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

In the past decade, ozone (O-3) pollution has become a severe environmental problem in China's major cities. Here, based on available observational records, we investigated the long-term trend of O-3 pollution in China during 2014-2020. The O-3 concentrations were slightly higher in urban areas than in non-urban areas. During these 7 years, the highest O-3 concentrations primarily occurred during summer in northern China, and during autumn or spring in southern China. Although O-3 precursors, including nitrogen oxides (NOx) and carbon monoxide (CO), continuously decreased, O-3 concentrations generally increased throughout the 7 years with a slower increasing rate after 2017. The long-term trend of O-3 concentrations differed across seasons, especially from 2019 to 2020, when O-3 concentrations decreased in summer and increased in winter. To analyse the causes of this observed trend, a photochemical box model was used to investigate the change in the O-3 sensitivity regime in two representative cities - Beijing and Shanghai. Our model simulations suggest that the summertime O-3 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 was in a transition regime in summer but in a VOC-limited regime in winter. This study helps to understand the distinct trends of O-3 in China and provides insights into efficient future O-3 control strategies in different regions and seasons.