Interpreting the <sup>13</sup>C/<sup>12</sup>C ratio of carbon dioxide in an urban airshed in the Yangtze River Delta, China
Abstract. Observations of atmospheric CO2 molar fraction and its 13C isotope composition (δ13C) in urban airsheds provide constraints on the roles of anthropogenic and natural sources in local and regional C cycles. In this study, we report observations of these quantities in Nanjing at hourly intervals from March 2013 to August 2015 using a laser-based optical instrument. Nanjing is the second largest city located in the highly industrialized Yangtze River Delta (YRD), Eastern China. The mean CO2 molar fraction and 13C were 439.7 ppm and −8.48 ‰ over this observational period. The peak monthly mean δ13C (−7.44 ‰, July 2013) was 1.03 ‰ higher than that observed at the Mauna Loa Observatory. The highly enriched 13C signal was attributed to the influence of cement production in the region. By applying the Keeling plot and the Miller–Tans method to midnight and midday observations, respectively, we showed that the 13C signal of C sources in the Nanjing Municipality was 0.48 ‰ lower than that in the YRD. Flux partitioning calculations revealed that natural ecosystems in the YRD were a negligibly small sink of atmospheric CO2, consistent with the Carbon Tracker inverse modeling result.