3810 TABLE 2. JACOBI AND ANDRE Calculated Mean Values of Pb*° and Po? Concentrations and Their Ratio near Ground Level as a Function of the Removal Rate Removal Half-Life T, Activity Ratio Pb? Po™ Pb?! /Po?® 5 10 0.6 X10" 0.9 * 10-4 0.5 X 107 1.8 x 107 0.008 0.02 50 19 10-" 16 xX 107% 20 100 200 oo 1.3% 10-* 24x 104 2.7 *%104 3.5 * 10-4 6.0 * 107! 0.04 29 «* 107% 46 * 1075 9.8 * 10775 0.12 0.17 0.28 0.08 * Assuming a mean Rn”? exhalation rate of 0.4 atom /cm? sec. eo cals dene Spec. Activity,* c/m!? days wot Figure 7 shows the calculated equilibrium profiles of Pb™® and Po™ for different values of the half-life of removal 7’. The profile of Bi” (t,,. == 5.0 days) is approximately identical with the Pb” profile because both nuelides are practically in radioactive equilibrium. The distribution for T = o applies to the case with no washout or rainout, so that fallout is effected only by diffusion to the ground surface. All the calculated profiles show a slow increase of the Pb” and Po” concentrations with altitude in the troposphere. We have to keep in mind that the calculations are based on the assumption of a constant removal rate throughout the troposphere and lower stratosphere. Since the precipitation processes are restricted to the troposphere, the actual profiles in the lower troposphere should correspond to profiles calculated for a finite residence time. Above the tropopause, however, a higher Pb™° and Po™ concentration should be expected. The maximum concentration should be reached at an altitude of about 20 to 30 km. An indicator of the fallout efficiency is the change of the Pb™/Rn™ and Po™/Pb”™ratios with altitude. The theoretical values of these ratios are shown in Figure 8. The ratios are approximately constant up to an altitude of about 1 km. Above this altitude the Pb™/Rn™ ratio increases rapidly aecording to the deerease of Rn™ concentration and exceeds a value of 1 in the lower stratosphere. The Po™/Pb™ ratio rises more slowly with increasing height, until radioactive equilibrium is reached approximat in the lower stratosphere. The theory shows the important result t! rather low ratios must be expected in the tro] sphere even without any removal of Pb™ a Po” by washout or rainout. In this case Pb Rn™ ratios of about 10° and Po™/Pb™ rat of about 0.3 are obtained in ground-level 4 This result can be understood by considering t buildup of the steady-state distributions Pb™ and Po” atoms. The vertical distributi: of the production rate of Pb™° atoms must equal to the vertical profile of Bi™ or Rn™. B cause of their comparatively long residence tir in the atmosphere, the Pb™ atoms assume much more uniform vertical distribution th: their mother atoms. Supported by the diffusic of Pb™ atoms toward the earth’s surface ar their deposition thereon, this process results a lack of Pb” in the troposphere and in an e: cess with respect to Rn™ in the lower stratc sphere. The deviation of radioactive equilibriu: between Pb”™° and Po** without any washol. ean be explained in the same way. With dc creasing residence time of Pb™° and Po™ atom Le. with increasing washout efficiency, both re tios are lowered. The profiles of Pb™ and Po” shown in Fig ure 7 are standardized to a Rn™ exhalation rat of 1 atom/cm* sec. As was mentioned earlici this value is reasonable for the calculation c Rn™ profiles over continental areas when th horizontal advection of maritime air with lov Rn™ content can be neglected. Because of thei rather long residence times in the atmosphere this is no Jonger true for Pb™ and Po™*. As ar. approximation it can be assumed that horizonta. mixing leads to uniform Pb” and Po”® profile: over land and sea within the samelatitude belt. The land/sea ratio for middle latitudes of the northern hemisphere is about 0.4. The calculated absolute concentrations should be multiplied by this factor to get values that are com- parable with the observed average concentrations in this region. Table 2 summarizes the resulting theoretical mean concentrations of Pb and Po” near ground level for different valucs of the half-lives of their removal. Only a few measurements of Pb*™and Po™ are available for comparison with the calculations. Observed average values in ground-level air at several places in temperate latitudes of