| In recent years,as China’s economic strength continues to grow,urbanization continues to accelerate,global warming,people’s living standards continue to improve,air pollution has become an environmental problem faced by many cities with rapid economic development,high population density and industrial development,and the phenomenon of heavy pollution weather occurs frequently throughout the country,especially in the Beijing-Tianjin-Hebei region in the autumn and winter seasons.However,since the national outbreak of the new crown epidemic in January 2020,China has taken strict quarantine measures in response to the epidemic prevention and control,shutting down a large number of factories and transportation,etc.,and implementing nationwide work stoppages,production shutdowns and travel reductions,which can be regarded as an ideal anthropogenic source reduction control measure.Few domestic and foreign scholars have analyzed the combination of PM2.5pollution and atmospheric circulation.In this paper,the spatial and temporal characteristics of PM2.5 in the Beijing-Tianjin-Hebei region are explored in greater depth,and the influence of different meteorological elements on the diffusion of fine particles and the circulation situation in the Beijing-Tianjin-Hebei region are analyzed.Using the PM2.5 concentration data observed at stations in the Beijing-Tianjin-Hebei region from 2019 to 2022 and the TAP dataset,the characteristic patterns of PM2.5 spatial and temporal distribution during the study period are analyzed.Firstly,the global reanalysis data from the National Centers for Environmental Prediction and the National Center for Atmospheric Research were calculated using the Lamb-Jenkinson fractal method to obtain the geostrophic wind speed,wind direction,geostrophic vorticity values and Atmospheric circulation index for the study area in 2019-2022.The daily circulation situation in the study period is calculated according to the fractal criteria,and the changes of the circulation situation on different time scales are analyzed.Two typical PM2.5 heavy pollution processes in Beijing-Tianjin-Hebei region were selected to analyze the weather background of the circulation situation and the change characteristics of meteorological elements during the occurrence,maintenance and receding phases of the heavy pollution process.The main findings of this paper are as follows:（1）The high season of PM2.5 pollution in Beijing,Tianjin and Hebei is winter,and the overall distribution is characterized by high in the south and low in the north.Except for January 2020,the monthly average value of PM2.5 concentration in Beijing-Tianjin-Hebei region did not decrease compared with the same period in 2019,the monthly average value of PM2.5 concentration in Beijing-Tianjin-Hebei region in winter of 2020-2022 decreased compared with the same period in winter of 2019,and the atmospheric fine particulate matter PM2.5 pollution in Beijing-Tianjin-Hebei region improved significantly.2019-2022 PM2.5concentration in Beijing-Tianjin-Hebei region in winter concentration mean value is the maximum for four seasons,and the whole southern Beijing-Tianjin-Hebei region reaches the maximum annual concentration,and the minimum value area in winter is the northern part of Chengde City.The southern cities of Hebei Province are more polluted than other cities in the study area,and most of the areas along the Taihang Mountains have relatively low average annual PM2.5 values.The northern cities are less polluted and have the lowest annual average PM2.5 concentrations relative to the central and southern regions.From 2019 to 2022,the highest PM2.5 concentrations are found in the study area in winter,followed by autumn,spring,and summer.Several heavy air pollution events occurred in some cities in the Beijing-Tianjin-Hebei region during the study period,but the average PM2.5 winter concentrations showed a significant yearly decreasing trend,with relatively larger decreases in PM2.5 concentration values from 2019 to 2022,and fewer areas where heavy pollution occurred year by year.（2）Compared with 2019,air pollution with PM2.5 as the primary pollutant improved year by year from 2020 to 2022.The year with the highest frequency of mild pollution was2020,the year with the highest frequency of moderate pollution was 2019,and the year with the highest frequency of severe pollution or above was 2019.The number of PM2.5 heavy pollution in the Beijing-Tianjin-Hebei region is gradually decreasing,and the air pollution with PM2.5 as the primary pollutant is improving year by year.（3）The heavy pollution weather in the Beijing-Tianjin-Hebei region is easily affected by Atmospheric circulation classification.The frequency of heavy pollution of PM2.5 or above is the highest under the conditions of northerly atmospheric circulation,followed by anticyclonic circulation with north wind pattern,cyclonic circulation with south wind pattern,cyclonic circulation and anticyclonic circulation.The change of wind speed and direction in the study area indicates that the prevailing easterly wind in northeast China and North China during the study period gradually shifted to southeast wind and northeast wind,from convergence in the middle to diffusion at both ends,and there were small local wind shear phenomena.The wind speed center of the occurrence stage is located in the southwest of Hebei and the Fen-Wei Plain,the center of the maintenance stage is transferred to Inner Mongolia and Gansu,and the center of the extinction stage appears in the Liaodong Peninsula,the Taiwan Strait and part of the East China Sea.In the process of the high relative humidity in the study area gradually changing from the northeast part to the south and southeast part,the enhancement of relative humidity not only promotes the condensation and settlement of fine particles,but also enhances the possibility of their transformation into other pollutants or even non-pollutants,assisting fine particles in the whole region to reduce PM2.5concentration through their own physical and chemical methods. |
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