Diagnosing ozone formation regimes in Shandong province, China (2019–2023): Implications for volatile organic compounds control policies during the warm season and COVID-19 lockdowns

Ground-level ozone (O₃) concentrations in Shandong Province have exhibited a dynamic trend, characterized by an initial decline followed by a rebound. To investigate the drivers of these changes, we combined satellite-derived formaldehyde-to-nitrogen dioxide ratios (HCHO/NO₂) with ground-based O₃ measurements to establish a diagnostic threshold for O₃ formation sensitivity. Results reveal a transition from NO_x-limited/transitional regimes to VOC-limited regimes from 2019 to 2021, followed by a gradual reversal to transitional regimes from 2021 to 2023. Satellite observations corroborate this trend, showing a rebound in HCHO columns alongside a sharp decline in NO₂ columns between 2019 and 2023. These findings highlight that while stringent NO_x emission controls have succeeded in reducing NO₂ levels over the past five years, insufficient VOC mitigation has enabled O₃ resurgence. During COVID-19 lockdowns, we found that abrupt NO_x reductions did not shift Shandong to NO_x-limited regimes. Unbalanced emission reductions lead to O₃ increases, with at least 60 % VOC reductions required to offset O₃ rises. This suggests that future measures (e.g., transport electrification) may be insufficient without synergistic VOC/NO_x control.