Abstract: The offaxis integrated cavity output spectroscopy technique was used to measure air CO₂ concentration, stable carbon (δ¹³C) and oxygen (δ¹⁸O) isotope ratios on the Fourth Ring Road (FRR) and in the green space system of Beijing Institute of Landscape Architecture (BILA) in summer and winter seasons. The variations of CO2 concentration, δ¹³C value, δ¹⁸O value and the differences of them between the FRR and the BILA, which were correlated with traffic volume and meteorological factors, were analyzed at halfhour timescale. The results showed that traffic volume on the FRR was large both in summer and winter with obvious morning and evening rush hours, and more than 150 thousands vehicles were observed everyday during the observation periods. Diurnal variation of the CO₂ concentration showed a twopeak curve both on the FRR and in the green space system of the BILA. In contrast, diurnal variation of δ¹³C value was a twotrough curve while diurnal variation of δ¹⁸O value was a singletrough curve. The differences of CO₂ concentration, δ¹³C value and δ¹⁸O value between the FRR and the green space system of BILA in summer were greater than those in winter. The carbon isotope partitioning results showed that in summer vehicle exhaust contributed 64.9% to total atmospheric CO₂ of the FRR during measurement time, while heterotrophic respiration contributed 56.3% to total atmospheric CO₂ of the green space system in BILA. However, in winter atmospheric CO₂ from both the FRR and green space system mostly came from vehicle exhaust. Stepwise regression analysis indicated that differences of CO₂ concentration between the FRR and green space system were significantly related to vehicle volume and solar radiation at halfhour timescale, while solar radiation and relative humidity were the main meteorological factors causing δ¹³C and δ¹⁸O differences between the FRR and green space system. Plants in the green space system strongly assimilated CO₂ from fossil fuel burning by photosynthesis to maintain carbon and oxygen balance of urban area in the growing season, which played an important role in improving urban ecological environment.