Assessing the synergy between CO2 emission and ambient PM2.5 pollution in Chinese cities: An integrated study based on economic impact and synergy index

China's Carbon dioxide (CO2) emission and fine particulate matter (PM2.5) pollution have drawn widespread attention. Synergistic control of ambient PM2.5 pollution and CO2 emission is the core concern of China's environmental protection at present and in the next stage. This study developed a systematical method for assessing the synergy between PM2.5 pollution and CO2 emission. The methods integrated the assessment of PM2.5-related health impact and social costs for CO2 emission and the PM2.5-CO2 synergy index assessment, supporting a comprehensive evaluation from the perspectives involving absolute quantitative socioeconomic impact and relative bivariate correlation. Furthermore, the synergy between PM2.5 pollution and CO2 emission in 336 Chinese cities is evaluated as a case study. The results show PM2.5 pollution was associated with a total 1516.0 billion Yuan health loss in 336 Chinese cities 2020 while the social costs for CO2 emission were 1081.1 billion Yuan. High PM2.5-related health impacts and CO2 emission-related social costs mainly aggregated in Beijing-Tianjin-Hebei and surrounding areas (BTHS), Yangtze River Delta urban agglomeration (YRD), and specific center cities. The synergies between PM2.5 pollution and CO2 emission were more significant in 2017 and 2018 and key cities. High synergies were observed in the BTHS, the northern part of YRD, the northwest, and Qinghai-Tibet Plateau cities. This study provides a multi-perspective and classifying idea to assess the CO2-PM2.5 synergy while drawing a city-level map of China. The assessment, typological analysis, and synergistic control recommendations can be referenced by other countries that are bothered simultaneously by PM2.5 pollution and CO2 emission.