist ribution ‘tion rate of Pb*® atoms must be ertical profile of Bi™ or Rn™, Becomparatively long residenc e time here, the Pb” atoms assume a uform vertical distribution than toms. Supported by the diffusion toward the earth’s surfac e and 1 thereon, this process results in in the troposphere and in an exet to Rn™ in the lower stratolation of radioactive equ ilibrium und Po” without any washout od in the same way. Wit h de- 2e time of Pb® and Po™ atoms, ing washout efficiency, both ra- f Pb” and Po” shown in Figtdized to a Rn™ exhalatio n rate sec. As was mentioned earlier, EP Sonable for the calculati on of ° continental areas when the ion of maritime air with low be neglected. Because of their mee times in the atmosp here, ‘Tue for Ph®® and Po” As an ‘an be assumed that horizo ntal uiform Pb® and Po” pro files within the same latitude belt. 9 for middle latitudes of the "re is about 0.4. The calcucentrations should be multi' to get values that are comserved average concentra tions le 2 summarizes the resultan concentrations of Ph nd level for different values their removal. uTements of Pb”? and Por mparison with the cal cularage values in gTound -leve] } in temperate latitu des of " 1 14 km Altitude | ern hemisphere range from 0.5 « 10°“ towtoe c/m* for Pb™ [Biifford et al., 1952; Lehmann and Sittkus, 1959; see also US-AEC et al., 1955] and cosmic-ray-induced Be’ and P™ [Goel et al., 1959; Lal, 1959]. 1 the Burton and Stewart [1960] measure . Fallout Program, 1958, 1959, 1960]. Simultaneous measurements were made only by Burton and Stewart [1960] in Harwell and by Lehmann and Sittkus [1959] on the Schauinsland near Freiburg/Brsg. They obtained an average Po concentration of 5 x 10“ to 7 x 10° oft and a mean ratio of 0.05 to 0.1. These experimental data are in rather good agreement with Table 2 vertical distribution of Pb” and Po™ over England during a period of severalmonths. Mgure 9 shows the observed mean Pb™ profile an ne theoretical profiles (calculated for the censity profile in the ‘standard atmosphere ). The oe served slow increase in concentration with ne for a removal half-life of 20 to 30 days. A half- Above the tropopause the concentrationin life of this magnitude is well established by studies of the fallout of fission products [Stewart tude in the troposphere agrees rather re the profile calculated with T = 20 to 50 ays. creases more rapidly and approaches the c ia lated profile for T = oo. This effect should be