Atmospheric Boundary Layer Structure And Regional Transport Effects During Typical PM_2.5 Pollution In Northeast Chin

Due to its high latitude and the influence of continuous snow cover during winter,the atmospheric boundary layer structure in the Northeast region of China is unique.Its impact on the spatiotemporal variations of atmospheric pollution is an urgent atmospheric environmental issue that requires further understanding.In this study,we used meteorological and environmental ground observation data,reanalysis data,and November 2018 and January 2020data from atmospheric boundary layer-aerosol vertical enhancement experiments in major cities,along with atmospheric environmental numerical modeling,to investigate the evolution characteristics and underlying mechanisms of the atmospheric boundary layer structure during typical PM_2.5 pollution processes in the Northeast region of China,where local accumulation and regional transport are dominant.Using the atmospheric chemistry coupled model WRF-Chem,we designed a series of sensitivity simulation experiments to quantitatively evaluate the degree of influence of regional transport of atmospheric pollutants and winter snow cover on PM_2.5 pollution changes in Northeast China,and clarified the main mechanisms involved.The main conclusions of this study are as follows:（1）Atmospheric boundary layer structure characteristics and their impact mechanisms on local accumulation type PM_2.5 pollution in urban areas of Northeast China.Using environmental-meteorological observation data from major cities in Northeast China,including Shenyang,Changchun,and Harbin in January 2020,we studied the impact of multiscale meteorological conditions,especially the thermal structure of the atmospheric boundary layer,on PM_2.5 pollution in these cities.During the intensive observation period from January 1-20,2020,Harbin had the most severe PM_2.5 pollution levels among these cities.Compared with Shenyang and Changchun,Harbin had a stronger stability in the atmospheric boundary layer,which hindered the dispersion of atmospheric pollutants.Correlation analysis between near-surface temperature,near-surface PM_2.5 concentration,and atmospheric boundary layer height showed that temperature was positively correlated with the PBLH（correlation coefficient R=0.45,P<0.01）and negatively correlated with PM_2.5 concentration（-0.56,P<0.01）,indicating that the cold ground temperature in winter inhibits the development of the atmospheric boundary layer and exacerbates local accumulation of PM_2.5 in Northeast China.In addition,the high-altitude warm air flow driven by large-scale weather circulation has a significant inhibitory effect on the atmospheric boundary layer during the PM_2.5 pollution process in Northeast China,reflecting the impact of weather systems on PM_2.5 pollution in this region.（2）The impact of regional transport of air pollutants on the PM_2.5 pollution process in Northeast China and the characteristics of atmospheric boundary layer changes.Using the environmental and meteorological observation data from Shenyang,Changchun,and Harbin in November 2018,as well as model simulations,this study analyzed the quantitative impact of regional transport of air pollutants from North China on PM_2.5 pollution in Northeast China and the changing characteristics of atmospheric boundary layer structure.Due to the influence of the large-scale southwesterly wind at 500 meters above ground level,atmospheric pollutants were transported from the North China Plain to Northeast China.These pollutants remained in the residual layer at night and mixed vertically to the ground with the strengthening of turbulence after sunrise,leading to a significant increase in PM_2.5concentration near the ground.Backward simulations based on the FLEXPART-WRF model indicated that Hebei and Shandong provinces were the main source regions of atmospheric pollutant transport in the PM_2.5 pollution process in Shenyang and Changchun,which then continued to impact Harbin under the southwest wind drive.The contribution of regional transport of external PM_2.5 to Shenyang,Changchun,and Harbin was as high as 71.3%,84.9%,and 82.8%,respectively.On average,regional transport contributed 30.0-50.0%to the occurrence stage of PM_2.5 pollution and over 60.0%to the persistent stage of PM_2.5 pollution in Northeast China.（3）The degree of influence and meteorological mechanism of snow-covered underlying surface on the variation of PM_2.5 pollution in Northeast ChinaTo investigate the potential impact of snow-covered underlying surfaces on regional air quality changes in Northeast China,a local accumulation-type PM_2.5 pollution event from January 13 to 18,2020 in the region was chosen.The WRF-Chem model was used to conduct control and sensitivity experiments.In the sensitivity experiment,all of Northeast China was changed to have snow-covered underlying surfaces.Through a comparative analysis of the environmental and meteorological simulations between the sensitivity and control simulations,the contribution of winter surface snow cover to changes in PM_2.5 pollution was quantitatively evaluated.The influence of snow-covered underlying surfaces resulted in an increase in the regional area and degree of PM_2.5 pollution in Northeast China.The PM_2.5 concentration in the two major urban clusters of the Northeast Plain increased by 5.0～30.0μg·m-3.Specifically,the PM_2.5 concentrations in Shenyang,Changchun,and Harbin increased by 20.6μg·m^-3,5.4μg·m^-3,and 7.6μg·m^-3,respectively.Shenyang had the largest contribution rate at 18.8%,while Changchun and Harbin had smaller positive contributions,which may be related to the original snow-covered underlying surfaces.Analysis of the meteorological mechanism of the snow-covered underlying surfaces on the atmospheric boundary layer showed that the near-surface air temperature,atmospheric boundary layer height,and ventilation coefficient all decreased,which is conducive to the accumulation of PM_2.5 near the surface and aggravates winter PM_2.5pollution in Northeast China.