Modeling Of PM_2.5 Heavily Polluted Weather In Beijing-Tianjin-Hebei Region For Zhangjiakou Winter Olympics And Simulation Of Source Changes

The daily mean PM_2.5 concentrations and the meteorological data including temperature,relative humidity,wind speed and surface pressure in Beijing-Tianjin-Hebei（BTH）region during winters from 2014 to 2019 were comprehensively analyzed to systematically understand the temporal and spatial distribution characteristics and meteorological conditions of PM_2.5heavy pollution in winter in recent years.The pollution process of BTH region during winters from 2014 to 2019 was screened and classified.The synoptic patterns of the middle and low levels in BTH region and the interannual variation trend of pollution meteorological conditions were discussed.Several main weather situations affecting BTH region were determined,and the relationship between haze pollution and climate was clarified.The heavy pollution process of PM_2.5 in Zhangjiakou was analyzed,where the Winter Olympic Games will hold,the pollution type of Zhangjiakou in BTH urban agglomeration was also determined,and the main characteristics of PM_2.5 heavy pollution weather in Zhangjiakou was fully understood.Combined with WRF-FLEXPART diffusion model,the results revealed the spatial distribution and interannual variation characteristics of PM_2.5 potential impact area,quantified the relative contribution of PM_2.5 regional transmission and local emission,and provided scientific reference for air quality assurance of 2022 Beijing Winter Olympic Games.The main conclusions were as follows:1)Analysis on characteristics of heavy pollution and meteorological effects in winter in Beijing-Tianjin-Hebei region in recent yearsDuring the study period,PM_2.5 heavy pollution occurred frequently in winter in BTH region,with significant spatial distribution characteristics,seemed similar to the distribution characteristics of heavy industrial cities in the region.According to the daily mean PM_2.5concentration,13 cities of this region were divided into three different pollution types:"Good"（Zhangjiakou,Chengde,Beijing,Qinhuangdao）,"Light Pollution"（Tangshan,Langfang,Tianjin,Cangzhou）and"Heavy Pollution"（Hengshui,Xingtai,Handan,Baoding,Shijiazhuang）.The results showed that temperature,relative humidity,wind speed and surface air pressure were closely related to PM_2.5 concentration in BTH region.Wind speed andsurface air pressure were negatively correlated with PM_2.5 concentration,while relative humidity was significantly positively correlated with PM_2.5 concentration.There was a positive correlation between PM_2.5 concentration and temperature in"Good"and"LightPollution"urban agglomerations,and almost no correlation between PM_2.5 concentration and temperature in"Heavy Pollution"urban agglomerations.The heavy PM_2.5 pollution in winter in BTH region has the spatial distribution characteristics of high in the south and low in the north,high in the west and low in the east.The main meteorological conditions were lowwind and high humidity.The heavy PM_2.5 pollution weather in Zhangjiakou during the same period of the Winter Olympic Games was prone to occur at night.Due to fine diffusion conditions,PM_2.5 weather can dissipate rapidly.Wind speed was positively correlated with PM_2.5 concentration under heavy pollution in Zhangjiakou during the Winter Olympic Games,while temperature,relative humidity and surface pressure were negatively correlated.Dry and cold air was prone to heavy pollution in Zhangjiakou due to PM_2.5 regional transport.2)Analysis of synoptic pattern on PM_2.5 heavy pollution over the Beijing-Tianjin-Hebei region in recent 6 winters based on PCTThe PCT algorithm could provide more complete and more credible classifications of weather-patterns with longer observational data.According to the national standard of PM_2.5heavy pollution with the daily PM_2.5 concentration exceeding 150μg·m^-3,72 heavy PM_2.5pollution days were identified from urban air quality monitoring measurements over the Beijing-Tianjin-Hebei（BTH）region during winters from 2014 to 2019.By employing the PCT（Principle Components in T-mode）method with 0.25°×0.25°mean sea level pressure provided by ERA5 reanalysis data of meteorology,the synoptic patterns of the heavy PM_2.5heavy pollution days were objectively classified with five weather patterns:（1）FH（front of high pressure）,（2）DCF（depression in front of cold front）,（3）RH（rear of high pressure）,（4）UP（uniform pressure）,and（5）WL（weak low pressure）respectively accounting for 34.72%,20.83%,16.67%,16.67%,11.11%of total heavy pollution days over the BTH region.Furthermore,an analysis on a PM_2.5 heavy pollution process in the BTH region over February12—16,2017 revealed that the PM_2.5 heavy pollution levels,air pollutant concentrations,near-surface winds and air pollutant transport pathways were changed accordingly with the evolution of synoptic patterns during a heavy air pollution event.3)Climatic simulation on influence domains of ambient sources of air pollutants in Zhangjiakou in recent 10 yearsWith 10-year（2010-2019）WRF-FLEXPART simulation in February over Zhangjiakou region,the results showed that the local emission of Zhangjiakou is dominant,the contribution rate of PM_2.5 in northern Ulanqab,Shangyi County and Huai’an County of Zhangjiakou was higher,and the northern Ulanqab is the main source of external PM_2.5 in Zhangjiakou.The order of PM_2.5 contribution rate of local and surrounding cities to Zhangjiakou was:Zhangjiakou（56.02%）>Ulanqab（15.99%）>Datong（6.09%）>Hohhot（5.52%）>Ordos（3.05%）>Beijing（2.40%）>Shijiazhuang（1.64%）>Taiyuan（1.00%）.The main PM_2.5 transmission paths affecting Zhangjiakou winter air pollution can be divided into:1)Ordos→Hohhot→Southern Ulanqab→Zhangjiakou path;2)northern Ulanqab→Zhangjiakou path;3)Datong→Zhangjiakou path;4)northern Ulanqab→Zhangjiakou path.Except for the stable route Ordos→Hohhot→Southern Ulanqab→Zhangjiakou,the other three routes had obvious interannual variation characteristics.The heavy pollution process in Zhangjiakou was dominated by air pollutants driven by winter monsoon.Therefore,controlling the pollutant sources in Zhangjiakou,Ulanqab and the west direction could basically ensure that the air quality meets the standard during the 2022 Beijing Winter Olympic Games.