Characteristics of surface ozone in five provincial capital cities of China during 2014–2015

Ozone (O₃) pollution has become an increasing concern in China since elevated surface O₃ concentrations were observed in recent years. In this study, five provincial cities (Beijing, Shanghai, Guangzhou, Xi’an, and Hefei) located in different regions of China were selected to study the spatiotemporal variations and affecting factors of O₃ concentrations during 2014–2015. Beijing, Shanghai, and Guangzhou had suffered more severe O₃ pollution, yet Beijing had the highest number of days that exceeded the Chinese MDA8 (maximum daily 8 h average) standard of 160 µg m⁻³ . MDA8 O₃ exhibited different seasonal patterns among the five cities. In Beijing and Xi’an, MDA8 O₃ showed the highest in summer and lowest in winter. Guangzhou also had the highest O₃ concentration in summer, but had similar levels in other three seasons. The O₃ levels were similarly high in Shanghai during spring, summer, and autumn, while in Hefei, O₃ concentration peaked in autumn. No significant difference between weekend and weekday O₃ levels was observed in all the five cities. The diurnal cycle reached a maximum in the afternoon and a minimum in the early morning, which was consistent in the five cities. Correlation analyses showed that the associations between O₃ and the other five criteria air pollutants, as well as meteorological parameters, were substantially different among the five cities. Air mass cluster analyses during episodic days revealed that the short-distance transport of O₃ and its precursors had a greater impact for high O₃ pollution in the five cities. Overall, our results demonstrate that O₃ pollution exhibited great divergence among different regions and thus region-oriented control measures are suggested to reduce O₃ pollution in China.