Radioactivity stratospheric inventory

Mechanisms and rates of transport of nuclear test debris in the upper and lower atmosphere are considered. For the lower thermosphere vertical eddy diffusion coefficients of 3-6 X 106 cm.2 sec. 1 are estimated from twilight lithium enhancement observations. Radiochemical evidence for samples from 23 to 37 km. altitude at 31° N indicate pole-ward mean motion in this layer. Large increases in stratospheric debris in the southern hemisphere in 1963 and 1964 are attributed to debris from Soviet tests, transported via the mesosphere and the Antarctic stratosphere. Most of the carbon-14 remains behind in the Arctic stratosphere. 210Bi/ 210Pb ratios indicate aerosol residence times of only a few days at tropospheric levels and only several weeks in the lower stratosphere. Implications for the inventory and distribution of radioactive fallout are discussed. 

The first nuclear test took place in the United States in July 1945. Since then a number of other nations have tested nuclear devices both above and underground. The total amount of fission which took place in the atmosphere is estimated to be a little more than 200 megatons (MT). Most of the explosions occurred in the Northern Hemisphere, and most of the material produced was injected initially into the stratosphere. Careful and continuous monitoring and inventorying of the atmosphere since 1958 has indicated half-residence times for the fallout particulates of about one year in the stratosphere and about one month in the troposphere. Measurements in the stratosphere and upper troposphere showed that the fission product radioactivity was in or on particles with a diameter of. 03 jum or less. ... 

The test ban treaty of 1963, signed by the major nuclear powers at that time, virtually ended the large atmospheric inputs, although there has been a more or less continuous series of small and intermediate shots by non-signatories. Hence, using the isotope 90sr as an example, a total of some 12 megacuries (MCi) was deposited in the stratosphere. The peak inventory was about 6 MCi in early 1963 which decreased to less than. 01 in 1977. Except for the fraction which radioactively decayed, all of this material is now on the earth s surface. 

Continental margins are important areas to study concerning the distribution and deposition of pollutants, because coastal sediments accumulate most of those contaminants. The main source of radioactive pollutants is fallout from stratospheric nuclear testing performed from the late 1950s until the early 1960s. Northeastern Spain has also been affected by the 1986 Chernobyl accident, as the Cs/ Cs ratio calculated for seawater samples has proven. However, no sign of Cs has been detected in sediments collected in the area. In the Mediterranean Sea, the mean residence times in shallow waters are estimated as 11 years for plutonium and 5.5 years for americium. The inventory ranges from 9 Bq/m to... 

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