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 (NOx) 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 NOx-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.