Observed and simulated features of the CO2 diurnal cycle in the boundary layer at Beijing and Hefei,China

The mechanism of the CO₂ diurnal cycle is a basis for investigating the carbon budget and its impacts on climate and environment change. Regional diurnal variations in CO₂ concentration based on observations and modeling have been studied widely. However, few studies have focused on the pattern of the CO₂ diurnal cycle in China. In this study, a three-dimensional global chemical transport model, Goddard Earth Observing System (GEOS)-Chem, was applied to simulate the CO₂ concentration and its variation over China from 2004 to 2012. Meanwhile, we also analyzed the CO₂ concentration as observed by two eddy covariance flux observation towers, one located in Beijing (39°58′N, 116°22′E) and one in Hefei (31°55′N, 117°10′E), using LI-COR 7500A infrared gas analyzers. Observations showed the amplitude of the CO₂ diurnal cycle at Hefei to be larger than at Beijing, due to stronger ecological activities. GEOS-Chem successfully captured the main aspects of the diurnal cycle of the CO₂ concentration in the boundary layer observed at both Beijing and Hefei. However, some discrepancies between the model and observations did exist; specifically, the model tended to underestimate the amplitude of the CO₂ diurnal cycle. The data also showed that traffic emissions significantly enhanced the CO₂ concentration in the boundary layer.