A Study About Beijing's Common Tree Species' PM2.5 Absorption Abilities And Their Mechanisms

It is widely acknowledged that urban forest plants can significantly improve air quality by absorbing airborne particulate matter(PM)and thus safeguard the health of urban residents.However,it is notoriously difficult to accurately assess this ability for urban plants.This study picked Beijing's common urban tree species as subjects and discussed their dust detaining abilities over different scales It measured their dust-detaining ability for particulates like PM2.5 with wash-filter-weigh method based on leaves after natural/simulated rainfalls;the microstructures on leaf surfaces were observed with SEM,and saved images were processed by Fragstats and Matlab to quantitatively analyze these structures,to perceive plants' mechanism of fine particles absorption.The results of this study suggest that1.Plants' dust-detaining ability varies in different seasons,as it is influenced greatly by the climate and density of particulates in the air.When look at plants' detaining abilities for total suspended particulate(TSP),Pinus tabuliformis's performance in all seasons was significantly better than other species.For other species,Cephalotaxus sinensis(5236.66±153.75 mg/m2)and Ulmus pumila(4999.56±96.24 mg/m2)were better in spring;Populus tomentosa(7820.16±649.63 mg/m2)Ailanthus altissima(2818.21±39.40 mg/m2),Ulmus ppuila(2466.05±50.62 mg/m2),Taxus cuspidata(2392.13±141.22 mg/m2),and Metasequoia glyptostroboides(2011.73±3 1.6 mg/m2)performed better in summer;Populus tomentosa(2249.36±1 1 8.78 mg/m2),Koelreuteria paniculata(2230.19±116.09 mg/m2),and Quercus variabilis(2117.25±198.18 mg/m2)got better score in autumn;and Phyllostachys propinqua(6010.32±280.89 mg/m2)had a good dust detaining quantity in winter.The dust detaining ability on PM 2.5 were greatly different among the selected species.In spring it was Pinus tabuliformis(6163.03±630.43 mg/m2),Ulmuspumila(3511.12±198.63 mg/m2),and Cephalotaxus sinensis(1880.1±21 3.3 1 mg/m2)got the best scores,in summer it was Pinus tabuliformis(15325.76±1363.70 mg/m2),Populus tomentosa(3767.79±363.26 mg/m2),Ailanthus altissima(1505.91±1 16.58 mg/m2),Taxus cuspidata(1226.16±1 18.48 mg/m2),and Ulmuspumila(1 105.59± 112.46 mg/m2)performed best;in autumn it was Pinus tabuliformis(1509.22±90.82 mg/m2),Amygdalus triloba(846.17±74.16 mg/m2),Malus micromalus(672.9±66.15 mg/m2),and Populus tomentosa(664.68±49.20 mg/m2)achieved highest detained quantity;in winter it was Pinus tabuliformis(4934.64±212.13 mg/m2),and Phyllostachys propinqua(3532.02±316.8 mg/m2)detained most dust.2.Detained dusts' scour resistance varied greatly among species.For 29mm rainfall,Populus tomentosa recorded the best overall washing rate of 90.74±0.53%for TSP on leave surface,followed by Ailanthus altissima(82.94±0.40%)and Pinus tabuliformis(70.53±1.66%);they also recorded best washing rate for PM2.5,where rates were 90.78±0.74%,88.94±0.74%,65.78±3.34%,respectively.Medium rainfalls could wash off 20.74%-90.74%captured particulates(TSP)off the leaves,for PM>10 and PM2.5 the percentages were 11.92%-89.91%and 10.5%-90.78%,respectively.3.Based on the calculation using unit leaf area quantity of detained dust,leaf area index,lifespan of the leaves,and rainfalls,Beijing's total amount of dust detained by leaves in 2014 was 97k ton,where PM>10 was 52.8k ton,PM10-2.5 was 1.3k ton,and PM2.5 was 43.2k ton.4.The dust detaining ability of tree belt is influenced by many factors.As the experiment results showed,the edges were influenced mostly by changes in the seasons,where particulates were easily captured during spring and autumn,and were hard to be detained during summer due to winds and rainfalls.The plants in the belt that at 20m distant to the edge had a similar absorption rate for PM>10 during spring and summer and an increased absorption during fall,while their PM2.5 absorption rates grew in fall and drop in spring when compared with the summer absorption rates.There were notable differences between species for seasonal changes on PM2.5 absorption.5.From microscopic level:Plants like Tilia tuan,Malus micromalus,Armeniaca sibirica,Parthenocissus quinquefolia,Taipinghua had stripe-like structures on their leaves;plants like Ulmus pumila,Euonymus japonicus,Taxus cuspidata,Magnolia soulangeana,Malus micromalus,Eucommia ulmoides kept trench structures on their leaves.It was known that plants with trench structures on leaves had stronger PM2.5 detaining abilities.With this knowledge and analysis of SEM images by Fragstats,it was implied that leaves with smaller PARA,PAFRAC,PROX indexes and greater ENN index were more capable of detaining PM2.5.In summary,evergreen tree species could detain most TSP and PM2.5 in winter,then spring,then summer/autumn;for PM>10 absorption they could detain most in spring.For deciduous trees,their TSP,PM2.5,PM>10 detaining ability were all likely to be the higher in spring and autumn than in summer.When viewing from plant life-forms,evergreen trees were the best species for absorbing particulates of all sizes.It was also noted that when setting up forest belts,a width of no less than 20m was required for effective dust absorptions.Furthermore,rainfalls could wash off PM>10(coarse particles)easily,while detained quantities of PM2.5(fine particles)on leave surfaces were tend to be stable since such particulates were hard to remove.It was notable that medium rain of 29mm could wash of particulates of all sizes effectively.Study of the surface structures showed that plants with leaves that had surface structure like smaller,regulated patches were more capable of detaining PM2.5.