Variation of atmospheric ¹⁴CO₂ and its spatial distribution

The atmospheric ¹⁴CO₂ is usually presented in the Δ¹⁴C notation, which cannot reflect its absolute quantity change. This article presents the atmospheric radiocarbon activity concentrations (a_acn, reported in mBq/m³) in recent years at nine observation stations. The a_acn at Schauinsland decrease from 1977 to 1993 but between 1993 and 2003 keeps at a relative steady state. Atmospheric a_acns were higher in the northern hemisphere than that in the southern hemisphere. The a_acns in the northern hemisphere show clear seasonal cycle with higher value in winter and lower value in summer, while this seasonality is not obvious in the southern hemisphere. Vegetation plays as a role of sink in summer and a role of source in winter, and atmosphere-biosphere radiocarbon exchange might be the main driver of the a_acns seasonality. The annual mean a_acns in both hemispheres show slightly increasing trends since 2002, which may be mainly caused by decreasing air-sea ¹⁴C flux as the air-sea ¹⁴C gradient decline.