Long-term observations of atmospheric CO₂ and carbon isotopes at continental sites in Germany

A network for regional atmospheric CO₂ observations had already been established in Germany by 1972, consisting of 5 stations with basically different characteristics: Westerland, a coastal station at the North Sea, 2 regional stations, Waldhof and Deuselbach, as well as 2 mountain stations, Brotjacklriegel at the eastern boarder of Germany and Schauinsland in the Black Forest. In addition to CO₂ concentration observations, from 1977 onwards quasi-continuous 13CO₂ and ¹⁴CO₂ measurements were performed on samples from the Schauinsland site, and for the short period 1985-1988, ¹⁴CO₂ measurements were also made on Westerland samples. CO₂ data selection based on wind velocity allows for an estimate of the representative continental CO₂ level over Europe. The peak-to-peak amplitude of the seasonal cycles is between 12.1 ppmv (Schauinsland) and 17.6 ppmv (Waldhof). The phase of the seasonal cycles at the German sites is shifted if compared to maritime background sites with the concentration maxima occuring already between beginning of February and beginning of April, the minima in August. The long-term mean CO₂ increase rate in the last 20 years at Westerland and Schauinsland is 1.49 and 1.48 ppmv yr^-1, respectively. The mean δ¹³C of the seasonal source CO₂ at Schauinsland is calculated from unselected δ¹³C and CO₂ data to be – 25.1‰. From the ¹⁴C observations in unselected CO₂, we derive yearly mean fossil fuel contributions at Westerland of 4 ppmv, and at Schauinsland of only 2.5 ppmv. Based on the seasonality of the fossil fuel CO₂ component at Schauinsland and on concurrently observed atmospheric 222Radon activities, we derive a seasonal amplitude of the fossil fuel CO₂ source which is higher by a factor of 3 compared to emission estimates for Europe.