Coordinated effects of energy transition on air pollution mitigation and CO₂ emission control in China

China has made progress in energy transition to improve air quality, but still confronts challenges including further ambient PM_2.5 reduction, O₃ pollution mitigation, and CO₂ emission control. To explore the coordinated effects of energy transition on air quality and carbon emission in the near term in China, we designed 4 scenarios in 2025 based on different projections of energy transition progress with varying end-of-pipe control level, in each of which we calculated emissions of major air pollutants and CO₂, and simulated ambient PM_2.5 and O₃ concentrations. Results show that energy transition has disparate effects on emission reduction of different air pollutants and sectors, which largely depends on their current end-of-pipe control levels. The different effects on emission reduction may result in opposite variation tendencies of ambient PM_2.5 and O₃ concentration in a future scenario with aggressive energy transition policies and end-of-pipe control level in 2018. With the end-of-pipe control level strengthened in 2025, PM_2.5 and O₃ concentration could both reduce on the national scale, but the reduction of ambient O₃ lags behind PM_2.5, indicating the difficulty of O₃ pollution control. As to CO₂, national emission would go up in 2025 either implementing current or aggressive energy transition policies due to growing needs of electricity and on-road transportation, but emissions in most provinces could decline to below the 2018 level with aggressive energy transition policies because of substitution of clean energy in industrial, residential and off-road transportation sectors. The study results suggest strictly implementing restrictive end-of-pipe control measures along with energy transition to simultaneously reduce ambient PM_2.5 and O₃ concentration, and accelerating substitution of renewable energy in power sectors where electricity generation grows rapidly to synergistically control air pollution and CO₂ emissions. Furthermore, the projection of CO₂ emissions could provide references for short-term emission control targets from the perspective of air quality improvement.