Study On The Effects Of Greening Form On The Diffusion Of Vehicle Pollutant In Urban Street By Numerical Simulation

In order to clarify the effect of street canyon greening form on the vehicle pollutant diffusion,and guide the design of street canyon planting,we choose a large central city Wuhan,which is densely populated and heavily polluted by traffic,to carry out this study.We took the classified street canyon with dense population in central city as the research object,conducted street research and collected basic data.According to the difference angle to the leading wind direction,we selected 3 typical street canyon directions,orthogonal,oblique and parallel directions.We used PM10 as a line source pollutant,which has a significant impact on human health in vehicle exhaust.Using the urban microclimate software ENVI-met,we simulated 8 spatial arrangement scenarios of street green belts.The PM10 diffusion law of street canyon is analyzed.The rational spatial arrangement of green belts in the planning and design of street canyon is discussed.Then we selected four plant characteristic factors including leaf area density,crown height,crown shape and hedge height.The relationship between the change of characteristic factors and the PM10 diffusion of vehicle emission is simulated.To seek the characteristic factor of PM10 diffusion,in order to alleviate the PM10 pollution of street canyon footpath by controlling plant characteristic factor in the planting design.Comparing the simulation results of spatial arrangement of green belts in different street canyon directions,the following findings were found.(1)In the orthogonal direction and oblique direction,PM10 accumulated on the leeward side.The more the number of green belts,the more serious the accumulation.(2)In the oblique direction,there were obvious advantages and disadvantages in the same arrangement of green belts.When a single green belt was arranged,the tree planted on the windward side was the best,planted in the central area was the second,planted on the leeward side was the worst.When two green belts were arranged,planting trees was better than planting trees and hedges at the same time.When three green belts were arranged,in the middle of the street,planting trees were better than planting small trees and hedges at the same time.(3)In the Parallel direction,the pollution was relatively heavy when three green belts were planted.The second was that the trees planted on both sides.While the pollution was lighter when trees planted on the windward,planted on the leeward,trees and hedge planted on both sides,and trees planted in the central.The simulation results of plant characteristic factors in different directions were compared,and the following findings were obtained.(1)Along with the increase of leaf area density,the pollution of footpath was aggravated.The PM10 concentration in the orthogonal direction increased most rapidly.And he concentration of LAD tended to be slow after the LAD increased to 3m~2/m~3.(2)In the oblique and parallel direction of the street canyon,the average concentration of footpaths on both sides decreased with the increasing of crown height.When the crown height reached at 6m,the concentration changing on both sides tended to be gentle,and the PM10 concentration was no longer decreasing.(3)In the orthogonal direction,when the crown was in the shape of tower and cylindrical,the PM10 concentration of footpath was the highest.When the crown was in the shape of spherical,the PM10 concentration of footpath was the lowest.In the oblique and parallel direction of the street canyon,when the crown was in the shape of umbrella and tower.When the crown was in the shape of Obovate,the PM10 concentration of footpath was the lowest.(4)With the increasing of the hedge height,footpath PM10concentration decreased obviously only in the parallel direction street canyon.When the hedge height reached 0.9m,the increasing of PM10 concentration tended to be gentle.