| After the formation of the stable atmospheric boundary layer at night,the atmospheric pollutants on the ground are often less prone to diffusion and transmission due to weaker turbulent diffusion and smaller wind speeds in the stable boundary layer,but over the stable atmospheric boundary layer,the ground frictional effect is gradually weakened,causing the wind speed to increase with the increase of the height,so that the upper atmosphere has better transport and diffusion conditions of atmospheric pollutants than the ground.In order to understand the ability of high-wind-speed airflow over the nighttime boundary layer to transmit and diffuse atmospheric PM2.5,this study used FNL reanalysis data and ERA-interim reanalysis data in the WRF Model to simulates the changes in meteorological elements such as temperature,relative humidity and wind speed and the vertical distribution of wind speed,potential temperature and temperature during the four-time air heavy pollution process in Beijing from autumn to winter of 2017 to 2018.Then evaluate WRF model's performance on the simulation of meteorological elements.The WRF model simulation results with better performance were converted into the CMAQ model by MCIP.The vertical distribution characteristics of wind speed over the nighttime stable atmospheric boundary layer simulated by the WRF model and the vertical distribution characteristics of the atmospheric PM2.5 simulated by CMAQ were used to analyze the influence of the vertical distribution of wind speed on the atmospheric PM2.5 concentration in Beijing.By raising 50%and 90%of the surface source emission to 350m in the CMAQ model to analyze the variation of PM2.5 concentration in the ground atmosphere of Beijing and the possibility of using the vertical distribution characteristics of wind speed at night to improve the air quality in Beijing.The following conclusions were obtained:(1)The atmospheric boundary layer is relatively unstable during the daytime,and the mixing of pollutant is relatively uniform.After t the formation of the stable atmospheric boundary layer at night,the wind speed becomes smaller due to the rapid decrease of the turbulence intensity inside the boundary layer,and the surface block effect above the boundary layer decreases.As a result,the wind speed is increased,so there is a large wind speed difference between the airflow above the atmospheric boundary layer and the ground at night.In the process of heavy air pollution in the autumn and winter of 2017-2018 in Beijing,this phenomenon of high wind speed airflow above the stable atmospheric boundary layer occurs frequently.And the airflow with higher wind speed above the atmospheric stability boundary layer can transmits the air pollutants which have remained above the stable boundary layer due to the convection in the daytime,so that the PM2.5 concentration in the upper atmosphere is significantly decreased compared with the daytime,but the impact on the near-surface air pollutant concentration is small,and when the concentration of atmosphere PM2.5 is low,the high-speed airflow above the stable boundary layer has no obvious effect on the diffusion of atmospheric PM2.5.(2)Affected by the high wind speed airflow above the nighttime stable boundary layer,the elevated surface source emission height has a certain reduction effect on the PM2.5 concentration in the ground atmosphere of Beijing,and the higher the surface source uplift ratio,the stronger the reduction effect.When the 50%surface source emission is raised to 350 m,the average atmospheric PM2.5 concentration reduction rate in the four air heavy pollution processes is 25%to 26%,and the average reduction concentration is 16 ug/m3.For the elevation of 90%of the surface source to 350m,the average atmospheric PM2.5 concentration reduction rate during the four-time air pollution process is 38%to 43%,and the average reduction concentration is 26 ug/m3.However,when the atmospheric PM2.5 concentration is low overall,that is,when the atmospheric diffusion conditions are good,the elevation of the elevated surface source emission does not significantly reduce the atmospheric PM2.5 concentration.The four heavy pollution processes have similar atmospheric PM2.5 concentration reduction effects when raising a certain proportion of surface source emissions to a height of 350 m,indicating that the four heavy pollution processes have similar meteorological conditions in the vertical direction,so,the contaminants can diffuse quickly.(3)Both of the simulation results of WRF model driven by FNL reanalysis data and ERA-interim reanalysis data can better reflect the daily variation trend and value of ground temperature and relative humidity,and there is a certain deviation for the simulation of ground wind speed that is overestimated by WRF model.The simulation results of the WRF model driven by ERA-interim,although the simulation of the temperature in the steady state at night is relatively good,but there are different degrees of overestimation and underestimation at some moments,but the bias of simulations of wind speed and temperature are all within 4 K,which are acceptable,and can reflect the Inversion structures existing in the atmosphere.However,only the simulated performance of wind speed which below 200 m is close to the actual observation results.However,there are different degrees of overestimation and underestimation in more than 200 m.Generally,there are different degrees of overestimation above 800 m,up to 7 m/s to 8 m/s,but can basically reflect the phenomenon that there is a big difference between the upper and lower wind speeds in the boundary layer at night.The simulation of temperature roughly reflects the vertical variation trend from 200 m to 1000 m,but the performance of the simulation of temperature vertical variation which below 200m is poor and cannot accurately reflect the vertical distribution of the lower atmospheric temperature.Overall,the simulation results of ground meteorological elements performed driven by ERA-interim reanalysis data are better than the FNL reanalysis data. |
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