Variation of atmospheric (14)CO2 and its spatial distribution.

The atmospheric (14)CO2 is usually presented in the Δ(14)C notation, which cannot reflect its absolute quantity change. This article presents the atmospheric radiocarbon activity concentrations (aacn, reported in mBq/m(3)) in recent years at nine observation stations. The aacn at Schauinsland decrease from 1977 to 1993 but between 1993 and 2003 keeps at a relative steady state. Atmospheric aacns were higher in the northern hemisphere than that in the southern hemisphere. The aacns 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 aacns seasonality. The annual mean aacns in both hemispheres show slightly increasing trends since 2002, which may be mainly caused by decreasing air-sea (14)C flux as the air-sea (14)C gradient decline.