Fine particulate matter (PM
2.5) and Ozone (O
3) pollution have emerged as the primary environmental challenges in China in recent years. Following the implementation of the Air Pollution Prevention and Control Action Plan, a substantial decline in PM
2.5 concentrations was observed, while O
3 concentrations exhibited an increasing trend across the country. Here, we investigated the long-term trend of O
3 from 2015 to 2022 in Xinxiang City, a typical city within the Central Plains urban agglomeration. Our findings indicate that the hourly average O
3 increased by 3.41 μg m
−3 yr
−1, with the trend characterized by two distinct phases (Phase I, 2015–2018; Phase II, 2019–2022). Interestingly, the increasing rate of O
3 concentration in Phase I (7.89 μg m
−3) was notably higher than that in Phase II (2.89 μg m
−3). The Random Forest (RF) model was employed to identify the key factors influencing O
3 concentrations during the two phases. The significant dropping of PM
2.5 in Phase I could be responsible for the O
3 increase. In Phase II, the reductions in nitrogen dioxide (NO
2) and unfavorable meteorological conditions were the major drivers of the continued increase in O
3. The Observation-Based Model (OBM) was developed to further explore the role of PM
2.5 in O
3 formation. Our results suggest that PM
2.5 can influence O
3 concentrations and the chemical sensitivity regime through heterogeneous reactions and changes in photolysis rates. In addition, the relatively high concentration of PM
2.5 in Xinxiang City in recent years underscores its significant role in O
3 formation. Future efforts should focus on the joint control of PM
2.5 and O
3 to improve air quality in the Central Plains urban agglomeration.
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