EGUsphere EGUsphere EGUsphere EGUsphere Copernicus Publications Göttingen, Germany 10.5194/egusphere-2026-4203 Flux and Radiocarbon Evidence of Urban Carbon Emission Reductions under Climate Mitigation Policies Liang Dan 1 2 3 12 Zhang Hongxing 4 12 Niu Zhenchuan https://orcid.org/0000-0002-5566-6761 1 5 6 7 Wang Sen 2 8 Zhou Weijian 1 6 Li Xuxiang 5 Jia Jingjing 9 Tao Yifan 9 Wang Guowei 1 3 Feng Xue 1 3 Liu Wanyu 1 10 Jin Zhao 1 Han Yongming https://orcid.org/0000-0002-1282-1354 1 Lu Xuefeng 1 6 Pettersson Jan B. C. 11 Kong Xiangrui https://orcid.org/0000-0002-7205-0723 11 State Key Laboratory of Loess Science, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China College of Urban and Environmental Sciences, Northwest University, Xi’an 710127, China Xi’an Institute for Innovative Earth Environment Research, Xi’an, China State Key Laboratory of Regional and Urban Ecology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China Institute of Global Environmental Change, Xi’an Jiaotong University, Xi’an, 710049, China Shaanxi Provincial Key Laboratory of Accelerator Mass Spectrometry Technology and Application, Xi’an AMS Center, Xi’an 710061, China National Observation and Research Station of Regional Ecological Environment Change and Comprehensive Management in the Guanzhong Plain, Shaanxi, China Shaanxi Xi’an Urban Ecosystem National Observation and Research Station, National Forestry and Grassland Administration, Xi’an 710127, China State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, 100029, China Interdisciplinary Research Center of Earth Science Frontier, Beijing Normal University, Beijing, 100875, China Department of Chemistry and Molecular Biology, University of Gothenburg, Gothenburg 41390, Sweden These authors contributed equally to this work. 28 07 2026 2026 1 30 Copyright: © 2026 Dan Liang et al. 2026 This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/ This article is available from https://egusphere.copernicus.org/preprints/2026/egusphere-2026-4203/ The full text article is available as a PDF file from https://egusphere.copernicus.org/preprints/2026/egusphere-2026-4203/egusphere-2026-4203.pdf

Urban areas account for the majority of global CO<sub>2</sub> emissions, yet direct observational evidence of emission reductions under climate mitigation policies remains limited. Here we combine eddy covariance measurements with tree-ring radiocarbon (<sup>14</sup>C) to quantify changes in urban CO<sub>2</sub> emissions and attribute their sources in two megacities, Beijing and Xi&rsquo;an, from 2020 to 2024. We find that all urban sites remained net carbon sources but exhibited consistent interannual declines in CO<sub>2</sub> fluxes, with the strongest reductions during heating season and traffic peak hours. Compared to 2021, the CO<sub>2</sub> fluxes in Beijing and Xi&rsquo;an in 2024 reduced by 21.6% (1.1 kg C m⁻<sup>2</sup> yr⁻<sup>1</sup>) and 41.2% (0.7 kg C m⁻<sup>2</sup> yr⁻<sup>1</sup>), respectively. Independent constraints from tree-ring <sup>14</sup>C reveal a concurrent decrease in fossil fuel-derived CO<sub>2</sub> (CO<sub>2<em>ff</em></sub>), demonstrating that the observed emission reductions are primarily driven by declining fossil fuel combustion. Analysis of energy consumption data indicates that these reductions are associated with substitution of coal and petroleum with electricity and natural gas, as well as rapid adoption of new energy vehicles. Our results provide direct observational evidence that urban carbon emissions can decline under climate mitigation policies and highlight the central role of energy system transformation in achieving sustained emission reductions.