Spatio-Temporal Variation And Diffusion Mechanism Of Air Pollutants In Urban Streets And Planning Countermeasures

Climate change is rapidly emerging as the most serious development concern of the 21 st century,posing a serious threat to China’s urban air environment.As an important part of the urban built environment,streets are not only the most widely distributed public spaces in cities,but also the most frequent public places for urban citizens.Air pollution problems,which are directly caused by high-density buildings and traffic congestion,will continue to be a major challenge in the management of the urban environment until a complete transition to green transportation occurs.The purpose of this study is to explore the pattern of distribution and diffusion of Traffic-Related Air Pollution(TRAP)in urban streets and the planning response from interdisciplinary perspectives of urban and rural planning,environmental science,meteorology and geography.First,related concepts and research progress were sorted out through a literature review of related TRAP researches,the development environment and technical status of urban street’s TRAP research were familiarized,and the methodology was established.Second,Wuhan City was selected as a case to study the pattern of spatio-temporal variation of air pollution in urban streets through field measurements,the transmission diffusion mechanisms of TRAP in urban streets through CFD simulation,and the evaluation of the ventilation potential in urban streets was carried out through GIS spatial analysis.Finally,based on the above results,a planning response framework for improving natural ventilation and air quality of urban streets was constructed,and the planning strategies and design recommendations were proposed from both urban and neighborhood scales.The main research contents and results are as follow:(1)By selecting urban streets of different grades,the pattern of spatio-temporal variation of TRAP and the horizontal attenuation effect of ambient air pollution on main roads by combining data crawling and pretreatment technology,and the vertical and horizontal distribution characteristics of urban street microenvironment TRAPs were systematically analyzed.The contribution of urban traffics activitie to the pollution of different microenvironments of urban streets was explored.The results present that the field measurements of the air environment of multi-group urban streets with the Background Concentration(BC)can effectively identify the pattern of spatio-temporal variation of air pollution in urban streets and the potential pollution contribution of urban traffics.The variation of air pollution in different streets in the Central Area of Wuhan(WUCA)has obvious ‘Weekend Effect’,‘Weather Effect’,‘Horizontal Attenuation Effect’ and a certain degree of "travel peak effect".The potential contribution of urban traffics to the concentration of gaseous pollutants in different grades’ streets in WUCA is significant and stable,with an average of about 35%(30%-42%)above the BC,but there are no obvious differences in road class and microenvironmental openness in this effect,while the potential contribution to particulate pollutant concentrations has significant spatial variability.(2)In view of the key spatial elements such as the Building Interval(BI),Street Interface Density(SID),Length-Depth Ratio(L/D)and Driveway Layout(DL)in the street design management,the CFD simulation method verified with the wind tunnel experiment was used to analyze the movement process of the transmission diffusion mechanisms of the TRAP under different planning scenarios,the air environment assessment indicators such as Air Exchange Rate(AER),Purification Flow Rate(PFR),,Average Residence Time(ART),and Net Escape Velocity(NEV)were introduced to analyze the variation pattern of diffusion performance of urban streets and reveal the intrinsic correlation mechanism between important design control elements and urban street air pollution.(3)Based on the principle of urban morphology,the Street Consistency Coefficient(SCC)was carried out by using high-density meteorological site data and voronoi diagram,the openness of urban street space was assessed by using Sky Visibility Dactor(SVF)and Average Sspect Ratio(H/D),the diffusion potential assessment model for urban streets was constructed by using the SCC and H/D.The WUCA was selected as the study area to analyze and measure the diffusion potential of natural ventilation in street sacles.It shows that the SCC and H/D-based approaches for assessing the diffusion potential of natural ventilation in urban streets can effectively characterise the overall spatial distribution of the streets’ diffusion potential of natural ventilation at urban scale,which can provide a general reference for planning responses to natural ventilation and air pollution in neighbourhood-scale streets,as well as a reference for planning responses to natural ventilation and air pollution in urban-scale streets.The overall potential of the ventilation of streets in the WUCA presents a gradual upward trend from the city center to the periphery.This study promotes the excavation of traffic-based ventilation paths and the improvement of the overall ventilation network system in the city,and also provides a channel for the conduction of urban-scale ventilation planning and street design.Based on the above results of the analysis and evaluation in urban streets and the pattern of spatio-temporal variation of air pollution in urban streets,a planning response framework for improving natural ventilation and air pollution of urban streets was constructed,and the planning strategies and design recommendations for dealing with air pollution in urban streets were proposed from both urban and neighborhood scales.This study improves the understanding on the variation pattern of t spatio-temporal distribution and the transport diffusion mechanisms of TRAP in urban streets,expands the methodology of urban ventilation assessment and air pollution response.At the same time,this study compensates for the lack of consideration of the climate environment in urban spatial planning and design,and provides effective reference and guidance for scientific urban spatial planning and climate environment improvement and management.