City level CO2 and local air pollutants co-control performance evaluation: A case study of 113 key environmental protection cities in China

‘Co-control’, or synergistically reducing CO2 and local air pollutant emissions, is an important strategy for cities to achieve ‘low carbon’ and ‘blue sky’ simultaneously. However, there were few studies to evaluate and compare the level of co-control of CO2 and local air pollutants in cities yet. The present study proposed qualitative and quantitative methods to evaluate the level of co-control of CO2 and three local air pollutant (SO2, NOx, and particulate matter PM) emissions in key environmental protection cities in China over two periods (2012–2015 and 2015–2018). Statistical analysis found that, though three local air pollutant emissions positively correlated with CO2 emission, no significantly positive correlation was found between local air pollutants emission reductions and CO2 emission reductions, indicating that co-control effects were poor in general. By using the co-control effect coordinate system, qualitative evaluation showed that less than half of the sample cities could achieve co-control of the total amount of CO2 and local air pollutants. By employing the indicator of emission reduction equivalence (EReq), quantitative evaluation showed that the co-control level of the sample cities improved in 2015–2018 compared with 2012–2015. Further regression analysis showed that, reducing coal consumption and economic development significantly enhanced the co-control performance of the observed cities. The present case study proved that the proposed methods for evaluation and comparison of the city co-control performance works well, and can be used in other countries and regions to promote global cities racing to carbon and local air pollutants co-control.