Long-term changes of surface ozone and ozone sensitivity over the North China Plain based on 2015–2021 satellite retrievals

Despite the implementation of stringent pollution control measures, surface ozone (O₃) pollution remains a significant issue in the North China Plain (NCP) in recent years. Here we examined long-term changes in surface O₃ concentrations during 2015–2021 in the NCP. The mean summer maximum daily 8-hour average ozone concentration exhibited an annual increase of 12.77 µg m^− 3 (p < 0.01) from 2015 to 2019, followed by a decline from 174 µg m^− 3 in 2019 to 157 µg m^− 3 in 2021. Subsequently, satellite-based formaldehyde (HCHO) and nitrogen dioxide (NO₂) columns were used to track volatile organic compounds (VOCs) and nitrogen oxides (NO_x) emissions to evaluate the causes of O₃ changes. Results showed that the increase of HCHO column and the sharp decline of NO₂ column have indeed led to no decrease in O₃ concentration and even contributed to O₃ enhancement in most areas in 2015–2019. However, the sharp decline of HCHO and NO₂ columns contributed to the decline of O₃ concentration since 2020. The HCHO/NO₂ values showed a transitional regime ranging from 4.23 to 5.05 and the O₃ sensitivities were predominantly VOC-limited and transitional regimes in 2019–2021, varying among different mega-city clusters. The HCHO/NO₂ increased from 2.62 ± 2.41 in 2015 to 2.99 ± 3.49 in 2019 but decreased since then, facilitating O₃ formation regime transition from NO_x-limited to transitional (or from transitional to VOC-limited) regimes, which contributed to the decline in O₃ levels. The study determined that the optimal ratio of HCHO to NO₂ for reducing O₃ concentration is 4.66 for the NCP.