[Impacts of Leaf Surface Micromorphology Variation on the Ability to Capture Particulate Matter]

As the most important carrier of atmospheric particles captured by plants, the differences in micromorphology characteristics and leaf roughness are important determinants of particle capture capacity. Leaf micromorphology usually changes with growth (internal factor), and with environmental pollution intensity (external factor). The existing dust-spray simulation was always short; however, the leaf micromorphology changes had a clear delayed response, and therefore its results could not reflect the micromorphology changes caused by internal and external factors that influence the particulate capture capacity of leaves. In the present study, new and old leaves were used to study leaf growth, and different pollution source conditions were selected to study pollution intensity under natural conditions, to analyze the changes in leaf surface micromorphology and their impacts on particulate capture capacity. It was found that the amounts of TSP, PM₁₀, PM_2.5, and PM₁ on the old leaves of three evergreen trees (Taxus cuspidata var., Platycladus orientalis, and Pinus tabuliformis) were higher than those of the new leaves, and the amounts of the particles with respect to the old leaves increased with leaf growth. Moreover, there were significant differences between the new and old leaves regarding the captured amount of different-sized particles. The increase in needle roughness (Rq) of the three evergreen trees, caused by growth, was the main factor that led to an increase in particle capture capacity for old leaves. The TSP and PM₁₀ captured amounts of P. orientalis, P. tabuliformis, Sophora japonica, Populus tomentosa, and Ginkgo biloba were higher in the heavily polluted area than in the clean area. The amounts of PM_2.5 and PM₁ captured by P. tabuliformis, G. biloba, and P. orientalis in the heavily polluted area were higher than those in the clean area; however, the amounts of PM_2.5 and PM₁ captured by S. japonica and P. tomentosa in the clean area were higher than those in the heavily polluted area. Pollution intensity very significantly affected the capture capacity of TSP, PM₁₀, and PM_2.5 by leaves, as well as significantly affecting the capture capacity of PM₁. This was mainly caused by the leaf micromorphology changes found in the heavily polluted area, such as stomatal index decrease, waxy layer degradation, more irregular surface texture and boundaries of the epidermal cells, and longer and hardened trichomes. These changes caused the Rq values to be generally higher in the heavily polluted area than in the clean area, and the roughness of the abaxial surface increased more notably than that of the adaxial surface. These results will provide data support for further revealing the driving factors of particulate matter capture capacity of leaves and proposing more scientific urban forest management measures to improve their particulate matter removal function.