Background, aim, and scope To mitigate the global warming, more and more attention of the environmental scientists are focused on atmospheric CO_2 increase. Urban areas are hotspots of anthropogenic CO_2 emission sources. It is necessary to monitor the feature of CO_2 variations in urban areas. The data of CO_2 concentration and the stable carbon isotope ratio (delta~(13)C) are important for CO_2 reduction in urban areas. Therefore, the variation in CO_2 and delta~(13)C in urban areas are observed in this paper. The study presented the data of atmospheric CO_2 concentrations and delta~(13)C values at the urban sites in the inland city of Beijing and in the coastal city of Xiamen, to study the seasonal and diurnal variation in CO_2 concentration and delta~(13)C during 2014, and their key factors. Materials and methods In this study, we measured the CO_2 concentration and its delta~(13)C at the urban sampling sites in Beijing and Xiamen, respectively. Beijing is a typical inland city, which is located in the northwest of the North China Plain, surrounded by West Mountain and Jundu Mountain. It is the center of the Beijing-Tianjin-Hebei metropolitan region, with a population of more than 20 million. Xiamen is one of the cities in southeast coastal areas, facing the Taiwan Strait, with a resident population of more than 4 million. There is no direct pollution sources around the sampling sites in Beijing and Xiamen. We used aluminum foil sampling bags to collect the air samples and collected 144 urban air samples in this study. The CO_2 concentrations and delta~(13)C of samples were measured by the Picarro G2131-Ⅰ CO_2 Isotopic Analyzer (Picarro Inc.) with cavity ring down spectroscopy (CRDS). This analyzer has a characteristic of highly linearity and accuracy in CO_2 and delta~(13)C measurements (error < 0.1). The CO_2 and delta~(13)C value for each sample was measured for 6 minutes, and the average of the data from the last 4 min was used, to avoid equipment instability created by dead volumes when switching a new sample. Results The results showed that the CO_2 concentrations in Beijing and Xiamen were high in fall and winter, and low in spring and summer, but the delta~(13)C values are low in fall and winter, and high in spring and summer during the period of our study. Diurnal observations at the urban sites in Beijing and Xiamen showed low CO_2 concentrations and high delta~(13)C values in the daytime, while high CO_2 concentrations and low delta~(13)C values in the night. Additionally, morning and afternoon rush hour peaks were observed. Discussion The sampling site in Beijing and Xiamen displayed much higher CO_2 increase and lower delta~(13)C values than the background site of Waliguan. The obvious difference of CO_2 concentration and delta~(13)C values between sampling site and background station may be from the infl uence of regional emission, so it is necessary to analyze the regional emission sources. Keeling plot method by the additional stable carbon isotope ratio produced by a local source (delta_s) showed that the increased CO_2 in Beijing is mainly affected by coal combustion. The delta_s in Xiamen was influenced by fossil fuel like coal and oil in summer, and the reduction of respiration of plants and the change of weather condition during winter is the main reason for the variation in delta_s in Xiamen. Conclusions The reason for the variation atmospheric CO_2 in the inland city of Beijing and the coastal city of Xiamen were influenced by multivariate factors such as emission sources, height of the vertical mixing, wind direction and topography. Recommendations and perspectives Using stable carbon isotope technology can understand the main sources of increased atmospheric CO_2, but there is still some uncertainty. 14C tracing was needed to further quantify the fossil source CO_2 in the future.