Spatial Dispersion And Regulation Of Airborne Particulate Matter Under Tree Cover In Urban Street Canyon

The particulates emitted by vehicles in urban street canyons are one of the main sources of urban air pollution.Due to the low emission level of motor vehicles,the atmospheric particulates produced by vehicle exhaust can easily cause physical and mental health damage to pedestrians and people near street buildings.Numerous research results have shown that densely planted street trees on both sides of the street can adsorb particulate matter,have significant dust retention and pollution reduction effect,and can effectively improve the microclimate.However,recent studies have shown that the spread of particulate matter is blocked due to the canopy of roadside trees in the street canyon,which causes the accumulation of pollutants in the street canyon.The particle pollution of street canyons is not only related to the intensity of pollution sources,the characteristics of street canyons and meteorological factors,but also related to the characteristics of roadside tree coverage.However,it is still unclear how the coverage of roadside trees in street canyons affects the spatial dispersion of particulate matter,and how to adjust roadside trees' configuration in street canyons to alleviate particulate pollution in neighborhoods.In view of the uncertainty of the impact of roadside trees on the dispersion of particulate matter in street canyons,this study intends to analyze the daily variation and concentration of particulate matters under different roadside tree characteristics(canopy density,growth type)and street canyon characteristics(aspect ratio,orientation).The measurement is combined with the simulation of the ENVI-met model,to explores the response and regulation mechanism of the spatial dispersion of particulate matter in the street canyon to the coverage of roadside trees,and proposes a roadside tree configuration strategy for urban blocks based on particulate matter dispersion,in order to provide a reference for urban landscape planning.The main conclusions of this study are as follows:(1)The measurement of particulate matter concentration in the street canyon of low canopy density(? 35%),medium canopy density(35%-70%),and high canopy density(>70%)of roadside trees cover shows that,in the conditions of calm and breeze wind,the concentration of particulate matter in the street canyon under the coverage of high canopy density is the highest,leading to the accumulation of particulate pollutants in the street canyon;both low canopy density and medium canopy density are conducive to the dispersion of particulate matter in the street canyon.When the canopy density of roadside trees canopy is 30%-36%,the PM10 and TSP concentrations have the largest decrease compared with the control without canopy cover,decreasing by26.75% and 27.49% respectively;when the canopy density is 24%-36%,The PM2.5concentration showed the largest decrease(a decrease of 7.44%),demonstrate that the effect of particulate matter dispersion under low canopy density is the best.The ENVImet model simulation of particulate matter concentration under different canopy densities(9 gradients from 0% to 90%)further proves that roadside tree canopy density below 60% can reduce the concentration of PM2.5 and PM10 in street canyons,and canopy density exceeds 60% will lead to the expansion of high-concentration areas in the street canyon.As the leaf area density(LAD)of the sidewalk increases in the range of 0.5 m2 m-3 to 3 m2 m-3,the maximum and minimum concentration differences of PM2.5 and PM10 in the street canyon between the pedestrian layer,the bottom of the canopy and the upper of the canopy increase,and the distribution of particulate matter become more uneven.(2)The seasonal variation of the concentration and size of the particulate matter under different growth types of roadside trees shows that under calm and breezy conditions with the same canopy density level,the particulate matter(PM1,PM2.5,PM4,PM7,PM10,TSP)in the street canyon covered by deciduous broad-leaved trees are all higher than the evergreen broad-leaved tree in summer and winter,indicating that evergreen broad-leaved trees are more conducive to the dispersion of particulate matter in street canyons than deciduous broad-leaved trees.However,in summer,the small and medium-sized particles(PM1,PM2.5)are the main particle pollutant in street canyons covered by evergreen broad-leaved trees,and the larger particles(PM4,PM7,PM10,TSP)are the main particulate pollutant in street canyons covered by deciduous broad-leaved trees.And larger particles(PM4,PM7,PM10,TSP)with a relatively high concentration is the main particle pollutant in winter,whether street canyons is covered by evergreen or deciduous broad-leaved trees.The particulate matter in street canyons shows a seasonal trend of low concentration in summer and high concentration in winter,and the differences in the concentration of particulate matter in winter in street canyons covered by evergreen and deciduous broad-leaved trees have been reduced.(3)The measurement and the ENVI-met simulation of particle concentration under different street aspect ratios show that the concentration of various particles in deep street canyons with aspect ratio H/W ? 2 is high.The measured particle concentrations in summer and winter are the highest in street canyons with H/W ? 2 and lower in street canyons with H/W ? 0.5 or H/W ? 1.The more open the street canyon(the lower the aspect ratio),the lower the concentration of particulate matter,the more conducive to the dispersion of smaller particles such as PM1 and PM2.5;while the regular street canyon H/W ? 1 is conducive to large particles such as PM10 and TSP,it can be seen that regular street canyon(H/W ? 1)is the ideal aspect ratio for the dispersion of particles.The simulation of particle concentration in street canyons with different street aspect ratios shows that as H/W increases in the range of 0.5 to 4,the difference in particle concentration between roadside tree coverage and open spaces decreases;In street canyons with H/W = 4,50% canopy density of roadside tree coverage will only lead to increasing of the concentration of particulate matter(PM2.5,PM10)in the street canyon.(4)The vertical direction experiment of particulate matter at five heights of the street as: car exhaust emission source(0.3m),pedestrian layer(1.5m),under the canopy(6m),middle of the canopy(9m),and top of the canopy(12m)shows that the particle concentration of each particle diameter(PM1,PM2.5,PM4,PM7,PM10,TSP)all showed a trend of obvious decrease with the increase of height.There is a significant linear negative correlation between the particle concentration of each particle size and the height;the larger the particle size,the greater the difference in particle concentration between different heights,and there is a strong negative correlation between the total suspended particulates(TSP)and the height.The results of the ENVI-met model further confirmed that the concentration of particulate matter decreased significantly with the increase in height,and the difference in the concentration of particulate matter between different canopy density levels also decreased with the increase in height.(5)The measured result of particle concentration on both sides of the street canyon and the ENVI-met simulation value shows that the particle concentration in the leeward area of the street canyon in each aspect ratio is basically higher than that in the windward area.However,in the nearly regular street canyon with an aspect ratio of 0.5to 2,the concentration of large-particle particles PM7,PM10,and TSP on the leeward side in summer is significantly higher than that on the windward side,and the concentration of small particles of PM1,PM2.5 and PM4 on the leeward side is mostly higher than that on the windward side.The ENVI-met simulation of PM2.5 and PM10 concentrations in the east-west and north-south facing street canyons show that the leeward high concentration area and the upwind low concentration area are approximately symmetrically distributed in the two orientations,with only slight differences in the concentration value of particulate matter between the two orientations.(6)Correlation analysis between particulate matter concentration and environmental factors in street canyons showed that relative humidity was significantly positively correlated with particulate matter dispersion in the street canyons under the conditions of calm wind and breeze,and was the dominant environmental factor affecting particulate matter concentration.The ENVI-met model simulation under different street tree canopy density showed that,with the increase of street tree canopy density,the cooling area in the street canyon expanded,the difference between the average wind speed and the maximum wind speed in the pedestrian layer decreases and the relative humidity of the atmosphere increases.The wind speed of the street canyon with tree canopy density above 50% is lower than that of the street canyon without a tree canopy.When the roadside tree density is among 60% ~ 90%,the deep cooling area with a temperature difference of-2.5°C appeared in the pedestrian layer and the whole street canyon.A high humidification area(relative humidity increment > 10%)appeared in the street canyon when the roadside tree canopy density was 70% ~ 90%.The humidification zone and low wind speed zone were concentrated around the tree canopy,and their changes weakened with the increase of the distance from the canopy.In both east-west and north-south street canyons,the maximum wind speed difference is the highest in the deep street canyon(H/W = 4),and the wind speed in most areas of street canyons with roadside trees is lower than that of street canyons without roadside trees.It can be seen that the effect of street trees on the wind preventing the dispersion of particulate matter is particularly obvious in the deep street canyons with a high aspect ratio.The cooling effect of street trees and the area of the humidification zone decreased with the increase of aspect ratio.(7)Under the conditions of calm wind and light breeze,the sparse(? 35%)and medium(35% ~ 70%)canopy density indicator can be used as the ideal range of roadside tree configuration in the regular street canyon,and the canopy density in the street canyon with high pollution should be controlled at about 35%.The hindrance of dense canopy overlaps on particle dispersion can be improved by crown shrinkage and thinning pruning.The dust-catching effect of the combination of evergreen and deciduous broad-leaved street trees belt is better than green belt that of single roadside trees growing type.It is suggested to plant small evergreen broad-leaved trees as the main tree and large deciduous broad-leaved trees as the interval planting configuration.The deep street canyon environment that impedes the dispersion of particulate matter should be avoided in street planning.Areas with high concentration of small particles(PM1,PM2.5)should design open streets,increase building spacing or limit high-density buildings;in street areas with high concentration of large particles(PM10,TSP),the regular street canyon of aspect ratio H/W ? 1.0 can be used and airflow of wind direction vertical with canyon axis can be introduced by adjusting the street building vents.When new kindergartens,schools and hospitals are built,they should be located in the windward area to reduce the pollution exposure of vulnerable people in the leeward area of urban blocks.Vegetation with strong dust retention ability should be planted on the leeward roadside to absorb large particle size particles.