Atmosphere cosmogenic isotope concentrations

The atmospheric concentration of natural and bomb-produced radionuclides has been measured at ground level for several years at three locations throughout the world. The manner in which the concentration decreased suggested a half-residence time for stratospheric aerosols of 11.8 months at 46°N latitude. The annual spring concentration maximum occurred one to four months earlier at 71°N than at 46°N. Cosmogenic 7Be attained a maximum concentration before the bomb-produced radionuclides at 71° N and later than the bomb-produced isotopes at 46°N. The rate of increase toward the annual peak concentration for most radionuclides could be approximated by an exponential in which the concentration doubled every 60 days likewise, the rate of decrease from the maximum concentration could be approximated by an exponential with a half-time of about 40 days for most radionuclides except 7Be at 46°N, which shows a half-time of about 60 days. 

Carbon-14 ( " C, half-life = 5730 yr) is also formed cosmogenically by interactions with atmospheric nitrogen. This quickly gets mixed with the stable carbon pool and background concentrations are typically about 227 Bq kg stable carbon. Increasing fossil-fuel emissions, which are depleted in " C are diluting this carbon isotope so that the ratio of " C to is decreasing over time in the atmosphere. 

---