Observed and simulated features of the CO_2 diurnal cycle in the boundary layer at Beijing and Hefei,China

The mechanism of the CO2diurnal cycle is a basis for investigating the carbon budget and its impacts on climate and environment change.Regional diurnal variations in CO2concentration based on observations and modeling have been studied widely.However,few studies have focused on the pattern of the CO2diurnal cycle in China.In this study,a three-dimensional global chemical transport model,Goddard Earth Observing System(GEOS)-Chem,was applied to simulate the CO2concentration and its variation over China from 2004 to 2012.Meanwhile,we also analyzed the CO2concentration as observed by two eddy covariance flux observation towers,one located in Beijing(39°580N,116°220E)and one in Hefei(31°550N,117°100E),using LI-COR 7500A infrared gas analyzers.Observations showed the amplitude of the CO2diurnal 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 CO2concentration 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 CO2diurnal cycle.The data also showed that traffic emissions significantly enhanced the CO2concentration in the boundary layer.

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 varia- tions 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 concen- tration 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, dueto 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 Bei- jing 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 signifi- cantly enhanced the CO2 concentration in the boundary layer.