Abstract: In order to deal with the new environmental problems brought by the rapid industrialization and urbanization, online observations and comparative studies of ozone (O₃) and volatile organic compounds (VOCs) were simultaneously carried out by the National Joint Center for Air Pollution Prevention and Control in four cities (Beijing, Baoding, Langfang and Shanghai) in central-eastern China from June to September in the year 2021. The results are as follows: (1) It was found that the characteristic of O₃ pollution that used to be "significantly higher in rural and suburban areas than in urban areas" has been changing in central-eastern China where O₃ pollution was seriously worsened. The ozone concentrations were all higher in urban areas of the three northern cities (Beijing, Baoding and Langfang) than those in their adjacent rural (suburbs) areas in different degrees. A similar serious O₃ pollution level was observed in both urban and suburban areas of Shanghai. (2) There was little difference in VOCs concentrations between the four pairs of urban and rural (suburbs) areas. The most important components of VOCs were alkanes, accounting for about 70% in the three northern cities and for about 60% in Shanghai. (3) Aromatics and olefins were the major contributors to the ozone formation potential (OFP), particularly ethylene, m-xylene, toluene and isoprene. (4) The O₃ formation sensitivity analysis (EKMA curve) indicated that VOCs-limited regimes were mainly controlled in Beijing (both in the urban and suburban areas), Baoding (both in the urban and rural areas) and Shanghai (only in the urban area), while the others areas of this study belonged to a VOCs-NO_x transitional control regime. The formaldehyde to NO₂ concentration ratios (FNR) in 85 cities in central-eastern China showed a decreasing trend between 2018 and 2021, which indicated that the transition of O₃ formation sensitivity to a VOCs-limited regime was widespread. In summary, there is a profound change of the "O₃ urban-rural difference" compared to the past. This indicates that there is a trend of convergence in terms of O₃ concentration levels, formation sensitivity, and emission sources and spatial distribution of its precursors in central-eastern China.