Quantifying the synergies between air pollutant reduction and CO2 mitigation in the transportation sector of Guangdong, China.

Transportation control measures are widely promoted as a means of simultaneously reducing CO2 and air pollutant emissions, yet their synergistic effects remain insufficiently quantified. In this study, we develop an integrated framework that couples air quality modeling with health impact assessment to quantify the air pollution-related health and CO2 mitigation benefits of real-world transportation control measures implemented in Guangdong, China. We introduced a synergy index (SynI) to capture both the magnitude and balance of health and CO2 mitigation benefits, whereas a coordination degree (CD) captures only their relative balance. The results show that in 2030, transportation control measures are estimated to avoid 1730 (95% confidence interval (CI): 1482-2001) PM2.5-, 5364 (95% CI: 2722-10,425) NO2- and 86 (95% CI: 45-128) O3-related premature deaths, while reducing annual CO2 emissions by 39.46 Mt yr-1. Spatially, densely populated and traffic-intensive central cities achieve the highest SynI but a lower CD, as nonlinear photochemical responses drive O3 increases that partially offset the health benefits from PM2.5 reductions in our simulations. In contrast, peripheral cities exhibit a higher CD but smaller absolute benefits. Among individual measures, improving energy efficiency is estimated to achieve the highest SynI in 18 of 21 cities but ranks poorly in CD, whereas eliminating high-emission vehicles shows the highest CD across most cities but limited SynI owing to smaller emission reductions. The energy transition performs best in Foshan, Dongguan, and Zhuhai, achieving both a high SynI and CD. This study reveals spatial variations in synergy levels and measure priorities, providing scientific support for region-specific strategies to jointly mitigate air pollution and CO2 emissions in regions undergoing rapid transportation transformation.