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.

Automated summary

In the past decade, ozone pollution has become a severe environmental problem in China's major cities. This study investigated the long-term trend of ozone pollution in China from 2014 to 2020 based on observational records. The study found that ozone concentrations were slightly higher in urban areas compared to non-urban areas. The highest concentrations occurred during summer in northern China and during autumn or spring in southern China. Despite the decrease in ozone precursors, ozone concentrations generally increased over the 7-year period, with a slower rate of increase after 2017. There were seasonal differences in the long-term trend, with ozone concentrations decreasing in summer and increasing in winter from 2019 to 2020. Additionally, a photochemical box model analysis suggested a shift in the ozone sensitivity regime in urban areas of China during the study period.