Study On The Influence Of Urban Road Green Space On PM_2.5 Diffusion

The atmospheric PM_2.5 pollution problem had attracted much attention,how to effectively control the harm of PM_2.5 to urban residents has been a research hotspot.As an important measure to alleviate atmospheric pollution,the regularity and influencing factors of atmospheric PM_2.5 concentration in urban road green spaces was studied to reduce the harm of PM_2.5 pollution to urban residents,and provide theoretical and practical significance for urban green space system planning.In this study,through the combination of online collection and field surveys,four main green roads with main trees and shrubs and hedges in Nanjing were selected as research objects.Three monitoring points were set on both sides of the road and in non-green space.Vertical monitoring heights of 0.3 m,1.5 m,3 m,and 5 m were set for each monitoring point,and horizontal monitoring positions for driveways,non-motored lanes,and sidewalks were monitored.Atmospheric PM_2.5 concentrations and meteorological elements were studied to investigate the change regularity.Pearson correlation and boosted regression tree?BRT?were used to analyze the influence of meteorological elements on PM_2.5 concentration.A stepwise regression method was used to construct a PM_2.5 concentration prediction model.The urban microclimate simulation model ENVI-met was used to simulate the change and distribution of atmospheric PM_2.5 concentration with the aspect ratio?the ratio of building height to road width,H/W?of 0.5,1.0,and 1.8,and in the four kinds of plant structures?without vegetation,trees+hedges,trees,hedgerows?so as to explore its mechanism of the change.The main conclusions were as follows:?1?The diurnal variation of atmospheric PM_2.5 concentration in the green spaces of the four typical urban roads basically showed a"double-peak single-valley"type,falling first and then rising,which meant PM_2.5 concentration were higher in morning and night than during the daytime,the peak appeared at 8:00-9:00 in the morning and 18:00-19:00 in the evening.,the valley appeared from 11:00 to 14:00.?2?The concentration of atmospheric PM_2.5 in the road green spaces showed a fluctuating upward trend in the vertical direction?0.3 m-5 m?.Among them,the PM_2.5 concentration at the1.5 m of the green spaces was always the lowest;in addition,the ratio of PM_2.5 to PM₁₀ also showed an increasing trend with height..?3?The concentration of atmospheric PM_2.5 in the road green spaces showed higher in blank sapces than in green spaces in the horizontal direction;among them,the concentration of atmospheric PM_2.5 in the blank spaces was sidewalk>non-motored lane>driveway,and there is a hedgerow road.The concentration showed a tendency of driveway>non-motored lane>sidewalk.The hedgerows had a certain retarding effect on the atmospheric PM_2.5 concentration in the road.?4?The Pearson correlation test showed that there was a significant correlation between meteorological factors and PM_2.5 concentration:a significant positive correlation with relative humidity,and a significant negative correlation with temperature,wind speed,solar radiation intensity,atmospheric pressure,and mixed layer depth.?5?The analysis results of boosted regression tree?BRT?indicated that the contribution rate of meteorological elements to the change of atmospheric PM_2.5 concentration in the road green spaces was:atmospheric pressure?65.9%?>relative humidity?17.5%?>solar radiation intensity?6.5%?>wind speed?5.5%?>temperature?3.2%?>mixed layer depth?1.5%?.?6?Taking atmospheric PM_2.5 concentration as the dependent variable and meteorological elements as independent variables,a stepwise regression method was used to establish a multiple linear regression model based on meteorological elements:,the model had good effect?R²=0.77?,was atmospheric PM_2.5 concentration,w was the wind speed,t was the temperature,h was the relative humidity,and p was the atmospheric pressure.?7?The results of ENVI-met numerical simulation showed that the atmospheric concentration of PM_2.5 with low aspect ratio was lower,and gradually increased with the decrease of road width.The highest concentration occured near the pollution source.The farther away from the pollution source,the lower the atmospheric PM_2.5 concentration.Except for the road structure with an aspect ratio?H/W?of 1,the concentration of PM_2.5 in the green spaces was higher than that in the blank space,contrary to the previously observed results.In the road structure with H/W=0.5,when the wind direction was perpendicular to the road,atmospheric PM_2.5 concentration showed trees>hedges>trees+hedges>no vegetation,when the wind direction was parallel to the road,there is hedge>tree>tree+hedge>no vegetation;In the road structure with H/W=1,atmospheric PM_2.5 concentration presented tree>hedge>no vegetation>tree+hedge;In the road structure with H/W=1.8,atmospheric PM_2.5 concentrations presented tree>tree+hedge>hedge>no vegetation.It could be concluded that the road green plant?tree,hedge?would hinder the diffusion of atmospheric PM_2.5 proliferation,causing increase of the atmospheric PM_2.5 concentration in the road green spaces.?8?The simulation results showed that when the wind direction was perpendicular to the road,the atmospheric PM_2.5 concentration on the leeward side of the road was higher than that on the windward side,and the arrangement of vegetation with tree showed a consistent trend of increasing atmospheric PM_2.5 concentration in different road structures.In the hedge-only configuration,atmospheric PM_2.5 concentration showed near-field measurement>leeward>windward,indicating that hedge had a certain retarding effect on atmospheric PM_2.5,which verified the aforementioned measured results.?9?The simulation results showed that when the wind direction was parallel to the road,the change of atmospheric PM_2.5 concentration in different vegetation configurations all showed near-field measurement>south side of the road>north side of the road.