Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 9 1 2009 10.5194/acpd-9-2501-2009 http://www.atmos-chem-phys-discuss.net/9/2501/2009/ http://www.atmos-chem-phys-discuss.net/9/2501/2009/acpd-9-2501-2009.html http://www.atmos-chem-phys-discuss.net/9/2501/2009/acpd-9-2501-2009.pdf 2501 2531 2009-01-27 ³⁶Cl bomb peak: comparison of modeled and measured data U. Heikkilä ulla.heikkila@bjerknes.uib.no J. Beer J. Feichter V. Alfimov H.-A. Synal U. Schotterer A. Eichler M. Schwikowski L. Thompson EAWAG, Dübendorf, Switzerland MPI-MET Hamburg, Germany ETH/PSI, Zurich, Switzerland Division of Climate and Environmental Physics, Physics Institute, Univ. of Bern, Switzerland PSI, Villigen, Switzerland School of Earth Sciences, The Ohio State University, USA now at: Bjerknes Centre for Climate Research (BCCR), Allegaten 55, 5007 Bergen, Norway The extensive nuclear bomb testing of the fifties and sixties and the final tests in the seventies caused a strong ³⁶Cl peak that has been observed in ice cores world-wide. The measured ³⁶Cl deposition fluxes in eight ice cores (Dye3, Fiescherhorn, Grenzgletscher, Guliya, Huascarán, North GRIP, Inylchek (Tien Shan) and Berkner Island) were compared with an ECHAM5-HAM general circulation model simulation (1952–1972). We find a good agreement between the measured and the modeled ³⁶Cl fluxes assuming that the bomb test produced global ³⁶Cl input was ~80 kg. The model simulation indicates that the fallout of the bomb test produced ³⁶Cl is largest in the subtropics and mid-latitudes due to the strong stratosphere-troposphere exchange. In Greenland the ³⁶Cl bomb signal is quite large due to the relatively high precipitation rate. In Antarctica the ³⁶Cl bomb peak is small but is visible even in the driest areas. The model suggests that the large bomb tests in the Northern Hemisphere are visible around the globe but the later (end of sixties and early seventies) smaller tests in the Southern Hemisphere are much less visible in the Northern Hemisphere. The question of how rapidly and to what extent the bomb produced ³⁶Cl is mixed between the hemispheres depends on the season of the bomb test. The model results give an estimate of the amplitude of the bomb peak around the globe. 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