Sixteen Years of Ozone Changes: Photochemical Pollution Control Experience in Beijing

 In recent years, air pollution control in China has advanced substantially. While these efforts have led to pronounced reductions in particulate matter concentrations, ozone (O₃) levels have increased significantly. Many previous studies have suggested that reductions in nitrogen oxides (NOₓ) may be a major driver of the observed increase in O₃, thereby highlighting the importance of controlling volatile organic compounds (VOCs). Using Beijing, a Chinese megacity, as a case study, this work analyzes long-term observational data from 2009 to 2024 to investigate the temporal evolution of O₃ and its precursors (NOₓ and VOCs) and their interrelated response characteristics.

The results show that during 2009–2015, both the maximum daily 8-hour average ozone (MDA8 O₃) concentration and total oxidant (Oₓ) increased rapidly, at rates of 8.9% yr⁻¹ and 5.4% yr⁻¹, respectively. After 2015, the growth rates slowed markedly and both metrics exhibited high-level fluctuations (O₃: 2.9% yr⁻¹; Oₓ: −0.9% yr⁻¹). Further stratification by pollution level and temperature reveals that, since 2015, the 90th percentile of O₃ and O₃ concentrations under high-temperature conditions (≥25 °C) have shown declining trends (90th percentile: −0.47% yr⁻¹; ≥25 °C: −0.76% yr⁻¹), with the decreases mainly occurring during midday high-ozone periods.

A combined analysis of the response relationships between O₃ and NOₓ, together with photochemical reactivity indicators, indicates that 2015 represents a turning point at which O₃ formation sensitivity in Beijing shifted from a VOC-limited regime toward a transitional, co-limited regime. Looking ahead, further reductions in NOₓ emissions from natural gas–fired power plants and mobile sources—particularly diesel vehicles and non-road mobile machinery—will be critical for effective O₃ pollution mitigation in Beijing. This study provides valuable insights and practical experience for photochemical air pollution control in megacities worldwide.