Fossil CO2 emissions have been diluting the global 14C/C ratio of atmospheric CO2 (Suess effect). We estimated the 14CO2 amount in the atmosphere (and its trend) utilizing the calculated 14CO2 activity concentration in the atmosphere (aacn
, reported in mBq m–3). This parameter, calculated from Δ14CO2 and the CO2 mixing ratio (reported in micromoles of CO2 per mole of air), is connected with the 14CO2 quantity in the volume or mass unit of air, which is not influenced by the Suess effect. This parameter can only be influenced by processes linked to 14CO2 emissions/uptake, e.g. associated with atmosphere-biosphere or atmosphere-ocean CO2 exchange as well as by anthropogenic emissions of 14CO2. Results obtained from measurements at Schauinsland station, Germany, indicate a stable amount of 14CO2 in the atmosphere since the early 1990s.