Four Decades of Road Transport Carbon and Air Pollution emissions across Chinese Cities: Trends, Drivers, and Synergistic Mitigation

Abstract: Rapid motorization has established urban road transportation as a dominant contributor to both carbon dioxide (CO₂) and air pollutants. However, the long-term co-evolution and potential for synergistic mitigation of greenhouse gases and air pollutants remain to be further quantified at the high-resolution city-fleet scale. Here, we present a comprehensive, vehicle-fleet-based emission dataset for 338 Chinese cities spanning 1985–2023, quantifying CO₂, seven air pollutants (NO_x, CO, PM_2.5, VOC_s, SO₂, OC, BC), and specialized organic aerosol precursors (I/S/LVOC_s ) with 1 km spatial resolution. Our analysis shows that the relationship between urban carbon emissions and pollutant emissions has undergone a profound transition from early synergistic growth to a subsequent period of divergent decoupling. Driven by the stringent implementation of China I to VI emission standards, the seven pollutants have peaked and entered a sustained decline, achieving reductions of 35.7%–84.2% by 2023 relative to their historical peaks. Conversely, while CO₂ emissions have not yet peaked, they have begun to decouple from the exponential growth of the vehicle population (VP).  We further reveal that this divergence is driven by unbalanced contributions of key factors: the substantial negative contribution of pollution intensity (pollutant/CO₂) has effectively offset the pressures from motorization, whereas carbon intensity (CO₂/VP) remained a primary driver of emission growth until 2015. By constructing a carbon-pollution peaking matrix at the city scale, we find that only 6.5% of cities—predominantly megacities—have achieved dual peaking of carbon and pollutants. In contrast, 57.6% of cities exhibit a pollutant-peaked and carbon-plateaued pattern, where the effectiveness of pollutant governance has been overwhelmed by the scale effect of vehicle intensity (VP/GDP), resulting in a temporal inconsistency between air quality improvements and climate targets. We propose that the transition toward synergistic mitigation is achievable as supported by scenario analysis: an integrated policy package combining accelerated vehicle electrification, obsolete vehicle phase-out, and freight structure optimization could achieve a 55.7% reduction in CO₂ and a 57.9% reduction in air pollutants.  These findings provide a robust evidence base for city-specific governance, highlighting the urgency for regions with inadequate carbon-pollutant synergy to proactively implement vehicle emission reduction strategies through structural transformation to align local air quality efforts with national carbon neutrality goals.