Analysis And Simulation Of Chemical Composition And Vertical Structure In The Boundary Layer Of PM_2.5 In Kunshan City In Winter

PM_2.5,one of the most important air pollutants,is still at a high level in winter in the economically developed Yangtze River Delta region.Therefore,the exploration of PM_2.5chemical composition pattern and boundary layer structure characteristics in winter in the Yangtze River Delta region is of great significance for the accurate prevention of haze in the region.In this paper,the chemical composition of PM_2.5and boundary layer structure characteristics in the winter of Kunshan City from 2017 to 2019 were analyzed and simulated by using the chemical composition data,lidar data,wind profile radar data,microwave radiometer data and emission inventory data,and the following conclusions were acquired in this study:(1)The concentration of PM_2.5 in Kunshan showed a downward trend,while the concentration of O₃ showed a slight upward trend,which resulted in the enhancement of atmospheric oxidation.The concentration of NO₂ decreased slightly,and the concentration of SO₂ decreased year by year,and its decline was far more than the reduction level of NO₂concentration.The PM_2.5pollution in Kunshan in winter was mainly transport-type pollution.(2)The concentration of PM_2.5 and chemical components under local pollution conditions was higher than that under regional transport conditions,which was mainly caused by the rapid increase in the concentration of secondary components caused by local gas-particle conversion.The increase in the proportion of local pollution events in the total pollution events will lead to the increase of PM_2.5 concentration throughout the whole winter.The increase of atmospheric oxidation caused by the increase of O₃ concentration still leaded to the increase of secondary aerosols,which weakened the effect of the reduction of PM_2.5 concentration caused by emission reduction.The diurnal variation of the concentration of chemical components at the clean time corresponded well with the development process of the boundary layer,while at the polluted time,the diurnal variation of the concentration of chemical components was more related to the emission and transportation process.(3)There was a good correspondence between the near-ground extinction coefficient and the ground PM_2.5 concentration.The diurnal variation of physical and chemical properties in the boundary layer was obvious at the cleaner time,but gradually became less obvious with the increase of pollution.The vertical distribution of wind speed,temperature and humidity all had certain influence on the vertical distribution of extinction coefficient,and humidity was the main meteorological factor affecting the vertical distribution of pollution in Kunshan area.EC was the most sensitive to the change of boundary layer height,and SO₄^2-was the least sensitive,which was related to the fact that EC was the absorbing aerosol and SO₄^2-was the scattering aerosol.Whether from the perspective of improving air quality or slowing down global warming,it was very meaningful to reduce the emission of absorbent aerosol EC.(4)The simulation effect of WRF-CHEM3.9 on the chemical components in PM_2.5 needed to be further improved,although the simulation effect on the vertical structure in the boundary layer was better.Relative humidity was the most likely meteorological factor to influence the simulation effect of PM_2.5and its chemical component concentration.