Atmosphere-based estimations of CO2 emissions at city and county scales and comparative analysis: a case study in Yangtze River Delta Urban Agglomerations (YRDUA).

Accurate quantification of carbon emissions at the city and county scales will help achieve China's carbon mitigations. Large uncertainties remain in the estimates of CO2 emission at city and county scales in China. As a case study, we selected 71 counties in 11 cities in the Yangtze River Delta Urban Agglomeration (YRDUA). A new observation site was established to measure CO2 mole fractions. The study used two atmospheric transport models to quantify regional emissions. Combining the observations, two models, and the Bayesian inversion, we quantified CO2 emissions in YRDUA in 2023 as 0.80 ± 0.28 Gt. The inversion results showed that Huzhou, Suzhou, Changzhou, Taizhou, and Wuxi have lower emissions than the inventory results, and Ningbo, Shaoxing, and Jiaxing have higher emissions than the inventory results. The top ten CO2-emitting counties accounted for 30.3 % of the CO2 emissions in the YRDUA in 2023 but contributed to 44.2 % of the estimated differences between emission inventories and our inversion results. In addition, this study provides two directions to improve the accuracy of CO2 emission estimates at the county scale: 1. Calibration of activity data regarding multiple data sources; 2. Increase the density of regional as well as county observation networks. Efforts in these suggested directions will result in more accurate estimates of CO2 emissions to support specific county governments in formulating policies toward the goal of carbon peaking.