Temporal And Spatial Changes In Ozone Concentrations In Three Megacity Clusters Of Eastern China During 2015-2020:Implication For Their Cause(s)

In recent years,thanks to strong emission control policies,the concentrations of PM_2.5,SO₂,NO₂,and CO have been significantly reduced,while ozone（O₃）has become a major air pollutant in eastern China in the past decade.Therefore,this study is of great significance to explore the temporal and spatial changes and influencing factors of ozone concentration in three mega urban agglomerations in eastern China.This thesis firstly uses the air pollutant data of the National Environmental Monitoring Center of China to analyze the temporal and spatial characteristics and changing trends of ozone concentrations in the Beijing-Tianjin-Hebei,Yangtze River Delta and Pearl River Delta regions from 2015 to 2020,and analyzes the main contribution of the change trend from the perspective of ozone exceeding days.Secondly,based on the method of ozone pollution level to explore the changes of ozone pollution,and finally analyze the difference of meteorological factors and ozone precursors between ozone exceeding days and those on non-exceeding days to explore the main reasons for the increase in ozone concentration in recent years.The results show that the three major urban agglomerations in eastern China,namely Beijing-Tianjin-Hebei（BTH）,Yangtze River Delta（YRD）and Pearl River Delta（PRD）,have a significant increase in the daily maximum sliding 8-hour mean（MDA8）annual mean ozone concentration.25%,10%and 19%respectively in 2015-2020.In addition,a significant increase in the number of ozone exceeding days（defined as MDA8 O₃>75 ppb at 273 K）was also observed in all three megacity clusters during the same period,as indicated by an analysis of the upward trend in the annual mean concentration of MDA8 O₃.This trend is almost entirely attributable to an increase in consecutive ozone exceeding days.Taking the Beijing-Tianjin-Hebei region as an example,based on the method of ozone pollution level,we know that the number of days with lower ozone concentration levels generally decreases significantly,while the number of days with higher concentration levels increases significantly,especially the strongest level of ozone pollution,namely Bin 10（average ozone concentration greater than approximately 100ppb）increased most significantly.This indicates that ozone pollution has shifted from weak to strong,and there is a good positive correlation between the number of consecutive ozone exceeding days and the number of Bin 10 days（R=0.84,P<0.05）.The increase in the number of Bin10 days may lead to an increase in the number of ozone exceeding days and consecutive ozone exceeding days in the Beijing-Tianjin-Hebei region,which has an important impact on the rising trend of ozone concentration in recent years.In order to identify the key factors for the significant increase in ozone exceeding days,we compared the differences of meteorological factors and ozone precursors between ozone exceeding days and non-exceeding days.There is no significant difference of ozone precursors（such as NO）between the two,while meteorological factors such as surface net solar radiation（SSR）,daily maximum temperature（T-max）have significant differences between ozone-exceeding days and non-exceeding days,as well as the statistical finding that the number of days with strong solar radiation or high temperature has increased significantly in recent years,suggests that changing meteorology/climate is the primary driving force are the main driving force behind the increase in ozone concentration.