Long-term trend of ozone pollution in China during 2014-2020: distinct seasonal and spatial characteristics and ozone sensitivity

In the past decade, ozone (O₃) pollution has become a severe environmental problem in major cities in China. Here, based on available observational records, we investigated the long-term trend of ozone pollution in China during 2014–2020. Ozone concentrations were slightly higher in urban areas than in non-urban areas. During these seven years, the highest ozone concentrations primarily occurred in summer in northern China, and in autumn or spring in southern China. Although ozone precursors, including nitrogen oxides (NO_X) and carbon monoxide (CO), continuously decreased, ozone concentrations generally increased throughout the seven years with a slower increasing rate after 2017. The long-term trend of ozone concentrations differed across seasons; especially from 2019 to 2020 when ozone concentrations decreased in summer and increased in winter. To analyze the causes of this observed trend, a photochemical box model was used to investigate the change in ozone sensitivity regime in two representative cities – Beijing and Shanghai. Our model simulations suggest that the summertime ozone sensitivity regime in urban areas of China has changed from a VOC-limited regime to a transition regime during 2014–2020; by 2020, the urban photochemistry is in a transition regime in summer but in a VOC-limited regime in winter. This study helps to understand the distinct trends of ozone in China and provides insights into efficient future ozone control strategies in different regions and seasons.