Abstract
We evaluated surface-based analysis for assessing the possible relationship between the microstructural properties and particulate matter (i.e., two size fractions of PM2.5 and PM10) adsorption efficiencies of their leaf surfaces on ten woody species. We focused on the effect of PM adsorption capacity between micro-morphological features on leaf surfaces using a scanning electron microscope and a non-contact surface profiler as an example. The species with higher adsorption of PM10 on leaf surfaces were Korean boxwood (Buxus koreana Nakai ex Chung & al.) and evergreen spindle (Euonymus japonicus Thunb.), followed by yulan magnolia (Magnolia denudata Desr.), Japanese yew (Taxus cuspidata Siebold & Zucc.), Japanese horse chestnut (Aesculus turbinata Blume), retusa fringetree (Chionanthus retusus Lindl. & Paxton), maidenhair tree (Ginkgo biloba L.), and royal azalea (Rhododendron schlippenbachii Maxim.). There was a higher capacity for the adsorption of PM2.5 on the leaf surfaces of B. koreana and T. cuspidata, followed by A. turbinata, C. retusus, E. japonicus, G. biloba, and M. denudata. In wax layer tests, T. cuspidata, A. turbinata, R. schlippenbachii, and C. retusus showed a statistically higher PM2.5 capturing capacity than the other species. Different types of trichomes were distributed on the adaxial and abaxial leaves of A. turbinata, C. retusus, M. denudata, pagoda tree (Styphnolobium japonicum (L.) Schott), B. koreana, and R. schlippenbachii; however, these trichomes were absent on both sides of the leaves of G. biloba, tuliptree (Liriodendron tulipifera L.), E. japonicus, and T. cuspidata. Importantly, leaf surfaces of G. biloba and S. japonicum with dense or thick epicuticular leaf waxes and deeper roughness revealed lower PM adsorption. Based on the overall performance of airborne PM capture efficiency, evergreen species such as B. koreana, T. cuspidata, and E. japonicus showed the best results, whereas S. japonicum and L. tulipifera had the lowest capture. In particular, evergreen shrub species showed higher PM2.5 depositions inside the inner wall of stomata or the periphery of guard cells. Therefore, in leaf microstructural factors, stomatal size may be related to notably high PM2.5 holding capacities on leaf surfaces, but stomatal density, trichome density, and roughness had a limited effect on PM adsorption. Finally, our findings indicate that surface-based microstructures are necessarily not a correlation for corresponding estimates with leaf PM adsorption.
Highlights
Particulate matter (PM) is an air pollutant in urban and industrial areas and PM fractions with a diameter of 10 μm or less often exceed air quality standards and cause serious concerns on ecosystems and humans including a wide range of respiratory and vascular diseases [1,2,3,4]
Our results showed that there were substantial differences among species for the PM10 amount encapsulated in leaf wax layers in the following order (Figure 3): Taxus cuspidata (8.4 mg/m2 ), Aesculus turbinata (6.0 mg/m2 ), Euonymus japonicus (5.7 mg/m2 ), Rhododendron schlippenbachii (5.1 mg/m2 ), Chionanthus retusus (3.7 mg/m2 ), Buxus koreana (3.6 mg/m2 ), Ginkgo biloba (3.2 mg/m2 ), Styphnolobium japonicum (2.2 mg/m2 ), Magnolia denudata (2.1 mg/m2 ), and Liriodendron tulipifera (2.0 mg/m2 )
An increasing number of studies have demonstrated that the ability of plants to adsorb and retain PM depends on species, leaf and branch density, and the microstructural properties of leaf surfaces
Summary
Particulate matter (PM) is an air pollutant in urban and industrial areas and PM fractions with a diameter of 10 μm or less often exceed air quality standards and cause serious concerns on ecosystems and humans including a wide range of respiratory and vascular diseases [1,2,3,4]. Expanding tree canopies across a city is very helpful for adsorbing PM particles and reducing air pollution to improve the urban environment [4]. Reducing PM emissions that come from vehicles or industrial operations is not an economically viable solution in highly urbanized areas. Plants have been proposed as a sustainable way of reducing artificial PM in urban environments, and they can effectively improve air quality by adsorbing PM10 onto leaf surfaces or encapsulating PM2.5 within the wax layer [3]. The ability of plants to retain fine particulates depends on the species, leaf and branch density, and surface properties related to the microstructure of leaves [11]
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