The atmospheric carbon dioxide (CO.sub.2) mixing ratio and its carbon isotope ([delta].sup.13 C-CO.sub.2) composition contain important CO.sub.2 sink and source information spanning from ecosystem to global scales. The observation and simulation for both CO.sub.2 and [delta].sup.13 C-CO.sub.2 can be used to constrain regional emissions and better understand the anthropogenic and natural mechanisms that control [delta].sup.13 C-CO.sub.2 variations. Such work remains rare for urban environments, especially megacities. Here, we used near-continuous CO.sub.2 and [delta].sup.13 C-CO.sub.2 measurements, from September 2013 to August 2015, and inverse modeling to constrain the CO.sub.2 budget and investigate the main factors that dominated [delta].sup.13 C-CO.sub.2 variations for the Yangtze River delta (YRD) region, one of the largest anthropogenic CO.sub.2 hotspots and densely populated regions in China. We used the WRF-STILT model framework with category-specified EDGAR v4.3.2 CO.sub.2 inventories to simulate hourly CO.sub.2 mixing ratios and [delta].sup.13 C-CO.sub.2, evaluated these simulations with observations, and constrained the total anthropogenic CO.sub.2 emission. We show that (1) top-down and bottom-up estimates of anthropogenic CO.sub.2 emissions agreed well (bias 6 %) on an annual basis, (2) the WRF-STILT model can generally reproduce the observed diel and seasonal atmospheric [delta].sup.13 C-CO.sub.2 variations, and (3) anthropogenic CO.sub.2 emissions played a much larger role than ecosystems in controlling the [delta].sup.13 C-CO.sub.2 seasonality. When excluding ecosystem respiration and photosynthetic discrimination in the YRD area, [delta].sup.13 C-CO.sub.2 seasonality increased from 1.53 0/00 to 1.66 0/00. (4) Atmospheric transport processes in summer amplified the cement CO.sub.2 enhancement proportions in the YRD area, which dominated monthly [delta].sub.s (the mixture of [delta].sup.13 C-CO.sub.2 from all regional end-members) variations. These findings show that the combination of long-term atmospheric carbon isotope observations and inverse modeling can provide a powerful constraint on the carbon cycle of these complex megacities.