Introduction
Contaminants in air are classified into particulate substances and gaseous substances, and dust, which is a particulate contaminant, is classified according to the particle size based on total suspended particles, PM10 which is 10 μm or less in diameter and PM2.5 which is 2.5μm or less in diameter. PM10 and PM2.5 can cause or worsen acute and chronic respiratory diseases, increase the risk of cardiovascular, cerebrovascular diseases and arrhythmia and lead to early death in children and elderly and vulnerable people (
Popek et al., 2013;
Shin, 2007). As such, a range of measures are needed to protect the national health from high PM10 and PM2.5 concentrations.
Plants can adhere, absorb and eliminate contaminants from the above ground part, and planting-based air purification by microorganisms is possible (
Beckett et al., 1998).
Speak et al. (2012) carried out an experiment to remove PM by four green roof speceies – creeping bentgrass (
Agrostis stolonifera), red fescue (
Festuca rubra), ribwort plantain (
Plantago lanceolata) and sedum (
Sedum album) – that were normally used for extensive rooftop greening in downtown Manchester, England and predicted that the total rooftop greening area would be 325 ha if the rooftop greening was applied to the downtown area, and that such area could remove approximately 2.3% of the total amount of PM10 released in the city. The total leaf area of trees by tree age and the amount of PM2.5 adhesion were calculated using the parameter estimated value of the biomass model and the relative outgrowth table by tree part targeting pine trees, nut pine and oak trees, which were the species that were planted the most frequently in the country. The PM2.5 reduction effects caused by trees and economic benefits from such reduction were analyzed and the total economic benefits in terms of reduced medical expenses for respiratory and cardiovascular diseases from the 1 ton reduction of PM2.5 emissions was almost 100,001,872 KRW (
Shin and Lim, 2018).
The air purification effect of plants has been known since 1980, but for a long time plants were not actively used for such purpose due to a number of problems, including insignificant purification volume and the requirements of plant care. However, as the advantages of plants including eco-friendliness and sustainability have recently been emphasized, various studies have been carried out to prepare basic data related to the capacity of plants to remove indoor pollution sources such as indoor formaldehyde and carbon dioxide (
Park et al., 2008a;
Park et al., 2008b).
It is considered that the use of appropriate gardening plants as biological filters can improve air quality and enhance heath.
Yan et al. (2016) experimented with PM adhesion ability targeting
Rosa xanthina which was a shrub,
Broussonetia papyrifera which was a broad-leaf tree and
Pinus bungeana which was a needle-leaf tree in Beijing, China, and reported that shrubs displayed the highest PM adhesion effect, the shape of PM adhered to the leaves included particles with
gentle boundary surface (86.4 – 93.8%) and spherical particles (23.4%), and the amount of PM adhesion varied according to the shape of the PM.
Sæbø et al. (2012) investigated the amount of PM accumulated in the leaves targeting 47 woody plant species that were common in Norway and Poland in Northern Europe, and obtained a result indicating that no correlation was shown between the PM accumulation and the surface roughness and size of a leaf.
Chen et al. (2015) investigated PM accumulated in the leaves targeting 24 woody plant species collected in Wuhan, China, and obtained a result indicating that even the same plant species showed different PM accumulation effects between the control group with relatively good air quality and the polluted area. In other words, the plant species that showed high PM accumulation in a highly polluted area was different from the plant species in the control group that showed high PM accumulation. It was reported that the plant species that showed high PM accumulation also varied according to the size of the PM.
Wang et al. (2015) carried out a study on the structure of leaves and PM accumulation in the leaf development process using three deciduous species (
Ulmus pumila L.,
Salix babylonica L.,
Ginkgo biloba L. ) and reported the result, which indicated that more PM accumulation was shown in the upper side of the leaf, and that leaves with a smaller elevated surface scale were more effective in terms of PM2.5 accumulation.
Przybysz et al. (2014) carried out a study on the accumulation of particle material according to plant pollution level of evergreen species (
Taxus baccata L.,
Pinus sylvestris L., and
Hedera helix L.), rainfall and the passage of time and presented a result indicating that it was necessary to consider plant species and the dynamics of deposition and leaf washing by rain during the season when evaluating the total PM removal effect of vegetation. In a study on PM removal efficiency according to the morphological characteristics of plant leave types, such as needle, compound, small and large leaves,
Son et al. (2019) asserted that PM adhesion occurred frequently in the wax layer, which meant that needle-leaf trees with high wax layer content displayed high PM10 removal amount.
Cho (2019) pointed out the problem that domestic and overseas studies on plants for PM reduction have mainly focused on plants’ absorption function rather than their adhesion function, and as a result tree species for PM reduction were selected very unsystematically and studies were carried out mostly based on indoor foliage plants such as air purification plants selected by NASA.
In preceding studies on the selection of an effective plant for reducing fine dust, the leaves of trees planted mainly on roadsides in parks were collected and the amount of fine dust was measured and compared (
Sæbø et al., 2012;
Wang et al., 2015;
Perini et al., 2017). Depending on the meteorological environment, PM may float longer or spread in the air instantly, or may sink immediately due to rainfall, so there is a large variation in the amount of PM accumulated in leaves. Vegetation also varies by country, so a study on plants that are abundant and can be utilized in our country is necessary. According to the Convention on Biological Diversity (
Bell, 1992), which asserts the need to recognize the sovereignty over native species and mandates the preservation and monitoring system of native species, there is a growing interest in native plants as well as the utilization of species in the southern part of our country due to environmental changes such as global warming.
Therefore, 12 woody plant species including Dendropanax morbiferus, Pittosporum tobira and Viburnum odoratissimum var. awabuki, which were tree species in the southern species, were exposed to PM, and the amount of PM by hour was analyzed in order to select appropriate gardening plants for fast air purification after the occurrence of higher concentrations of PM.
Results and Discussion
The characteristics and total leaf area of each plant are as shown in
Table 1.
Viburnum odoratissimum var.
awabuki, an evergreen broad-leaf tree, showed the largest total leaf area at 6690.5 cm
2, followed by
Actinidia arguta (3619.4 cm
2) and
Fraxinus rhynchophylla (3308.8 cm
2), while
Dendropanax morbiferus (866.2 cm
2), Schisandra chinensis (763.2 cm
2) and
Rhapis (700.0 cm
2) showed a relatively smaller total leaf area.
When the mosquito repellent incense was initially injected into the chamber based on 1027.7 ± 27.2
μg for the amount of PM10, the amount of PM2.5 was 916.7 ± 29.9
μg which was approximately 89% of the amount of PM10 and the amount of PM1 was 232.3 ± 6.7
μg which was approximately 23% of the amount of PM10 (
Fig. 1). The ratio of PM2.5 and PM1 increased gradually over time. When the experiment was 50% (1/2) completed, the amount of PM10 was 282.8 ± 28.2
μg and the amount of PM2.5 was 272.8 ± 29.1
μg, which was approximately 94% of the amount of PM10, and the amount of PM1 was 172.0 ± 15.6
μg, which was approximately 61% of the amount of PM10. When 300 minutes had passed (at the end of the experiment), the amount of PM10 was 105.3 ± 30.5
μg and the amount of PM2.5 was 104.2 ± 30.4
μg, which was approximately 94% of the amount of PM10; the amount of PM1 was 172.0 ± 15.6
μg which was approximately 61% of the amount of PM10. This is because PM1 settled more slowly and was segmented less, as its particle size was smaller in comparison to PM10, whose amount decreased rapidly and was segmented easily.
In a Pearson’s correlation analysis between the total leaf area and the amount of PM in the chamber after 300 minutes, the correlation with PM10, PM2.5 and PM1 was −0.740, −0.741 and −0.738 respectively, showing a negative correlation (
Fig. 2). As the total leaf area of a plant in the chamber was larger, the amount of residual PM was less, and this result indicated that the total leaf area of a plant affected the PM reduction in an area of the same volume, a finding that agrees with the result of a previous experiment on PM reduction according to the existence of a plant (
Lohr and Pearson-Mims, 1996), and also indicates that PM was effectively removed when a plant was present in comparison to the control group with no plants.
While the experiment on PM reduction using a plant mainly measures the amount of PM extracted from collected plant leaves and compares the amount of PM adhesion per unit area, the comparison of trees for PM reduction effect by urban forests in the whole city calculates the amount of PM by estimating the total leaf area in consideration of tree age and crown (
Liang et al., 2016), so the vine plant group with many leaves was effective in PM reduction in a sealed space such as a chamber. The result of comparing the amount of PM in the chamber after 300 minutes by converting the initial injection amount of PM10, PM2.5 and PM1 into a percentage is as shown in
Table 2. Since the leaf development status of 30 cm high plants for the experiment varied depending on the plant properties, plants were classified into three groups including vines, trees, and shrubs and small trees, and their PM reduction effects were compared. For the average amount of PM10, PM2.5 and PM1, the vine group showed 7.917%, 8.796% and 30.275% respectively, the shrubs and small trees group showed 10.142%, 11.133% and 36.448% respectively and the trees group showed 11.475%, 12.892% and 40.421% respectively. Based on the same height, vines that showed fast leaf development were more effective in the PM reduction. In a study carried out by
Chen et al. (2015) on PM accumulation targeting 24 woody plant species,
Parthenocissus tricuspidata was effective for PM2.5 and PM0.2 accumulation, showing the same result as this experiment, indicating that 4 vine species were effective in terms of their PM reduction. In addition, it is considered that
Actinidia arguta, which showed the highest PM reduction effect among vines in this experiment, could play the same role with
Hedera helix L, a vine frequently used for landscaping decoration, heat reduction and air purification in foreign studies on wall greening and rooftop greening (
Sendo et al., 2010;
Cuce, 2017;
Przybysz et al., 2014).
Viburnum odoratissimum var.
awabuki in the shrubs and small trees group showed the largest total leaf area and the smallest residual PM amount among target 12 plant species for the experiment.
Perini et al. (2017) indicated that the amount of PM with particle sizes between 0.1
μm and 20
μm was the largest for
Trachelospermum jasminoides, followed in order by
Hedera helix,
Cistus and
Phlomis fruticose, when these 4 plant species were planted on the vertical greening system and the amount of PM accumulation was quantified. Trachelospermum jasminoides showed the largest amount of wax among these 4 plant species. In this experiment,
Viburnum odoratissimum var.
awabuki with hard and thick leaves and vines including
Actinidia arguta,
Vitis coignetiae,
Schisandra chinensis and
Parthenocissus tricuspidata also showed good results, indicating that these plant species were excellent for PM removal.
Sgrigna et al. (2020) indexed 9 items including the micro-characteristics and morphological characteristics of a leaf including roughness, pore density, hair, leaf shape, time of leaf growth and leaf size, and determined the PM accumulation index by adding all these indexes.
Sgrigna et al. (2020) also asserted that not one but all microscopic and macroscopic characteristics of a leaf were combined and interacted with each other, affecting the PM accumulation in the tree. In a study by
Chen et al. (2015) on PM accumulation targeting 24 woody plant species,
Pittosporum tobira leaves showed a small amount of PM0.2 accumulation, and the leaves used in this study also showed less of a PM reduction effect, and low PM reductione effect was caused by the total leaf area and other leaf characteristics of
Pittosporum tobira. With regard to
Fraxinus rhynchophylla and
Salix koreensis,
Fraxinus rhynnchophylla had a larger total leaf area than
Salix koreensis, but Salix koreensis showed better PM10 and PM2.5 accumulation effects than
Fraxinus rhynchophylla, so that there was a difference in the reduction effect according to the particle size of PM.
Conclusion
In this study, the amount of PM accumulation over time was investigated after contaminants were injected into the chamber in order to investigate the PM removal efficiency of 12 woody plant species. The 12 plant species used in this study were divided according to plant characteristics into 3 groups, which were vines, trees, and shrubs and small trees, and the comparison showed that the vine plant group was more effective in PM removal than the other two groups. There was a negative correlation between the total leaf area and the amount of PM in the chamber. Viburnum odoratissimum var. awabuki, which had the largest total leaf area, showed the smallest amount of PM remaining in the chamber after 5 hours for three PM types including PM10, PM2.5 and PM1 among the 12 tree species. It is considered that the relatively slower leaf development of the trees group than the other two groups affected its PM removal at the tree height of 30 cm.
As such, it is considered that the PM reduction response of these 12 woody plant species can be used as basic data for selecting an appropriate tree species according to the size and floating characteristics of PM when planting trees as biological filters for yellow dust and contaminant leakage from factories, which have recently been frequently occurring. Various plant characteristics affect PM reduction, but in the simplest case, it is effective to select a tree species with a large total leaf area, and it is necessary to consider that plants should grow properly so that their leaves can be developed actively in order to achieve PM reduction effects using such plants.