Ultrasonic Based Investigation On Particle Size Distribution And Retention Efficiency Of Particulate Matters Retained On Tree Leaves Of 5 Species Of Native Trees In Beijing

The pollution of urban particulates is increasingly serious in our country,forestry can regulate and eliminate atmospheric particulates in a variety of ways,so it has attracted widespread attention in recent years to mitigate the pollution of atmospheric particulates by means of urban trees.Quantitatively comprehensive assessment of the amount of PM retained on plant leaves is very important for accurately assessing the air purification ability of urban forests in different scales(leaf,individual tree,and stand)and screening tree species with higher PM retention capability.In this paper,we took five widely planted greening tree species in Beijing,as the research objects of study.First of all,we optimized the quantitative determination techniques for different trees,and then used this technology to collecte leaves of these trees that experiencinged two different dust retention periods as the test materials to carry out the relative research.The research content mainly included:(1)Rapid and complete collection of the atmospheric particles retained on leaves is the premise of precisely evaluating the retention of tree leaves to atmospheric particles.(2)to compare and analyze the particle size distribution and its dynamics of particles retained on leaves of different tree species and its dynamic change of particles retained on leaves of the five tree species.(3)to estimate the retention capacitefficiencyyefficiency of different tree species in retaining different-sized particles on leaf and individual tree scales.The results showed that:(1)under ultrasonic powers of 200,300,400 and 500 W,the best ultrasonic cleaning time for Ginkgo biloba was 5,5,5 and 3 min,respectively,that for Pinus tabulaeformis was20 min for each.The optimal cleaning procedure was demonstrated to be "soak&stirring-scrubing-ultrasonic power of 500 W(3 min for Ginkgo biloba leaves/5 min for Pinus tabulaeformis leaves)"in our laboratory.After routine cleaning procedures,further cleaning with ultrasonic under power of 500 W could make the leaves more clean,and washed off at the maximum about 50%of the particles retained on leaves.Then we get two concepts:"Easily removable particulate matter(ERP),in this study,PM on leaf surface that could be eluted by soak and stirring was defined as ERP,which was similar to that eluted by natural rainfall.The retention capacity of plants for ERP is a renewable PM retention capacity under natural conditions."Difficultly removable particulate matter(DRP)" in this study,PM eluted by srubing and ultrasonic cleaning from leaf surface was defined as DRP,which was the kind of PM that was tightly retained on leaf surface and difficult eluted by natural rainfall and wind.Therefore,the retention capacity of plants for DRP was difficultly renewable under natural conditions.(2)The size distribution of particulate matters was almost the same after experiencing 2 dust duration.Salix matsudana,Sophora japonica and Sabina chinensis showed bimodal curves,whereas the particulate matters retained on Ginkgo biloba and Pinus tabulaeformis exhibited unimodal size distribution in Dadu city ruins park.However,the size distribution of particulate matters was not the same for the same tree in two areas.Pinus tabulaeformis showed bimodal curves in Dadu city ruins park,whereas the particulate matters retained on Pinus tabulaeformis exhibited unimodal size distribution in Zhiwuyuan area.Thus,the characteristic of PM distribution on leaves may be have connection with the various kinds of pollution source and climate at sampling location(3)The ERF is of 20%to 50%of the total DRF and the total retention efficiency of small size particles in this study is generally higher than other studies.Among the five investigated greening tree species,Ginkgo biloba,Pinus tabulaeformis,Sophora japonica,Salix matsudana and Sabina chinensis had higher efficiency in retaining PM10 than previous studies(134.87?106.14?346.27?415.23 and 344.41 mg/m2·d-1,respectively),wheras Sophora japonica and Salix matsudana had higher efficiency in retaining PM2.5 than previous studies(197.56 and 172.87 mg/m2·d-1,respectively).(4)Based on the analysis of ERF and DRF,this paper provided suggestions on high dust-tolerant tree species in different polluted areas.For Xizhimen traffic pollution control point,the car exhaust gas is the main source of air pollution,in this study,Salix matsudana,Sophora japonica and Sabina chinensis showed better comprehensive dust retention capacity.For the botanical garden of such suburban environmental evaluation point,which is less affected by the exhaust,Pinus tabulaeformis,Sophora japonica and Sabina chinensis showed better comprehensive dust retention capacity.