O ₃ –NO _<i>x</i> –VOCs sensitivity in major Chinese regions: detailed insights from GEMS satellite hourly observations

Abstract. Ozone (O3) pollution remains a critical issue facing China. Between 2020 and 2024, the proportion of cities exceeding national standards stood at 22.5 % and 24 % respectively, representing an annual increase of 4.8 %. O3–NOx–VOCs sensitivity analysis is central to ozone pollution control. Satellite observations from the OMI and TROPOMI provide daily HCHO and NO2 data but cannot capture hourly ozone variability. In this study, we use hourly observations from the Geostationary Environment Monitoring Spectrometer (GEMS), in combination with locally estimated scatterplot smoothing (LOESS), to investigate the diurnal evolution of ozone formation sensitivity over China during the warm season (April–September) from 2021 to 2023. The results show a strong correlation between the HCHO / NO2 ratio and ozone concentrations (R2 &gt; 0.78), highlighting the complex nonlinear interactions within the O3–NOx–VOCs chemical system. At the urban agglomeration scale, ozone formation shifts to NOx-limited or transitional regimes in the Beijing–Tianjin–Hebei (BTH), Yangtze River Delta (YRD), and Sichuan–Chongqing (SC) regions in the afternoon, while it shifts to NOx-limited in Pearl River Delta (PRD). Further inter-city comparisons reveal distinct controlling mechanisms. Beijing remains VOC-limited in the morning due to high NOx emissions combined with strong solar radiation. Nanjing exhibits a persistently complex transitional regime. Chengdu maintains a predominantly VOC-limited regime throughout the day. In contrast, Guangzhou, characterized by active VOC emissions, shows a greater tendency to shift toward a NOx-limited regime. This study provides the first regional-scale characterization of diurnal ozone formation sensitivity, supporting time-resolved and region-specific emission control strategies.