Background variations of atmospheric CO₂ and carbon-stable isotopes at Waliguan and Shangdianzi stations in China

Using observational data from 2007 to 2010 at the Waliguan and Shangdianzi stations in China, atmospheric CO2, its δ13C composition, and their potential relationship with sources and sinks are studied. Results suggest that at Waliguan (WLG) station, both CO2 and δ13C possess long-term trends and seasonal cycles that correlate well with each other. CO2 and δ13C interannual variations indicate terrestrial ecosystem source-sink seasonal features in the midlatitude to high-latitude Northern Hemisphere. CO2 annual means vary from 384.0 ppm to 390.2 ppm and increase in an approximately linear manner with a mean annual growth rate of 2.1 ± 0.1 ppm. The δ13C annual means vary from 8.30‰to 8.35‰and decrease almost linearly with a mean annual rate of 0.02‰± 0.001‰. Under the given conditions of terrestrial biosphere and anthropogenic activities at Shangdianzi (SDZ) station, the CO2 annual means vary from 385.1 ppm to 390.6 ppm and approximately increase linearly with a mean annual growth rate of 1.8 ± 0.1 ppm. The peak-to-peak annual seasonal amplitude is 23.0 ppm. The δ13C annual means vary from 8.27‰ to 8.36‰ between 2009 and 2010. Mean values of 25.44‰±0.72‰ and 21.70‰±0.67‰ for the respective sources are obtained at WLG and SDZ. The estimated δs values are more negative in winter and spring than in summer and autumn at WLG. While because substantial C4 photosynthesis taking place in summer and biomass burning strongly contribute in winter, the estimated δs values at SDZ are unusually heavier throughout the year and more positive than those at WLG