25 Years of Trospheric 14C Observations

Abstract

A long-term mountain station series of tropospheric 14C data for the period 1959 to 1984 is presented. This series is considered representative of the higher altitude 14C level over central Europe. Even tree-ring 14C levels from a rural ground level site in southern Germany are consistently lower (by Delta-14C depression = -15 per mil if compared with the mountain station summer average in atmospheric CO2). The rural tree-ring series is considered to represent the additional continental Suess effect at ground level without local contamination. This Suess effect decreases gradually with the distance from the ground (ie, source) level. We therefore estimate the additional continental Suess effect in the vegetation period to be Delta-14C depression = -5 per mil for the mountain station and -20 per mil for a rural ground level site, respectively. Based on this assumption, yearly mean tropospheric 14C levels corrected for fossil fuel contamination and representative of the Northern Hemisphere are provided for use in global carbon cycle models.