Effects Of Regional Transport On Air Pollution In Urban Agglomeration In The Middle Reaches Of The Yangtze River

In recent years,with the rapid development of economy and urbanization,air pollution in urban agglomeration in the middle reaches of the Yangtze River has become increasingly challenging,and it tends to spread around,from the previous local pollution to current complicated regional pollution.Therefore,it is necessary to characterize the regional air pollution and to investigate the impacts of air tranport on a long time scale,which would be helpful for making policy for environmental pollution prevention and control in urban agglomeration in the middle reaches of the Yangtze River.This paper collected the real-time environmental monitoring data of PM_2.5 and O₃and meteorological data in urban agglomeration in the middle reaches of the Yangtze River from the year of 2015 to 2019,and analyzed the temporal and spatial variability of pollutant concentrations and the relationship between meteorological factors and pollutant concentrations.The Lamb-Jenkinson weather classification method was used to calculate and classify the circulation patterns in the study area.For each circulation pattern,the near-ground meteorological and pollution characteristics were discussed.The forward and backward trajectory simulated by HYSPLIT model revealed the features of air transport in the study area.With the help of the trajectory clustering function in the model,the potential source regions and affected regions were explored for heavy pollution periods.The main conclusions are summarized as follows:（1）The concentration of PM_2.5 exceeds the standard limit seriously,but the overall trend is decreasing;the seasonal variability is obvious:high in winter and low in summer,middle in spring and autumn;the peak occurs mainly in January and December,and the trough usually occurs in June,July and August.The concentration of O₃ increases year by year,showing an obvious seasonal variability with the highest in summer,followed by spring and autumn,and the lowest in winter.In terms of spatial distribution,PM_2.5 and O₃ show the same geographical gradient distribution,i.e.Wuhan urban agglomeration>Chang-Zhu-Tan urban agglomeration>Poyang Lake urban agglomeration.The concentrations of PM_2.5 and O₃ are greatly influenced by meteorological conditions.PM_2.5concentration is negatively correlated with temperature,humidity and wind speed,while O₃ concentration is strongly positively correlated with temperature,and negatively correlated with humidity and wind speed.These two pollutants are insignificantly associated with atmospheric pressure.（2）According to the meteorological characteristics,the circulation patterns are divided into three categories:（1）SW,W,and S types having the best transport conditions,with the average wind speed greater than 2.4 m/s and the relatively high humidity;（2）N,NE,NW,and C types having moderate transport conditions,with the average wind speed greater than 2.0 m/s and the higher humidity for C type,and the lower humidity for N,NE,NW types;（3）E,SE,A,UM types having the poorest transport conditions,with the average wind speed less than 2.0 m/s,The SE,UM types have higher humidity,and the humidity for E and A types is lower.The circulation patterns of A,SE,E and UM are associated with PM_2.5 accumulation;the circulation patterns of S and SW are associated with PM_2.5 transport;and the circulation patterns of NE,C,NW,N and W are associated with PM_2.5 cleaning.Nevertheless,UM,E and SE circulation patterns are linked to O₃ accumulation;and N,NW,C,NE and W circulation patterns are linked to O₃ transport.Among them,N,NW,C and NE circulation patterns also have meteorological characteristics beneficial to O₃ formation.The circulation patterns of SW,S,A are O₃ clean circulation patterns.（3）The impacts of pollution transport and diffusion is restricted within the adjacent regions of urban agglomeration in the middle reaches of the Yangtze River,with relatively low trajectory frequency ranging from 0.05 and 0.4.Meanwhile,the local accumulative effect is more obvious.