Knowledge of variations and sources of urban atmospheric CO₂ is important to determine energy saving and emission reduction policies. In the present study, air CO₂ concentration and stable carbon (δ¹³C) isotope ratios on the Fourth Ring Road (FRR) and in the green-space system 100 m from FRR were measured using an off-axis integrated cavity output spectroscopy technique in heating and non-heating seasons. The CO₂ variations and different source contributions were analyzed with the Keeling plot method and IsoSource software. The results showed CO₂ from different sources had significantly different δ¹³C values. The δ¹³C values from high to low were noted from soil respiration (-18.92‰), plant respiration (-23.40‰), coal combustion exhaust gas (-24.10‰), motor vehicle exhaust gas (-28.14‰), and natural gas (-33.34‰). The CO₂ concentrations of the FRR and green-space system in the heating season were 26.2% and 41.2% higher than those in the non-heating season, respectively. There was a significant difference of CO₂ concentration between the FRR and green-space system in the non-heating season, but no difference in the heating season. The CO₂ concentration had an obvious daily change and two peaks at 6:00 and 20:00 in the heating season. The CO₂ concentration of FRR was highest at the bottom of the observation tower and decreased with increased height, but the CO₂ concentration of the green-space system was highest at 8 m. In the heating season, the CO₂ concentration had two peaks at 8:00 and 19:00, with a similar daily trend to that of traffic volume. CO₂ concentrations of both the FRR and green-space system decreased with increasing height. However, there were obviously different sources between the FRR and green-space systems. The CO₂ of the FRR was mainly from motor vehicle exhaust gases, but that of the green-space system mostly came from soil and plant respiration in the non-heating season. However, there was no difference in CO₂ sources between the FRR and green-space system, and most sources were coal-fired emissions and motor vehicle exhaust gases.