Co-benefits in carbon reduction from freight mode shifts under China's "Clean Air Action"

China's air pollution mitigation policy promotes the shift of bulk cargo transportation from roadways to railways and waterways, which might also influence carbon emissions. In this study, we developed a novel emissions assessment framework encompassing three truck types, rail, and waterways at the provincial level. Using counterfactual scenarios during the "Clean Air Action" (2013-2020), we quantitatively examined the co-benefits of freight mode shifts on mitigating CO2 and air pollutant emissions. Our findings indicate that nearly 6.4 Mt of CO2 emissions in the freight sector, equivalent to 2.4% of the total anthropogenic emissions (266.8 Mt, 95% CI: 192.1-320.0) in 2020, are avoided in China. Medium-duty and heavy-duty trucks contribute the most to this reduction over different phases. Rail transportation acted as the most effective mode for simultaneous emissions mitigation. However, while co-reductions are observed in HC (hydrocarbon, 3.9 Kt), CO (carbon monoxide, 83.6 Kt), and NOx (nitrogen oxides, 158.6 Kt) emissions, PM2.5 (particulate matter with an aerodynamic diameter ≤2.5 μm) emissions increased slightly due to increased share of the waterway freight. More than 80% of provinces in China benefited from freight mode shifts in carbon reductions during the second phase (2017~2020) due to the ambitious goals. To enhance freight mode shifts' co-benefit, attention should emphasize controlling waterway emissions, expanding railway capacity, and optimizing energy structures. These strategies will amplify policy impact on reducing air pollutant emissions from freight transport and contribute to China's journey towards carbon neutrality.