Analysis Of Vertical Distribution Characteristics And Sources Of Ozone In Shijiazhuang

In recent years,ozone（O₃）has become the primary pollutant affecting surface air quality in summer in Shijiazhuang.In addition to photochemical generation,surface O₃concentrations are also influenced by the physical conditions of the upper atmospheric mixing and vertical boundary layer,and as the height of urban buildings continues to increase,researches on high-altitude pollution are receiving increasing attention.However,the existing studies on O₃ pollution in this region were mainly focused on the ground level,and there is a lack of studies on O₃ generation at higher altitudes.Therefore,this study analysed the vertical distribution characteristics of O₃ and the causes and sources of heavy pollution based on vertical observations of O₃ from gradient monitoring stations and ground-based Li DAR,and explored scientific issues such as the factors influencing O₃ in the vertical direction,photochemical effects and the evolution of sensitivity using the Generalized Additive Model（GAM）,Observation-based Box Model（OBM）,combined with methods and tools such as Relative Incremental Reactivity（RIR）and pollutant source analysis models.The analysis showed that the average O₃ concentration in the peak season（the highest daily maximum 8-hour average O₃ concentration for six consecutive months,May～October）was 1.95 times higher than that in the non-peak season（November～April）,and O₃ exhibited a positive gradient distribution in the range of 20～200 m in the non-peak season,while the highest O₃ concentration was found at 116 m in the peak season.In the height range of0.3～3.0 km during the heavy pollution period in summer（June～August）,O₃ decreased with increasing altitude in the range of 0.3～3.0 km,and showed positive gradient distribution characteristics in 0.37～0.76 km,and the O₃ concentration was most stable and highest in the 0.76～1.16 km,after which the O₃ concentration decreased in a wave-like manner with increasing height.Correlation analysis showed that the meteorological factors with the highest correlation with O₃ were temperature and UVB in peak and non-peak seasons,respectively,and among the pollutant factors,NO and NO₂ were significantly negatively correlated with O₃.The GAM results also showed that NO was a major influencing factor for O₃ variation throughout the year,and the most important meteorological factors for the peak and non-peak seasons of O₃ were temperature and UVB,respectively.Simulation results based on the OBM model indicated that strong photochemical reactions were an important driver for O₃ pollution and atmospheric photochemistry was more important for the surface O₃.Through the calculation of RIR,it was determined that alkenes were the precursors with the greatest impact on O₃,and among typical species,isoprene and ethylene have the highest RIR values in the peak and non-peak seasons,respectively,indicating the influence of plant emissions and traffic emissions.In simulations of different pollution scenarios,it was found that increasing NO concentrations favoured O₃pollution mitigation,and the abundance of·OH was a significant feature of O₃photochemical reactions and dominated the effect of changes in NO concentration on O₃.The analysis of the causes of heavy O₃ pollution showed that local photochemical generation combined with vertical transport was an important reason for heavy O₃ pollution,and the pollution trajectory that had a greater impact on the ground came from the local area of Shijiazhuang,with the contribution of urban areas to O₃ concentration in Shijiazhuang exceeding 70μg·m^-3 at different heights,while external transport sources including Xingtai,Handan,Cangzhou in Hebei province and Yangquan in Shanxi province.