36Cl in modern atmospheric precipitation

The Cl-36/Cl ratios of annual precipitation samples, collected at 28 meteorological stations over the globe were measured with accelerator mass spectrometry. Local Cl-36 fallout rates were obtained from the experimental data and compared with theoretical fluxes deduced from atmospheric cosmogenic production and an atmospheric transport model. Most of the measured fluxes were found to be larger than the predicted cosmogenic ones, up to about one order of magnitude. The deviations are largest in the northern hemisphere and for low latitudes. Recycling of Cl-36 from nuclear weapon tests from the biosphere into the troposphere as CH3Cl is proposed as a possible source of the observed difference and can explain the measurements as simulation calculations showed.