Measurement report: Airborne observation of CO ₂ and CH ₄ in the urban atmospheric boundary layer in Eastern China

Abstract. To characterize the concentrations of CO2 and CH4 in the urban atmospheric boundary layer (ABL), this study conducted airborne measurements over four cities in Eastern China, obtaining full vertical profiles (ground to 2 km) over Beijing and Nanjing, partial profiles over Hengshiu and Shangqiu. Results showed that the CO2 and CH4 concentrations in the ABL were consistently higher than those in the free atmosphere, with the highest values observed near the surface (Beijing and Nanjing). In Beijing, the daytime and nighttime inversion jumps in the CO2 concentration were −25.2 ± 0.4 and −18.0 ± 0.2 ppm, respectively. In Nanjing, the corresponding values were −9.5 ± 0.1 and −10.0 ± 0.4 ppm. For CH4, the inversion jumps were −171.4 ± 0.4 ppb during the day and −202.6 ± 2.3 ppb at night (Beijing); in Nanjing, they were −140.7 ± 0.1 and −108.0 ± 2.1 ppb, respectively. Change in the airmass trajectory altered the free-atmospheric CO2 concentration over Nanjing by 3 ppm in a matter of a few hours. The EDGAR CH4:CO2 emissions ratio was within measurement uncertainty of the nighttime ABL value in Beijing, but about 80 % higher in Nanjing, indicating that the inventory may have missed the recent energy transition from gasoline and natural gas to electric in the transport sector. The experimental data is available at https://doi.org/10.7910/DVN/ZPVSVU (Wang et al., 2026).