| Air,necessary to our lives,is of great demand nowadays.However,along with the fast pace of urbanisation,a large number of particles released from industrial manufacturing,transportation and other emission resources lead to the further deterioration of air,causing great harm to environment and human health,and making city air pollution increasingly prominent.According to the researches and reports related,fine particles,the primary pollutant of atmospheric pollution in the city,will lead to reduced visibility,haze and other adverse weather.It has made a great threat to production,construction and transportation,it can also cause developmental disease of human beings and damages of almost all kinds of physical systems,and even lead to death.Urban forest,different from common primeval forests,its ecological effect is closely related to human life and has great ecological values.The city forests can not only fix carbon,release oxygen,block wind,fix sand,reduce noise,but also have another important function which is to reduce and alleviate the air pollutions.It is generally believed that the dry deposition process is the main mechanism for plants to purify the atmosphere.Particles fall to the surface of the leaves,which can cause the adsorption of particles on the leaf surface due to their own properties and the nature of the leaves themselves.Due to the current domestic and foreign researches mainly focusing on the macro-scale city forests on air purification efficiency,many researches centre on the atmospheric pollutants purification quantity due to different component of tree species,different scale of patches,so that the particulate matters qualities and quantities of purification in the future can be calculated,and its ecological value evaluated.But the studies on the micro-scale mechanism of how particles are adsorbed on the surface of the leaves is still in its infancy.The present researches mainly centre on the leaf surface by electron microscopy observation,and the possible association between plant leaves and leaf morphology of dust retention ability are put forward.But there are lacking in scientific specific quantitative indicators,and other related indicators for the study samples are not rigorous.Therefore,it is urgent to establish a systematic model between leaf properties and its mechanism of dust-retenting,so as to study the effects of different indexes on the dust retention abilities,and provide a theoretical basis for the future selection of greening tree species.Based on the existing research made previously,19 kinds of typical greening trees species were selected in Shanghai,and through the use of aerosol generator?QRJZFSQ-I?,leaves PM2.5 dry deposition velocity were measured seasonally from the summer of 2015 to the spring of 2016,and the corresponding seasonal leaf wax content,leaf surface free energy and its polar component and dispersion component,leaf surface roughness indexes were also determined,At last,the relationship of the leaf dry deposition velocities of PM2.5 and the various factors were respectively studied by one-dimensional linear regression and multiple linear regression,Moreover,the interaction of various indicators were also conducted,which can help filter out the main factors affecting the effectiveness of dust-retenting abilities of plants.The main contents and results are as follows:?1?Dry deposition velocity of plant leaf PM2.5 in different seasonsThe leaves for test were put into in aerosol generator?QRJZFSQ-I?,and then through a certain wind speed the particles on the surface of leaves were blown into the air,thereby the particle concentration was measured,and calculated,then was divided by the local atmospheric background concentration,the dry deposition velocity on the leaves of PM2.5 can at last be calculate.The results showed that the dry deposition velocity of PM2.5 in summer was the lowest in one year,while the one of autumn was slightly higher than the one in summer,and the highest PM2.5 dry deposition velocity appeared in winter and spring.?2?Study on the PM2.5dry deposition velocity and leaf wax content of leavesThrough extraction of dissolving leaf wax in chloroform,wax content could be obtained in per unit mass of leaves?mg/g?.The study found that the wax contents of conifers were generally higher than the ones of broad-leaf plants.Through one-dimensional linear regression,it was found that the dry deposition velocity of PM2.5 on plant leaves was significantly negatively correlated with evergreen and coniferous plants,and the correlation coefficients relatively were-0.211 and-0.291.?3?Dry deposition velocity and surface free energy and component of PM2.5 in plant leavesThe leaf surface free energy was measured by using the instrument of optical contact angle measurement,the contact angles of water?polar?and diiodomethanein?non-polar?formed on leaves were measured and photographed,and the leaf surface free energy and its components were calculated through the corresponding calculation software.It was found that the proportion of dispersion component in free energy of leaf surface is larger than polar one.The result of one-dimensional linear regression showed that the dispersion component and PM2.5 dry deposition velocities,in addition to deciduous plants,were significant correlated to the rest of category of plants,so that the dust retention ability of plants is greatly influenced by the force of dispersion.Because of the universal existence of dispersion force,it may become a major factor affecting the dust retention ability.?4?Dry deposition velocity and surface roughness of PM2.5 on plant leavesThrough the use of bio atomic force microscopy,the leaf surface roughness?Ra mainly?could be scanned and two-dimensional,three-dimensional related images obtained.Due to the fact that surface roughness of the sample can be varied with the perspective of different scanning areas,two different perspectives of 5?m×5?m and 20?m×20?m were selected for research.It was found that the leaf surface roughness conifer varied within the range of 20 nm40 nm despite of the scanning areas,while the roughness of broad-leaved plants increased with the expansion of scanning area.Though one-dimensional linear regression,it was found that the PM2.5dry deposition velocity has a significant positive correlation with the leaf roughness of leaves within 5?m×5?m scanning area,and the correlation coefficient is 0.466.Within the 20?m×20?m scanning area,there is a significant negative correlation between PM2.5 dry deposition velocity and leaf roughness of broad-leaved plants,and the correlation coefficient of is-0.466.?5?Dry deposition velocity of PM2.5 on leaf and leaf comprehensive indexIt was found that PM2.5 dry deposition velocities of leaves have no obvious correlation with leaf wax content;the correlation level of PM2.5 dry deposition velocity and leaf surface free energy is 0.009,which is less than0.050 and 0.01,so they have a greatly significant correlation,and the correlation coefficient is 0.224;the correlation level of PM2.5 dry deposition velocity and the dispersion component of leaf surface free energy is 0.011,which is less than 0.050 but larger than 0.01,so they have a significant correlation,and the correlation coefficient is 0.217;the correlation level of PM2.5 dry deposition velocity and leaf surface roughness is 0.004,which is less than 0.050 and 0.010,so they have a significant correlation,and the correlation coefficient is-0.200. |
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