Carbon content constitutes a major fraction of atmospheric particulate matter (PM) and directly influences the earth’s climate and human health. The stable carbon isotope ratios (δ¹³C) can be used to track potential sources and atmospheric processes of carbonaceous aerosols. Previously, determination of δ¹³C was always conducted in offline carbonaceous aerosol samples. The poor time-resolution results cannot provide information regarding the temporal evolution of δ¹³C at a short-time scale. In this study, we developed a new system for online measurements of δ¹³C in atmospheric carbonaceous aerosols by combining a semicontinuous organic carbon/elemental carbon (OC/EC) analyzer and online cavity ring-down spectroscopy (CRDS) (OC/EC analyzer-CRDS). To provide better stability in the determination of δ¹³C, a carrier gas with CO₂ (∼200 ppm) in “balance gas” was used, and Keeling analysis was employed to separate the δ¹³C signal of the sample from background CO₂ gas. Our results showed that the accuracy and absolute precision of the δ¹³C measurements by the OC/EC analyzer-CRDS system were better than 0.1‰ and 0.5‰, respectively, for the samples containing carbon content more than 5 μg. Furthermore, we employed this system to monitor δ¹³C (δ¹³C-TC) in particulate total carbon (TC) with a time resolution of 2–4 h over Beijing in late summer and early autumn, 2019. During the sampling period, the TC concentrations varied from 0.1 to 12.0 μg m^–3 with a mean value of 6.0 ± 2.4 μg m^–3. The δ¹³C-TC ranged from −28.2 to −24.2‰ (mean value was −25.9 ± 0.9‰) without significant diurnal variations, suggesting similar contributing sources to TC. Comparing the δ¹³C signatures of different emissions, we found that liquid fuels and primary and secondary C₃ plants were likely the dominant sources of particulate TC. Finally, we found that atmospheric heavy precipitation washed out the aged aerosols from the polluted air, resulting in significant depletion (∼2.4‰) of δ¹³C-TC in the atmosphere. This paper described a novel system for conducting online measurements of δ¹³C in atmospheric carbonaceous aerosols and provided us information to better understand the temporal evolution of emission sources and atmospheric processes of carbonaceous aerosols.