| The double inversion layer structure of urban atmosphere is very important for the spatial and temporal distribution of urban air pollutant concentration.In this paper,we applied the Multi-Scale Urban Canopy Model(MSUCM)independently developed by our research group and coupled it to the WRF-Chem atmospheric chemistry model to numerically simulate the process of typical atmospheric pollution events in Nanjing in the four seasons in2015.Through sensitive numerical experiments by the urban canopy physical process,the influence of the temporal and spatial variation characteristics of the two near-surface temperature inversion structures,the atmospheric boundary layer and the night-time atmospheric residual layer,on the distribution of atmospheric pollutants is discussed,and the mechanism of effect is analyzed.This article discusses the impact of the dual inversion structure on the urban surface air circulation structure and the vertical distribution characteristics of pollutant concentration in the pollution process mainly caused by local accumulation and external transportation,as well as the impact on the surface air pollution concentration.The results show that:(1)In the absence of obvious weather system,the diurnal variation of the height of the urban atmospheric boundary layer top is obvious.After the evening,the atmospheric boundary layer regenerates near the ground and the inversion is strong,while the inversion of the residual layer top becomes weak gradually and disappears in the morning of the next day,and the diurnal variation of the height is not obvious;(2)Compared with the results of the original WRF-Chem model,the MSUCM simulation results obtains a more physically realistic night-time urban heat island atmospheric circulation structure and range of effects.The results of the diurnal variation of the urban atmospheric boundary layer circulation,including the height of the boundary layer and the inversion layer,are closer to the observation characteristics,which also have obvious influence to the simulation results of the spatial and temporal characteristics of the concentration distribution of atmospheric pollutants,and more consistent with the observation data;(3)The WRF-Chem-MSUCM simulated urban atmospheric boundary layer top height at night is about 100-300 m higher than the original WRF-Chem control group simulation result,and the urban residual layer inversion stratification above it is also slightly stronger than the control group.The residual concentration effect of pollutants in the vertical space between the two temperature inversion structures is more obvious.Before noon the next day,as the atmospheric boundary layer rises,this part of the pollutants re-enter the new atmospheric boundary layer.The higher the concentration of pollutants in the residual layer at night,the more pollutions will enter the atmospheric boundary layer the next day,and vice versa.(4)In the simulation of pollution events mainly caused by local accumulation,the concentrat ion of pollutants near the surface of the city at night in the MSUCM experimental group is usually lower than that of the UCM result,because the atmospheric boundary layer of the city at night in the experimental group is higher than that in the control group,so there is larger environmental capacity slows down the rise of pollutant concentration;At the same time,the concentration of pollutants remaining above the boundary layer and below the residual layer is also lower than the UCM result,which is due to the strong double inversion structure over the city in the experimental group has a certain obstructive effect on the vertical mixing of air pollutants near the ground;In the simulation of heavy pollution process for several consecutive days,the daily cumulative increase of near surface concentration in the experimental group was lower than that in UCM.(5)In the simulation of pollution events mainly caused by regional transportation,the rate of increase in the concentration of surface pollutants at night in cities and suburbs is faster than that of UCM in the simulation results of MSUCM,and the pollutant concentration in the residual layer of the MSUCM result higher than that near the ground.This is because the heights of the atmospheric boundary layer and residual layer over the cities in the experimental group are higher than those over the surrounding suburbs,which is significantly higher in the experimental group than that in the control group,so it is conducive to the vertical diffusion of pollutants transported from high altitude and near the ground;After the next morning,with the increase of boundary layer height,the beginning time of pollutant diffusion in the experimental group is earlier than that in the control group,and the speed of concentration mixing is faster.These findings are of great significance for us to increase the impact of double inversion layer structure on air pollution and improve environmental pollution. |
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