Evidence for Coordinated Control of PM_2.5 and O₃: Long-Term Observational Study in a Typical City of Central Plains Urban Agglomeration

Fine particulate matter (PM_2.5) and Ozone (O₃) 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 O₃ concentrations exhibited an increasing trend across the country. Here, we investigated the long-term trend of O₃ from 2015 to 2022 in Xinxiang City, a typical city within the Central Plains urban agglomeration. Our findings indicate that the hourly average O₃ 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 O₃ 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 O₃ concentrations during the two phases. The significant dropping of PM_2.5 in Phase I could be responsible for the O₃ increase. In Phase II, the reductions in nitrogen dioxide (NO₂) and unfavorable meteorological conditions were the major drivers of the continued increase in O₃. The Observation-Based Model (OBM) was developed to further explore the role of PM_2.5 in O₃ formation. Our results suggest that PM_2.5 can influence O₃ 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 O₃ formation. Future efforts should focus on the joint control of PM_2.5 and O₃ to improve air quality in the Central Plains urban agglomeration.