The Dust Retention Effect Of Landscaping Trees And Their Influence On The Concentration And Component Of Particulate Matter Such As PM_2.5

With the rapid development of the society,environmental pollution especially air pollution is becoming increasingly serious in our life,and it has brought severe damage to both human health and the ecological environment.Therefore,landscaping plants are crucial to this problem.They can retain and adsorb the particulates in the air effectively and thus improve the air quality while beautifying our environment.With Hangzhou,a representative city in East China as the object of study,this paper aims to probe into the influence law of different plant communities?cultivated communities?on the concentrations of PM_2.5 and other particulates and elemental carbon/organic carbon?EC/OC?;compares and analyzes the capability of different landscaping trees to retain particulates of different sizes(TSP,PM10,PM_2.5);applies scanning electron microscopy?SEM?to observe the morphology of leaf surfaces and explores the relationship between the leaf micromorphology and the quality of retained particulates;combines energy dispersive X-ray detector?EDX?and inductively coupled plasma optical emission spectrometer?ICP-OES?to measure the elements of retained particulates and the soil of the sample area so as to analyze the source of the particulates.Here are the main findings:?1?According to the research on the seasonal variation of PM_2.5 concentrations in pure forest of Cinnamomum camphora,pure forest of Koelreuteria bipinnata var.integrifoliola,pure forest of Prunus lannesiana ‘Hatzakura',pure forest of Rhododendron pulchrum and lawn,the concentrations in all the plant communities followed the seasonal ranking: winter > autumn > spring > summer except the pure forest of Cinnamomum camphora whose PM_2.5 concentrations minimized in spring.The daily variation of PM_2.5 concentrations in different sample areas increased after showing the first decreasing trends inall the seasons except spring.Basically,PM_2.5 concentrations reached the maximum at 8:00 a.m.and fell to the minimum in the afternoon or evening.In summer?leafy period?,the PM_2.5 concentration was positively correlated with canopy density,average height and planting density,and there was a stronger correlation between the PM_2.5 concentration and structural features in plant communities in summer than in winter?leafless period?.In winter,pollution source and meteorological factors can exert larger influence on the variation of PM_2.5 concentrations.Studies showed that the PM_2.5 concentration was strongly negatively correlated with temperature and wind speed,and strongly positively correlated with relative humidity.The correlation between the PM_2.5 concentration and meteorological factors followed the ranking: relative humidity > temperature > wind speed.?2?EC concentrations of all the plant communities followed the seasonal sequence: winter >autumn > spring > summer except the pure forest of Prunus lannesiana whose EC concentrations in summer were higher than those in spring and autumn.The ranking of OC concentrations in five sample areas was: winter > autumn > spring > summer.The daily variation of EC and OC concentrations in five plant communities showed two peaks in all seasons except in winter.The correlation coefficients between EC and OC was respectively 0.85 in spring,0.78 in summer,0.81 in autumn,and 0.53 in winter.It can be seen that the coefficients were high in all seasons except in winter,which indicated that the source of EC and OC of PM_2.5 was similar.However,the source in winter was complicated,such as coal burning,biomass burning and vehicle exhaust.The concentration ratio of OC/EC was 2.92 in spring,2.67 in summer,3.11 in autumn and 3.07 in winter.It can be shown that the ratio was over 3 in autumn and in winter,which indicated that there was SOC in the air.?3?Based on the research on the retention ability of 21 species of landscaping trees,it can be concluded that Ternstroemia gymnanthera,Myrica rubra and Rhaphiolepis umbellata had stronger ability to retain TSP while Chimonanthus praecox,Koelreuteria bipinnata var.integrifoliola and Acer palmatum ‘Dissectum' were weaker in spring.In autumn,the ability of Firmiana platanifolia,Pterocarya stenoptera and Myrica rubra to retain TSP was stronger than that of Acer palmatum‘Dissectum',Mahonia fortunei and Cinnamomum camphora.In spring and autumn,the ability of Elaeagnus pungens ‘Aurea',Pterocarya stenoptera,Myrica rubra and Pseudolarix amabilis to retain PM10 and PM_2.5 was stronger than that of Koelreuteria bipinnata var.integrifoliola and Acer palmatum‘Dissectum'.The proportion of the size of the retained particulates that exceeded 10 ?m was higher than50% except those retained by Pseudolarix amabilis,Cinnamomum camphora,Acer palmatum‘Dissectum' in spring and Pterocarya stenoptera in autumn,which indicates that most trees have strong retention ability.In the two seasons,the leaf surface,wax layer and blade had stronger ability to adsorb TSP than PM10 and PM_2.5.The average quality of TSP,PM10 and PM_2.5 retained by arbors was higher than that of shrubs.?4?The microstructures on leaf surfaces exert a great influence on the retention ability.Sinojackia xylocarpa,Firmiana platanifolia,Myrica rubra and Cinnamomum camphora can retain a large quantity of PM_2.5 of high quality.There are multiple crisscrossed branch structures,hollows,wrinkles and grooves on leaves of Pterocarya stenoptera,Celtis sinensis,Acer buergerianum,Elaeagnus pungens‘Aurea',Mahonia fortunei,Hibiscus mutabilis and Deutzia ningpoensis.And these structures can retain a large amount of PM_2.5 in the bottom,which shows that these hollows,wrinkles and grooves on the leaf surfaces are beneficial to PM_2.5 retention.The leaf surfaces of Myrica rubra,Firmiana platanifolia and Rhododendron pulchrum are covered with secretion,and the quality of their retained TSP and PM_2.5 was higher,which indicateds that mucus and grease can contribute to particulate retention.Besides,the different tomentum on the leaf surfaces can also contribute to particulate retention.However,the retention ability of the tomentum varies due to different structures.Relatively speaking,the tomentum can retain fine particulates such as PM_2.5 more easily.?5?Studies found out that there were totally twelve elements in the particulates on both leaf surfaces in spring,and seven of them were overlapped.There were three more elements on the adaxial leaf surfaces than the abaxial leaf surfaces.In autumn,the total elements reached thirty-seven and there were four more elements on the adaxial leaf surfaces than the abaxial leaf surfaces.There were elements in the retained particulates overlapped with those in the soil both in spring and autumn,and there were eight more overlapped elements in autumn than in spring.There was a weak correlation between the elements of retained particulates on the leaf surfaces and soil element content,and the correlation was stronger in spring than in autumn.Studies showed that the elements of the retained particulates in spring were more consistent with those in the soil.Therefore,it can be inferred that the particulates on the leaf surfaces mainly come from the soil.In autumn,the elements of the retained particulates contained Nb,Cl,Br,Pd,Ga,Rb,etc.in addition to some of the soil elements.These elements came from unnatural environment such as building cement dust,soft coal dust,etc.This study monitored and analyzed the PM_2.5 concentrations and carbon fraction variation in different plant communities?cultivated communities?,compared the dust-retention capacity of different tree leaves,observed the micromorphology of leaf surfaces,and measured the elements composition of the retained particulates.The research can provide theoretical basis for the application of fine landscaping plants and plant landscape design in areas with the same climate zone and vegetation forms against the background of increasingly polluted city air.