Has China achieved synergistic reduction of carbon emissions and air pollution? Evidence from 283 Chinese cities

Confronted with the dual pressures of carbon emissions and air pollution, examining their synergistic effects becomes increasingly paramount in China. This study verifies and quantifies the bidirectional synergistic effects between reductions in carbon emissions and air pollutants (SO₂ and PM_2.5) at the city level. We also analyzed the spatial spillover of this effect among cities, investigating the dynamic evolution, heterogeneity, and underlying mechanisms of both synergistic reduction and increase effects across different time scales. Key findings include: 1) Carbon emission mitigation causes greater synergistic reduction effect than pollution reduction; 2) Carbon emission reduction has a more significant synergistic effect on SO₂ than PM_2.5; 3) Synergistic effects vary across city types. Cities with higher resource endowments, lower administrative levels, smaller populations, and lower economic status show greater synergistic effects; 4) The synergistic effects demonstrate significant positive spatial spillovers across cities, with local reductions in carbon and sulfur emissions spurring sulfur emission reductions in neighboring regions; 5) Synergistic reduction effects between carbon and air pollutant emissions exhibit temporal delays, while their synergistic increase effects are pronounced in both short and long terms; 6) R&D investment mitigates synergistic increase effects, while the impact of energy efficiency on synergistic reduction effects shows an inverted U-curve relationship. Our findings offer valuable insights for policymakers seeking to simultaneously tackle climate change and environmental pollution more effectively.