Transport of SF6 and (CO2)-C-14 in the atmospheric general circulation model ECHAM4

Two tracers, SF6 with sources at the earth's surface and (CO2)-C-14 with a source in the lower stratosphere, are used to investigate the simulation of global scale transport in an atmospheric general circulation model. The si mulated mixing ratios of SF6 in the troposphere are generally in close agre ement to observations revealing a realistic description of the large scale tropospheric transport. The interhemispheric exchange time for SF6 is calcu lated to be 0.9 years, indicating a slightly too strong interhemispheric ex change. In the lowermost stratosphere the simulated vertical gradient of SF 6 is in good agreement with observations within the Ist 4 to 5 km above the tropopause indicating that the flux from the troposphere to the lowermost stratosphere is captured by the model. On the other hand, downward transpor t of (CO2)-C-14 from the stratosphere into the troposphere is found to be o verestimated. From a comparison with observations it is concluded that it i s the downward transport in the subtropics that is overestimated, at high l atitudes the vertical gradients in the tropopause region are close to obser vations. Finally, the tracer tests show that the transport into the uppermo st two levels, above 20 km, is underestimated as these levels serve as spon ge layers and not as layers with a reasonable transport. Consequently, the tracer concentrations in that altitude interval are underestimated, by up t o a factor 2.