ALFRED ‘A", Vow. 68, No. 13 JouRNAL OF GEOPHYSICAL RESEARCH Jury 1, 1903 The Vertical Distribution of Radon 222, Radon 220 and Their Decay Products in the Atmosphere 407921 W. Jacopr anp K. ANDRE Hahn-Meitner-Institut fiir Kernforschung, Berlin-Wannsee, Germany Abstract. The vertical equilibrium distributions of Rn™, Rn™, and their decay products in a horizontally isotropic atmosphere are calculated from diffusion theory with the help of a digital computer. The change of the turbulent diffusion coefficient with altitude and the re- moval of Pband Po*” by fallout were taken into account. Vertical profiles of the natural radionuclides in the atmosphere are presented for several typical profiles of the turbulent diffusion coefficient. Agreement with available experimental data is rather good. The results indicate the importance of the natural radioactive tracers in the study of mixing processes in the troposphere and in the mechanism of transport from the troposphere into the lower stratosphere. method can be applied to all decay products of INTRODUCTION Rn™ and Rn™ by taking into consideration their removal from the atmosphere bydiffusion and washout. The computations involved in this method are considerable. They could be done Our knowledge of the processes of diffusion of aerosols in the atmosphere and removal from it is rather limited, although these processes are of considerable importance in estimating the haz- only on a fast electronic computer. ards following the release of radioactive and inactive toxic materials in the free air. Rn™, Rn™, and their decay products are valuable, THE Dirrusion Equation anv Its Bounpary CONDITIONS except for the boundarylayer near groundlevel only a few measurements of Rn™, Pb™°, and Po”? are available at present. The spreading out of Rn™ and Rn™ in the atmosphere after their exhalation from the ground is caused by turbulent diffusion and convection and is limited only by the radioactive decay of the two nuclides. In contrast to natural tracers for the study of these processes in the troposphere and lower stratosphere, but For a general view of the vertical distribution of Rn™, Rn™, and their decay products in the atmosphere under different conditions of turbulence, theoretical computations on the basis of the exchange theory are useful. Previous attempts in this direction were made by Hess and Schmidt [1918], Schmidt [1926], Priebsch [1932], Philip [1959], and Malakhov [1959], whocalculated the vertical distribution of Rn” assuming a constant value or a power law for the increase of the vertical diffusion coefficient with altitude. Under these assumptions analytical solutions of the diffusion equation for Rn™ and Rn™ a. possible, but they represent only rough appro’imations to the real conditions in the atmosphere. For morerealistic results we solved the diffusion equation by a numerical method which enabled ns to use any vertical profile of the turbulent diffusion coefficient. Furthermore, this 209 the emanations, the decay products, being iso- topes of heavy metals, become readily attached to aerosol particles and precipitation clements. The distribution of these decay products *s thus controlled not only by radioactive decay and by diffusion but also by sedimentation and washout, 1e., by the removal of aerosols to the earth’s surface. Because of these processes the concentration of a nuclide in the decaychain at a point having horizontal coordinates z and y st an altitude z above ground level (z = 0) is g:,en by the general expression on, ot 3799 = V (KVn,) —u Vn, + 9, on + Y-imi- — Ox + A,)n, (1)