Variation Characteristics Of PM_2.5 Pollution And Sources In Zhangjiakou Under The Background Of Clean Atmosphere In The Mountains In The Northwestern Margin Of Beijing-Tianjin-Hebei

The outdoor events of the Beijing-Zhangjiakou Winter Olympics in February 2022 will be held in Chongli,Zhangjiakou.Air pollution control and atmospheric environmental protection are particularly important.However,a series of atmospheric environmental issues such as air quality changes in Zhangjiakou,air pollution sources and regional transmission and their meteorological effects under the background of clean atmospheric environment in the mountains in the northwestern margin of the Beijing-Tianjin-Hebei region still lack a complete understanding.Therefore,this article first uses the winter environment-meteorological observation data of the past four years to analyze the changes in the clean atmosphere of Zhangjiakou,a mountain city on the northwestern margin of the Beijing-Tianjin-Hebei region,as well as the characteristics of PM_2.5 pollution and meteorological effects in Zhangjiakou in winter;then,use two types of reanalysis weather data-driven WRF-CAMx model simulates PM_2.5 changes in Zhangjiakou City in February 2019,compares and analyzes the PM_2.5simulation performance,and carry out the integration of the two PM_2.5 simulation results and their effect evaluation.Finally,based on the WRF-CAMx-PSAT source analysis simulation,evaluate the contribution of the main source areas and main source types in the PM_2.5 changes in Zhangjiakou in February 2019,aiming to fully understand the air pollutants in the surrounding area.The extent and mechanism of regional transmission on the monthly PM_2.5pollution changes in Zhangjiakou Winter Olympics will provide scientific support for the effective emission reduction control and air quality forecast guarantee of Zhangjiakou's air pollution divisional sources during the Winter Olympics.The main research conclusions of this paper are as follows:(1)The characteristics of PM_2.5 pollution changes in winter in Zhangjiakou city in recent years under the background of clean atmospheric environment in the mountains in the northwestern margin of Beijing-Tianjin-HebeiZhangjiakou in the northwestern mountainous region of the Beijing-Tianjin-Hebei region has an average PM_2.5 ground concentration of 30.8?g·m^-3 in the winter from 2016 to 2019,which meets the national clean air quality standard,and its anthropogenic air pollutant emission intensity is also at a low level,indicating that Zhangjiakou,the host city of the 2022 Winter Olympics,has a clean atmosphere in winter.However,the average daily PM_2.5 in Zhangjiakou winter exceeded 75?g·m^-3 for 34 days in the past four years.PM_2.5 pollution lasted more than two days and 8 times,the longest lasting 3 days,and the PM_2.5 ground concentration exceeded150?g·m^-3 and heavy pollution process mostly lasts 6-10 hours.Under the background of clean atmospheric environment,Zhangjiakou also experienced PM_2.5 pollution and even heavy pollution process changes in winter,which made it more difficult to guarantee the atmospheric environment of the Winter Olympics.The analysis of the impact of changes in meteorological conditions on PM_2.5 in winter shows that the PM_2.5 concentration during the clean period of the atmospheric environment is negatively correlated with the wind speed near the surface,and the PM_2.5 concentration and wind speed are positively correlated during the PM_2.5 pollution process,and its positive correlation becomes larger from the PM_2.5 cleaning period to the pollution period,this reflects the important role of the regional transmission of atmospheric pollutants driven by strong winds in aggravating PM_2.5 pollution.Zhangjiakou,a mountainous city on the northwestern edge of Beijing-Tianjin-Hebei,has a unique regional meteorological impact under the background of clean atmospheric environment.(2)Comparative analysis and integration of PM_2.5 changes in WRF-CAMx simulation driven by FNL and ERA5Using FNL and ERA5 weather-driven WRF-CAMx to simulate air quality in Beijing-Tianjin-Hebei and northern China with Zhangjiakou as the center in February 2019,compare and analyze the impact of two types of weather-driven fields on PM_2.5 concentration simulation,and carry out PM_2.5 concentration simulation integration effect evaluation.The simulation evaluation results of the two types of reanalysis data-driven models for the meteorological field are similar.The correlation coefficients of temperature,air pressure,and wind speed with the observed values all exceed 0.9,and the relative humidity exceeds 0.8.Therefore,it is believed that the two weather field simulation effects are better and can drive the CAMx-PSAT mode.The simulated PM_2.5 concentrations in Zhangjiakou area can reproduce the observed concentration changes and concentration levels well,and the consistency index of concordance are both above 0.7.The simulated PM_2.5 in the surrounding cities of Zhangjiakou can also reasonably reproduce the changes in the regional PM_2.5 concentrations.Carrying out the optimal integration of FNL and ERA5 weather-driven models to simulate the PM_2.5 concentrations and found that the correlation coefficient between the PM_2.5 concentrations and the observed value after the optimal integration was increased from 0.70 to 0.81 compared to the correlation coefficient driven by a single reanalysis data.And the root mean square error is significantly reduced.The OCF integration method can significantly improve the PM_2.5 simulation effect of Zhangjiakou,and has the potential for the application of air quality forecasts for the 2022 Winter Olympics.(3)WRF-CAMx-PSAT simulation analysis of the contribution of the main source areas and source types in Zhangjiakou PM_2.5 sourcesThe source analysis of WRF-CAMx-PSAT simulation shows that the average PM_2.5contribution of local sources in Zhangjiakou in February 2019 was 44.5%,and that of external sources(except ICBC and local sources)was 42.5%.It shows that surrounding sources dominate the PM_2.5 changes in Zhangjiakou.Ulanqab(9.8%)contributed the most PM_2.5 to Zhangjiakou.This is followed by Beijing(5.2%),then there is Datong(4.1%).There are three main directions to transport PM_2.5 from the surrounding source area to Zhangjiakou:1)the northwest direction from Ulanqab to Zhangjiakou;2)the southwest direction from Datong to Zhangjiakou;3)the Beijing-Tianjin-Hebei plain city passes through Beijing to Zhangjiakou.To the southeast(the canyon topography between Beijing and Zhangjiakou,the terrain height rises by steps).The major cities around Zhangjiakou should mainly reduce emissions from industrial sources,while Ulanqab should mainly reduce emissions from residential sources.Therefore,in the process of heavy air pollution,Zhangjiakou's upper windy area must be jointly prevented and reduced emissions.At the same time,we must also consider controlling local sources to reduce the background concentration of air pollutants in Zhangjiakou.The analysis of source types shows that the PM_2.5contributions to Zhangjiakou is sorted into industrial sources,residential sources,transportation sources,agricultural sources,and power sources,which are 34.5%,29.7%,15.7%,and 6.4%,respectively.The main types of pollution sources in Zhangjiakou are residential sources(17.6%),industrial sources(16.6%)and agricultural sources(8.8%).Residential and industrial sources in Zhangjiakou are key emission reduction sources.The main source of PM_2.5 in Zhangjiakou from Ulanqab is residents(3.5%).The emission reduction in Datong,Beijing and Tianjin should be mainly from industrial sources(4.3%).