Fossil fuel CO traced by radiocarbon in fifteen Chinese cities.

China is an important fossil fuel CO (CO) 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 (C) offers a different approach to quantify atmospheric CO derived from fossil fuel emissions. Here, we carry out a multi-year (2011-2016) CO tracing by C in Xi'an, and a three-year (2014-2016) CO tracing in 15 Chinese cities. The Xi'an results show that average CO 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 in Xi'an is mainly derived from local sources, and a source apportionment combined stable-carbon isotope showed that the CO in this city is dominated by coal combustion (72.6 ± 10.4%). Strong CO differences are found between January and July in most Chinese cities. High CO concentrations often correspond to severe haze episodes and there are generally positive correlations between CO and fine particulate (PM) concentrations. Our study provides scientific data to understand the effects of CO reduction strategies in China that can be applied to other countries as well.