Study On The Effect Of Leaf Surface Microstructure On The Particulate Matter Retention Capacity Of Plants

Atmospheric particulate matter（PM）is one of the main environmental air pollutants,but it can be retained by plant foliage.This study evaluated the PM retention ability of ten common tree species in China by quantifying the PM mass and number density on their leaves.The microstructure of the leaf surface was observed by using an electron scanning electron microscope（SEM）,and Image J software was employed to collect quantitative data about stoma and groove morphology.Meanwhile,confocal laser scanning microscopy（CLSM）and a contact angle（CA）measuring instrument were applied to characterize the leaf surface roughness and CA respectively so as to conduct a quantitative analysis on the relationship between the leaf surface microstructure and the particle retention capacity.Energy dispersive X-ray spectroscopy（EDS）was employed to analyze the elemental composition and content of the particles on the leaf surface and explore their possible sources.The main results are as follows:（1）There were significant differences in the capacity of retaining different sizes per unit leaf area among the ten tree species.The total mass of retained TSP per unit leaf area in three seasons followed the order:autumn（731.74μg/cm2）>summer（507.46μg/cm2）>spring（270.31μg/cm2）.In the three reasons,the total mass of retained TSP per unit leaf area followed the order:Metasequoia glyptostroboides>Platanus acerifolia>Sapindus mukorossi>Osmanthus fragrans>Amygdalus persica>Ulmus parvifolia>Eucommia ulmoides>Magnolia denudata>Ginkgo biloba>Schima superba.The total mass of retained PM_2.5 per unit leaf area in the three seasons followed the order:summer>autumn>spring.M.Glyptostroboides,O.fragrans and P.acerifolia exhibited high PM₁₀and PM_2.5 retention amount,while the amount of PM₁₀ and PM_2.5retained by M.denudata and S.superba was low.The mass of particulate matter retained on the surface of leaves and in the wax layer of the same plant was also different,and the mass of the retained PM on the surface of most plant leaves was higher than that in the wax layer.In summer and autumn,the leaves with relatively high total PM number densities included P.acerifolia and A.persica,while the PM number densities of S.superba and M.denudata were lower.The PM_2.5 mass retained on the leaves was small,but the PM number density was high.After the PM mass and PM number density retained on the leaves were compared,it was found that the order of the two was not completely consistent.（2）PM retention capacity can be affected by the surface microscopic morphology（eg.,grooves,stomata,folds,wax）,roughness,and wettability of leaves.Leaf surfaces with dense and narrow grooves,strip-like projections,high roughness and wettability had strong retention particle capacity.There was a significant positive correlation between the adaxial surface roughness and the total PM number density of the retained particles.The adaxial CA was significantly negatively correlated with the PM_2.5 and TSP mass retained by the leaves.（3）There were significant differences in the amount of the retained particulate matter per plant among different plants.P.acerifolia and U.parvifolia exhibited high TSP,PM₁₀ and PM_2.5 retention amount per plant,while the retention capacity of E.ulmoides M.denudata and A.persica was low.The ability of a single plant to retain particulate matter is not only related to the amount of particulate matter retained per unit leaf area of the leaf,but also to the total leaf area of a single plant,crown structure and leaf density.（4）There were nineteen and twenty-four elements on the leaf surface in summer and autumn respectively.The main sources of particulate elements are traffic pollution and soil dust.The research can provide theoretical references for the selection of plants with a strong PM retention ability for the green design of urban gardens.