| The regional flow field of the Ji'nan urban agglomeration where is located in the northwestern of Shandong Province is complicated.Once encountered unfavorable air diffusion condition,it will easily form a large area of continuous pollution belt,especially,in the heating season.In this thesis,the characteristics of average atmospheric flow field and the spatial-temporal distribution of air pollutants in the heating season of Ji'nan urban agglomeration were analyzed by numerical simulation and statistical analysis.And the evolution of the atmospheric flow field and the regularity of pollutant transport in the typical pollution process during the heating season were analyzed.The followings are conclusions of this thesis.The surface flow field in the heating season of the Ji'nan urban agglomeration is mainly affected by the Mongolian high pressure and the Aleutian low pressure.It 'is divided into three parts:the eastern branch of the northerly air current,the west branch of the northerly air current and the southern anticyclone air current.The 850hPa flow field is mainly affected by the winter monsoon in Siberia,and the area was controlled entirely by the northwestern air current.The northwestern direction of the Luzhong Mountains is in contact with the northwestern plain of Shandong Province,and the slope of the terrain is relatively large,which has a great influence on the airflow near the ground.The concentration of PM2.5 and SO2 in Ji'nan urban agglomeration was decreasing year by year.In 2016,the regional average concentration of PM2.5 was 98 ?g/m3,which was 25.8%lower than that in 2012,and the average SO2 concentration was 46 ?g/m3,which was 59.5%lower than that in 2012.The concentration of PM10 and NO2 also showed a decreasing trend.The seasonal variation of concentration of PM2.5 and SO2 in Ji'nan urban agglomeratio was obvious.The highest PM2.5 concentration appeared in winter,and the next is spring,then autumn,and the lowest concentration appeared in summer.SO2 concentration is higher in winter and spring,then autumn,and the lowest concentration also appeared in summer.All of the six basic pollutants in the heating season of the seven cities of Ji'nan urban agglomeration appeared obviously daily variation.The daily variation characteristics of PM2.5,SO2,and NO2 varies.Both PM2.5 and SO2 appeared two-peak-two-vale type or one-peak-one-vale type,and NO2 appeared two-peak-two-vale type or one-vale type.The daily variations of concentrations of PM10,CO and O3 in each city are consistent.with PM10 being two-peak type,CO being one-peak-one-vale type,and O3 being one-peak type.On the whole,the daily variation trend of pollutant concentration in Ji'nan urban agglomeration of has certain similarities.From the changes of the flow field in January 2017,East Asian trough strength was weaker than that in the same period,and zonal circulation is prevailing.Weaker cold air and more hazy weather in China as a result of it.At an altitude of 850 hPa,the winter monsoon weakened and the wind direction was southward.The airflow over the ground to reach the Ji'nan urban agglomeration area was weak,and the cold air gathered on the north side of the mountain range,forming a high-pressure center.A southwesterly flow appears in the northeastern direction of the mountain range,causing the eastern branch of the northerly wind entering the urban agglomeration to deflect eastward earlier.From the point of view of pollutant distribution changes,in January 2017,the PM2.5 and PM10 pollution belts moved northward,and high-concentration areas appeared in the line between the western part of Liaocheng and the central part of Zibo,where the concentration of PM2.5 reached more than 120?g/m3,and the concentration of PM10 reached above 200?g/m3.It is down from the same period in 2016.The SO2 and NO2 pollution levels have also been reduced.The SO2 high concentration areas are still concentrated in the central part of Zibo,with the regional concentration dropping to around 77?g/m3.The NO2 high concentration areas are concentrated at the border of central Zibo and Ji'nan-Dezhou-Liaocheng,and the regional concentration drops to about 65?g/m3.It can be seen from the typical pollution process that the simulated flow field of the WRF model is basically the same as the weather process represented by the actual monitoring data.It can be considered that both the simulated flow field of the WRF model and the actual monitoring data can be used to describe the characteristics of the large flow field during the pollution process.In the early stage of the typical pollution process,the rapid accumulation of pollutants was mainly due to the steady westerly circulation at the height of 500hPa,the moisture was brought by southwest transport channel at the height of 850hPa,the high moisture mass settlement process over the area,and the low wind speed and wind shear on the ground.During the pollution phase,during the stable weather,the pollution belt moves in the direction of near-surface airflow.During the clearing phase,the removal of strong cold air began at near ground and gradually developed toward high altitude.The role of pollutant removal was divided into stages.During this period,the weak cold air and the decrease of humidity caused the concentration of particulates to decrease,while the maintenance of the inversion layer allowed the particulates to be concentrated near the ground.The Ji'nan urban agglomeration usually has the following three high-concentration pollution belts during the stable weather during the heating season.The first type is the shear pollution belt formed by the wind shear on the ground.It is more common in plains with surface wind shear,and there is a certain lag in the distribution of pollution belts and changes in wind direction.The second type is the ahead of cold front pollution belt formed by the cold front crossing.This pollution belt changes with the moving position of the cold front,and it only exists in the cold front transit process.When the cold front is weak,the ahead of cold front pollution belt often exists in the line between western part of Liaocheng and the central of Zibo.The third type is the topographic pollution belt formed by topographic features,mainly in the Laiwu region. |
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