44 



BIOLOGICAL EFFECTS OF ATOMIC RADIATION 



debris in various parts of the atmosphere. It is generally agreed that the half-residence time of 

 radioactive particles in the troposphere is of the order of two to four weeks. It is likely that 

 the exact value depends on the location relative to the removal processes. Particles in or 

 below the rain-producing levels will survive for a shorter time than those in the upper tropo- 

 sphere. 



80 



TO- 



GO 



"1 1 1 r 



I I I I I n 



\ PREDICTED Sr ON GROUND, 1962-63 



- 10 



30 



20 



E 



u 



E 



30° 

 SOUTH 



60° 90° 



Figure I 



Significant new knowledge has been obtained on the storage times of stratospheric debris. 

 It is now generally recognized that the concept of a fixed fractional removal rate from the 

 stratosphere is untenable and that the removal rates depend on the latitude and altitude of in- 

 jection of the debris, the season, and upon stratospheric circulations, which have a spatial and 

 temporal variability. As a first approximation, the idea of a variable mean storage time may 

 be used. For equatorial injections the values range from about one to five years, the shorter 

 times applying to the lower stratospheric injections; for temperate and polar latitude injec- 

 tions, the time is under one year. (These estimates do not apply to debris injected into the 

 high atmosphere, above about 25 miles. ) The shorter residence times make the contribution 

 of shorter-lived isotopes in stratospheric fallout a more significant factor in environmental 

 contamination than was previously thought. 



Although there are still many unsolved problems concerning the exact mode of strato- 

 spheric transport and of the locale and mechanism of removal processes from the stratosphere. 



