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 delta C-13 composition, and their potential relationship with sources and sinks are studied. Results suggest that at Waliguan (WLG) station, both CO2 and delta C-13 possess long-term trends and seasonal cycles that correlate well with each other. CO2 and delta C-13 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 delta C-13 annual means vary from -8.30 parts per thousand to -8.35 parts per thousand and decrease almost linearly with a mean annual rate of -0.02 parts per thousand +/- 0.001 parts per thousand. 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 delta C-13 annual means vary from -8.27 parts per thousand to -8.36 parts per thousand between 2009 and 2010. Mean values of -25.44 parts per thousand +/- 0.72 parts per thousand and -21.70 parts per thousand +/- 0.67 parts per thousand for the respective sources are obtained at WLG and SDZ. The estimated delta(s) values are more negative in winter and spring than in summer and autumn at WLG. While because substantial C-4 photosynthesis taking place in summer and biomass burning strongly contribute in winter, the estimated delta(s) values at SDZ are unusually heavier throughout the year and more positive than those at WLG.