Abstract:

Objective: The aim of this study is to quantify the particulate matters (PM) retained per unit leaf area by different tree species and its spatio-temporal variation in Guiyang City, in order to provide a scientific basis for the rational selection of landscaping tree species with higher capacity of retaining PMs. Method: Fourteen commonly used landscaping tree species growing at 4 sites, urban square, urban park, suburban green area, and suburban forest, in Guiyang City each with different levels of pollution. Leaf samples were collected regularly from 3 trees of each species (some species are not present at certain sites) at the same time. The aerosol re-suspension method based on the wind erosion principle was applied to measure the retained PM quantity per unit leaf area (M). The micro-morphological and structural characteristics (stomata density, stoma size, stomata opening size, roughness, vein density, groove width, wax coverage index, epidermal hair length, epidermal hair density, etc.) of leaves were quantitatively analyzed using image analysis software. The influence of leaf morphology and structure characteristics on the leaf retained PM was analyzed by means of path analysis. Result: It showed that the site means of M_TSP of all studied tree species varied within a range of 1.56-11.14 μg·cm^-2, with non-significant difference between trees and shrubs, but a significant difference among tree species. The tree species of Loropetalum chinense var. rubrum showed the highest quantity of PM retention, followed by Cedrus deodara, Osmanthus fragrans, Rhododendron simsii, Bambusa multiplex, Ligustrum lucidum, and Magnolia denudata, while other tree species showed a weak capacity of PM retention. Among the structural morphological characteristics, the leaf surface roughness, the epidermal hair density, and epidermal hair length were the major factors affecting PM retention compared to other ones. Therefore, the leaves with higher epidermal hair density, bigger epidermal hair length, and rougher leaf surface presented a higher PM retention. The weight ratios of different PMs retained by leaves were not the same as that in the ambient air. The weight ratio of retained coarse and large PMs was 97.36%, while it was 80.29% in the ambient air, indicating that leaves tend to retain more PMs with bigger diameters (such as PM_2.5-100) but less fine PMs (such as PM_1-2.5 and PM₁). Most tree species showed a higher PM retention in winter and spring than in summer and autumn. For the same tree species, the PM retention in urban sites was higher than that in suburban sites. Among the 2 urban sites with a high air PM density, the PM retention quantity was higher in urban park than in urban square, probably due to the more concentrated distribution of trees in the urban parks which leads to a more favorable microclimate to capture more PM on leaf surface, i.e., an agglomeration effect. Conclusion: A significant difference in the PM retaining capacity exists among tree species. The main factors affecting the leaf PM retaining capacity are the leaf surface roughness and the epidermal hair density. The leaves tend to retain more large and coarse PM but less fine and ultra fine PM. The quantity of retained PM by leaves at more polluted sites is higher than that at less polluted sites. The micro-morphological and structural characteristics of leaves can be used as evaluation indicators for selecting tree species with high PM retention capacity. L. chinense var. rubrum, C. deodara, and O. fragrans can be used in urban greening for mitigating the PM pollution because of their high PM retention capacity.

Key words: urban landscaping tree species, quantity of PM retention, micro structure of leaf morphology