Measurement report: Airborne Observation of CO₂ and CH₄ 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 ppm and −18.0 ppm, respectively. In Nanjing, the corresponding values were −9.5 ppm and −10.0 ppm. For CH₄, the inversion jumps were −171 ppb during the day and −203 ppb at night (Beijing); in Nanjing, they were −141 ppb and −108 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 CH₄:CO₂ emissions ratio inferred from the nighttime ABL concentration data was lower than that obtained from the EDGAR inventory by 17 % in Beijing and 82 % 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.