Atmospheric CO.sub.2 mole fractions are observed at Beijing (BJ), Xianghe (XH), and Xinglong (XL) in North China using Picarro G2301 cavity ring-down spectroscopy instruments. The measurement system is described comprehensively for the first time. The geographical distances among these three sites are within 200 km, but they have very different surrounding environments: BJ is inside the megacity; XH is in the suburban area; XL is in the countryside on a mountain. The mean and standard deviation of CO.sub.2 mole fractions at BJ, XH, and XL between October 2018 and September 2019 are 448.4±12.8, 436.0±9.2, and 420.6±8.2 ppm, respectively. The seasonal variations of CO.sub.2 at these three sites are similar, with a maximum in winter and a minimum in summer, which is dominated by the terrestrial ecosystem. However, the seasonal variations of CO.sub.2 at BJ and XH are more affected by human activities as compared to XL. Using CO.sub.2 at XL as the background, CO.sub.2 enhancements are observed simultaneously at BJ and XH. The diurnal variations of CO.sub.2 are driven by the boundary layer height, photosynthesis, and human activities at BJ, XH, and XL. We also compare the CO.sub.2 measurements at BJ, XH, and XL with five urban sites in the USA, and it is found that the CO.sub.2 mean concentration at BJ is the largest. Moreover, we address the impact of the wind on the CO.sub.2 mole fractions at BJ and XL. This study provides an insight into the spatial and temporal variations of CO.sub.2 mole fractions in North China.