Study On Dispersion Mechanism And Model Of Vehicle Pollutants In Urban Road Network

With the increase of the motor vehicles and the proportion of motorized travel of residents in China,urban traffic has become one of the main sources of air pollution.The dense and towering building environment in the city further hinders the dispersion of motor vehicle pollutants to the peripheral areas,resulting in a more prominent impact of motor vehicle emissions on urban air quality.As an important urban traffic management measure,motor vehicle restriction can not only rapidly reduce the emission of motor vehicle pollutants from the source in a short time to alleviate the problem of air pollution,but also reasonably guide the choice of residents’ travel mode,optimize the urban traffic travel structure,and then improve the urban air quality from a long-term perspective.However,the building environment for the dispersion of motor vehicle pollutants in China’s cities is complex and diverse,and the impact of urban traffic on air quality varies in different regions.It is urgent to systematically analyze the building environment for the dispersion of motor vehicle pollutants,further clarify the diffusion mechanism of motor vehicle pollutants under different diffusion environments,and build a motor vehicle pollutant diffusion model suitable for a variety of diffusion scenarios at the city scale,So as to accurately describe the concentration distribution of motor vehicle pollutants in the city,improve the effectiveness and accuracy of motor vehicle traffic restriction measures,and make contributions to the optimization of urban air quality.To this end,this paper conducted researches about “ the dispersion mechanism and model of vehicle pollutants in urban network” based on the National key R & D plan strategic international scientific and technological innovation cooperation key projects “Information service and decision evaluation technology based on urban virtual transportation system”(2017090100).Based on the systematic analysis of the building environment for the dispersion of motor vehicle pollutants in typical urban road networks,this paper explored the dispersion mechanism of motor vehicle pollutants in a variety of dispersion building environments,and then quantitatively studied the impact of the characteristics of road section dispersion building environment on the dispersion of motor vehicle pollutants.On this basis,a comprehensive dispersion model for motor vehicle pollutants was constructed,which is applicable to the main dispersion environments and characteristics in urban road networks.The specific research content of this paper includes the following five parts:(1)Analysis on building environment of vehicle pollutant dispersion in urban road network.Based on the open source road network and building contour data,the structure of urban road network is obtained and simplified by using the Transtar software module,and a method for extracting the building environment data of vehicle pollutant dispersion in urban road network was proposed.The definition and calculation method of building environmental characteristics at road sections and intersections are described in detail.Based on this,the dispersion types of road sections and intersections are divided.The overall distribution of various dispersion types of road sections and intersections in the urban road network,as well as the frequency and spatial distribution characteristics of various diffusion building environmental characteristics are analyzed.(2)Study on dispersion mechanism of vehicle pollutants in urban road network.From the perspective of vehicle pollutant dispersion flow direction and concentration distribution,the research method of vehicle pollutant dispersion mechanism based on computational fluid dynamics was improved,which mainly includes the steps of geometric modeling,boundary condition setting,turbulence model and control equation selection,result output and statistics.By analyzing the wind field distribution,vehicle pollutant dispersion flow and concentration distribution under various dispersion scenarios,the dispersion mechanism of vehicle pollutants at the road sections and intersections of the main dispersion types was clarified.The road section dispersion types mainly include crevice road sections,open road sections and semi open road sections,and the intersection dispersion types mainly include closed intersections,crevice intersections,closed T-junctions and crevice T-junctions.(3)Research on the influence of building environmental characteristics on pollutant dispersion at road section.Pre experiments were conducted on other key factors affecting the dispersion of pollutants in the road section,mainly including wind direction,wind speed and traffic volume,to determine variable conditions for subsequent experiments.By analyzing the proportion of dispersion flow in the main flow direction and concentration distribution of pollutants in the porosity road section and semi open section,the specific impact of the characteristics of porosity and street size on the dispersion of vehicle pollutants is quantified.In the aspect of porosity,porosity and the difference of porosity between two sides of road were mainly considered.In the aspect of street size,street width height ratio and the difference of building height between two sides of road were mainly considered.(4)Study on comprehensive dispersion model of vehicle pollutants in urban road network.On the basis of expounding the basic framework and assumptions of the comprehensive dispersion model,the Gaussian plume model,the principle of the box model and the calculation method of the relevant dispersion parameters were introduced.According to the distribution of dispersion source intensity and dispersion characteristics of vehicle pollutants,macro dispersion models suitable were proposed,which is applicable to open road sections,intersections and T-junctions.Based on the numerical simulation experiment,the dispersion flow distribution data of vehicle pollutants under different dispersion building environmental characteristics in the dispersion scenarios of porosity road sections and semi open road sections are obtained,so as to fit the dispersion source intensity model of vehicle pollutants in the main flow directions of the road sections,and build a specific dispersion model according to the dispersion characteristics of each flow direction.(5)Verification and application of comprehensive dispersion model of vehicle pollutants in urban road network.Based on the Transtar simulation platform and a number of measured data,the validation data set of comprehensive dispersion model was constructed.The basic comparison model was selected from the perspective of different assumptions about the dispersion building environment,and the effectiveness of the model was verified based on the general evaluation indicators and standards.By analyzing the temporal and spatial distribution and time-varying law of urban air pollutant concentration,the implementation scope and time of regional tail number restriction measures were determined.The implementation effects of various traffic restriction measures are simulated by using Transtar software.The concentration changes of air pollutants before and after traffic restriction are calculated through the comprehensive dispersion model,and the air quality improvement effects of different traffic restriction measures are analyzed...