Emission Control and Sensitivity Regime Shifts Drive the Decline in Extreme Ozone Concentration in the Sichuan Basin During 2015–2024

In recent years, ozone (O3) pollution has become a prominent air quality concern in the Sichuan Basin (SCB). Based on surface O3 measurements from 22 cities between 2015 and 2024, this study investigates the evolution of extreme O3 pollution events and their underlying causes. While the average O3 concentration, the number of affected cities, and the total O3 pollution hours have all increased during the past decade, extreme O3 concentrations have shown a significant decline since 2020. These trends suggest that O3 pollution in the SCB has become more spatially extensive and less intense. Decomposition analysis attributed ~75% of the post-2020 decline in extreme O3 concentrations to precursor emission reductions, with meteorological variability explaining the remaining ~25%. Satellite observations of formaldehyde (HCHO) and nitrogen dioxide (NO2) column densities indicate a regional shift in O3 formation regimes across the SCB, with many areas transitioning from VOC (volatile organic compound)-limited to transitional or NOx (nitrogen oxide)-limited conditions. This shift likely contributed to the broader spatial extent and longer duration of O3 pollution in recent years. Model sensitivity simulations and Integrated Reaction Rate (IRR) analysis demonstrate that reductions in precursor emissions, particularly NOx, directly weakened daytime photochemical O3 production and disrupted NOx-driven radical propagation under transition and NOx-limited conditions, collectively driving the observed decline in extreme O3 concentrations.