O₃–NO_x–VOC_s Sensitivity in Major Chinese Regions: Detailed Insights from GEMS Satellite Hourly Observations

Abstract. Ozone (O₃) pollution continues to pose a severe environmental and public health challenge in China. Identifying whether ozone formation is more sensitive to nitrogen oxides (NOx) or volatile organic compounds (VOCs) is therefore fundamental to designing effective control strategies. This study investigates the diurnal evolution of O₃ formation sensitivity across major regions of China, utilizing high-temporal-resolution observations from the Geostationary Environment Monitoring Spectrometer (GEMS) from 2021 to 2023. By analyzing the formaldehyde-to-nitrogen dioxide ratio (HCHO/NO₂ or FNR) at an hourly scale (09:00–16:00 LST) during the warm season alongside ground-level O₃ measurements and meteorological reanalysis, we capture the dynamic daytime transitions in O₃–NOx–VOC chemistry. Results show distinct diurnal patterns: O₃ and HCHO concentrations generally increase through the afternoon, peaking around 15:00–16:00, while NO₂ declines with a morning rebound. Spatially, elevated precursor levels and complex sensitivity regimes are concentrated in key urban agglomerations (BTH, YRD, SC, PRD). The analysis reveals a systematic shift from VOC-limited regimes in the morning toward transitional or NOx-limited regimes in the afternoon, driven by intensified photochemistry. A comparative city-level analysis demonstrates that Beijing’s strong radiation under NOx-rich conditions sustains a morning VOC-limited regime, Nanjing remains in a complex transitional state, Chengdu’s basin topography reinforces a persistent VOC-limited condition, and Guangzhou’s active VOC emissions promote a shift toward NOx limitation. This study provides the first regional-scale, diurnally-resolved insight into O₃ formation sensitivity dynamics in China, offering a critical scientific basis for designing temporally precise and regionally tailored emission control strategies.