Fossil fuel CO₂ traced by radiocarbon in fifteen Chinese cities

China is an important fossil fuel CO₂ (CO_2ff) emitter and the international community is thus concerned with quantifying reductions in Chinese carbon emissions in the recent past. Compared to traditional statistical method, radiocarbon (¹⁴C) offers a different approach to quantify atmospheric CO₂ derived from fossil fuel emissions. Here, we carry out a multi-year (2011-2016) CO_2ff tracing by ¹⁴C in Xi'an, and a three-year (2014-2016) CO_2ff tracing in 15 Chinese cities. The Xi'an results show that average CO_2ff concentrations fell 35.9 ± 6.6% from 2014- 2016, compared to 2011-2013, and the timing of this decrease coincides with the implementation of nationwide carbon reduction measures in China, known as the Action Plan on Prevention and Control of Air Pollution. A WRF-Chem forward modeling simulation reveals that the CO_2ff in Xi'an is mainly derived from local sources, and a source apportionment combined stable-carbon isotope showed that the CO_2ff in this city is dominated by coal combustion (72.6 ± 10.4%). Strong CO_2ff differences are found between January and July in most Chinese cities. High CO_2ff concentrations often correspond to severe haze episodes and there are generally positive correlations between CO_2ff and fine particulate (PM_2.5) concentrations. Our study provides scientific data to understand the effects of CO_2ff reduction strategies in China that can be applied to other countries as well.