Interpreting the ¹³C  ∕ ¹²C ratio of carbon dioxide in an urban airshed in the Yangtze River Delta, China

Abstract. Observations of atmospheric CO₂ mole fraction and the ¹³C ∕ ¹²C ratio (expressed as δ¹³C) in urban airsheds provide constraints on the roles of anthropogenic and natural sources and sinks in local and regional carbon 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 mole fraction and δ¹³C were (439.7 ± 7.5) µmol mol⁻¹ and (−8.48 ± 0.56) ‰ over this observational period. The peak monthly mean δ¹³C (−7.44 ‰, July 2013) was 0.74 ‰ higher than that observed at Mount Waliguan, a WMO (World Meteorological Organization) baseline site on the Tibetan Plateau and upwind of the YRD region. The highly ¹³C-enriched signal was partly attributed to the influence of cement production in the region. By applying the Miller–Tans method to nighttime and daytime observations to represent signals from the city of Nanjing and the YRD, respectively, we showed that the ¹³C ∕ ¹²C ratio of CO₂ sources in the Nanjing municipality was (0.21 ± 0.53) ‰ lower than that in the YRD. Flux partitioning calculations revealed that natural ecosystems in the YRD were a negligibly small source of atmospheric CO₂.