Regional Contribution And Inter-City Transport Of Summertime Ozone Over Southern Hebei Cities

Based on the O₃ data of China National Environmental Monitoring Centre in southern Hebei（Shijiazhuang,Xingtai,Handan）summer（June-August）during 2013-2020,with Hebei University of Engineering monitoring site continuous monitoring of the meteorological factors（temperature,humidity,wind speed and direction）,O₃ and its precursor concentration were analysis to study the impact factor of O₃ pollution,the Weather Research and Forecasting model coupled with Chemistry（WRF-Chem）was applied over Hebei and surrounding regions in O₃ high concentration period,the results showed that WRF-Chem model had a great simulation effect and could reproduce the spatio-temporal variation characteristics of O₃ concentration in the simulated period.Then O₃ flux between Hebei and neighboring cities was calculated to understand the O₃transmission routes of Hebei cities,the regional contribution of surrounding areas to cities in southern Hebei is quantified by Brute Force method.From 2013 to 2020,the summer O₃ concentration in southern Hebei showed an overall upward trend with the concentration 91.8μg/m³.From 2013 to 2019,the annual average O₃ concentration increased from 79.5μg/m³ to 116.7μg/m³,increasing by 46.7%.O₃ concentration（103.7μg/m3）decreased in 2020.From 2016,the O₃ standard rate has increased rapidly,and the standard rate was the highest in 2018.Within a day,O₃concentrations showed a single peak,with a certain degree of weekend effect.During the monitoring period（June-July,2018）,the average concentration of VOCs in Handan is126.5μg/m³,which is positively correlated with O₃ concentration.NO_x（NO+NO₂）is negatively correlated with O₃ concentration,and the correlation coefficient is 0.98.OX（NO₂+O₃）was greatly affected by regional factors during daytime in summer,and the contribution of local pollution to OX was greater than that at night.Heavy pollution was more likely to occur during daytime in summer of Handan.Handan has a long photochemical age,and it was more seriously affected by the aging air mass,which is more conducive for the generation of O₃.The conducive climate conditions with wind speed of 0.4-0.6m/s,temperature 28-31°C,and relative humidity 42%-68%.Under the conditions of breeze and southwest wind,O₃ is mainly controlled by NO_x,while under the conditions of northeast wind,it is mainly controlled by the synergism of VOCs and NO_x.Different transmission channels can affect the sensitivity of O₃ in Handan.Olefin is a dominant species in O₃ generation.Automobile exhaust,industrial emissions and combustion are an important source of VOCs in the atmosphere of Handan and the key to O₃ pollution control.Controlling olefin,especially isoprene,can effectively reduce O₃ generation.The air mass tracks of each city are more in the southeast and less in the northwest,east of Shijiazhuang each air mass in June-south-southeast direction of trajectory is given priority to（75%）,O₃ pollution distribution of trajectory is the southeast,northwest less features,O₃ are greatly influenced by Shandong air mass transport,short distance transport（16%-35%）contributed to O₃ concentration in Handan transport（16%-35%）.The movement track of July is in the direction of east-southeast-south.In the direction of horizontal transmission,the cities in southern Hebei are export cities,and Handan is the main export city.The western Shandong cluster and the northern Henan cluster are the main input city clusters of Hebei,while the southeastern Shanxi cluster is the main output city cluster of Hebei.The O₃ flux value of upper layer（6-9 layer,320-1000 m）is slightly larger than that of the near layer（1-5 layer,0-320 m）.Cities in southern Hebei are always affected by the fluxes from south to north.In the vertical direction,the vertical inflow and outflow fluxes increase with the increase of height.In southern of Hebei,O₃ is mainly transported downward.Shijiazhuang and Xingtai mainly transmitted downward,with net fluxes of-0.9kt/d and-0.5 kt/d,respectively.Handan transmitted upward in total vertical direction,with net fluxes of 0.2 kt/d.Cities in southern Hebei,especially Handan,made the largest contribution in terms of regional contribution,followed by northern Henan and western Shandong.The concentration contribution of southern border cities is always greater than that of northern border cities,and that of eastern border cities is always greater than that of western border cities.Shijiazhuang and Handan are greatly affected by local source emissions,and Xingtai is affected by both local source emissions and surrounding area emissions.Therefore,it is very vital to implement regional joint prevention and control,cooperative management and reduce the regional transport of O₃.