Radioactivity in the Atmosphere

radioactivity caused by the presence of radioactive gases and aerosols in the atmosphere. Since these radioactive substances enter the atmosphere as a result of both natural and man-made processes, a distinction is made between natural and artificial atmospheric radioactivity. Natural radioactive gases are the isotopes of radon, namely, ²²²Rn (radon), ²²⁰Rn (thoron), and ²¹⁹Rn (acti-non), which are formed through the radioactive decay of ²³⁸U, ²³²Th, and ²³⁵U. These isotopes enter the atmosphere through the exchange of soil air with atmospheric air (exhalation) or through diffusion. The products formed in the radioactive decay of radon isotopes are aerosols because the chemical elements formed in the decay are metals and are nonvolatile under ordinary conditions (Po, Bi, and others).

Upon decay, ²²²Rn, with a half-life (T_½) of 3.8 days, is dispersed within the troposphere, and its long-lived decay products, ²¹⁰Pb(RaD), ²¹⁰Bi(RaE), and ²¹⁰Po(RaF), are found in the stratosphere. The content of ²²²Rn in the air over the oceans is two orders of magnitude lower than over the continents, and the concentration over the earth’s surface decreases by approximately one-half for each km of altitude. Thoron and actinon, for which T_½ is, respectively, 54 sec and 3.9 sec, are present only at the ground surface. The decay product of thoron, ^2l2Pb(ThB) with T_½ equal to 10.6 hr, is found in the lower troposphere. The isotopes ²²⁰Rn and ²¹⁹Rn, together with their decay products, are almost nonexistent in the air over the oceans.

Most of the natural radioactive isotopes—⁷Be, ¹⁰Be, ³⁵S, ³²P, ³³P, ²²Na, ¹⁴C, and ³H—which arise from the interaction of cosmic rays with the nuclei of the elements present in the air, are formed in the stratosphere, which is also the area of their highest concentrations.

Artificial radioactive aerosols are formed in nuclear explosions. Several tens of seconds after an explosion, these aerosols contain about 100 different radioactive isotopes. Of these, ⁹⁰Sr, ¹³⁷Cs, ¹⁴C, and ¹³¹I are considered the most toxic. The altitude that the radioactive aerosols attain depends on the power and altitude of the explosion. The way in which particles are dispersed depends on the size of the particles and on the altitude at which the scattering occurs. The largest particles, with dimensions of hundreds of microns and more, quickly fall from the atmosphere. These particles are deposited at distances of only hundreds of kilometers from the site of the explosion and constitute the local fallout. However, in the explosion of nuclear bombs with powers of tens of megatons, these particles reach the stratosphere and may traverse thousands of km in the atmosphere before settling to the surface of the earth. Fine aerosols, with dimensions not exceeding a few microns, enter the upper troposphere from explosions and are usually distributed along a latitudinal belt from west to east. The fine aerosols scattered into the stratosphere settle to the surface of the earth in one hemisphere and, in some cases, in both hemispheres. The settling of these aerosols constitutes the worldwide fallout.

Radioactive aerosols are removed from the atmosphere mainly by precipitation. The average time τ spent by a radioactive aerosol in the lower troposphere, that is, the total time before the precipitation falls to the earth’s surface, is of the order of several days; in the upper troposphere, the time varies from 20 to 40 days. Radioactive aerosols that reach the lower layers of the stratosphere have values for τ of the order of a year or more, and the magnitude of τ increases with the altitude to which aerosols are scattered in the stratosphere. Most of the radioactive fission products usually remain in the hemisphere in which the explosion occurs.

The concentration of fission products in the troposphere increases with altitude. An especially large increase is noted upon passing through the tropopause. The maximum concentrations of fission products in the stratosphere, according to measurements made up to the autumn of 1961, were noted at an altitude range of 19–23 km, a range similar to that of the maximum concentration of a nonradioactive aerosol. Radioactive pollution of the atmosphere from atomic industrial plants is usually local in character, although ⁸⁵Kr is distributed throughout the entire troposphere.

The study of the atmospheric distribution of natural radioactive aerosols and the products of nuclear explosions has made it possible to determine certain physical characteristics of the atmosphere. These characteristics include the rate at which aerosols are washed out of the atmosphere, the coefficient of large-scale turbulent diffusion, the rate at which the two hemispheric atmospheres are exchanged, and the rate at which the air of the stratosphere is exchanged for that of the troposphere.