| 27 Jan 2026
O3–NOx–VOCs 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.
This paper adds an important new data set to the discussion of ozone production and concentrations in rural and urban China. The observations and the description of diurnal variation are worthy of publication as a data set. However the aspects of those paper that interpret those observations are severely lacking and I cannot recommend publication. Interpretation of surface observations of ozone, VOC and nitrogen oxides have driven a wealth of publications and creative thinking about ozone in Chinese cities and in rural Chinese locations. This paper does little to characterize the state-of-the-art in our communities thinking about ozone production and as such it is almost impossible to understand if the new satellite remote sensing data confirms current understanding or contradicts it.
I note an active literature on the role of heterogeneous uptake of HO2 and thus a third dimension of aerosol control in addition to NOx and VOC control. I also note that it is important to be clear about whether the paper is interested in instantaneous ozone and ozone production or net production leading to high values in the afternoon. A small sampling of references are listed below. I do not attempt a comprehensive list. Other important references will be found in these papers and elsewhere.
T. Chen, B. Chu, J. Ma et al., Ozone Pollution in China: Current Status and Control Strategies, Engineering, https://doi.org/10.1016/ j.eng.2025.06.044
Dyson, J. E., Whalley, L. K., Slater, E. J., Woodward-Massey, R., Ye, C., Lee, J. D., Squires, F., Hopkins, J. R., Dunmore, R. E., Shaw, M., Hamilton, J. F., Lewis, A. C., Worrall, S. D., Bacak, A., Mehra, A., Bannan, T. J., Coe, H., Percival, C. J., Ouyang, B., Hewitt, C. N., Jones, R. L., Crilley, L. R., Kramer, L. J., Acton, W. J. F., Bloss, W. J., Saksakulkrai, S., Xu, J., Shi, Z., Harrison, R. M., Kotthaus, S., Grimmond, S., Sun, Y., Xu, W., Yue, S., Wei, L., Fu, P., Wang, X., Arnold, S. R., and Heard, D. E.: Impact of HO2 aerosol uptake on radical levels and O3 production during summertime in Beijing, Atmos. Chem. Phys., 23, 5679–5697, https://doi.org/10.5194/acp-23-5679-2023, 2023.
Li, C., Zhu, Q., Jin, X., and Cohen, R.C.: Elucidating Contributions of Anthropogenic Volatile Organic Compounds and Particulate Matter to Ozone Trends over China, Environ. Sci. Technol. 2022, 56, 18, 12906–12916.
Jie Li, Xueshun Chen, Zifa Wang, Huiyun Du, Weiyi Yang, Yele Sun, Bo Hu, Jianjun Li, Wei Wang, Tao Wang, Pingqing Fu, Huili Huang, Radiative and heterogeneous chemical effects of aerosols on ozone and inorganic aerosols over East Asia, Science of the Total Environment 622–623 (2018) 1327–1342 doi.org/10.1016/j.scitotenv.2017.12.041
Ye, X., Zhang, L., Wang, X., Lu, N., Hickman, S., Luo, G., and Archibald, A. T.: Deciphering the impacts of meteorology on surface ozone variability in eastern China using explainable machine learning models, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2026-74, 2026.