PM2.5 reduction effect of indoor plants
As a result of monitoring the change in concentrations 2 hours and 4 hours after putting 300 μg·m
−3 of PM into the chamber with plants and the empty chamber, all chambers showed the similar trend in which PM2.5 concentrations decreased with time (
Fig. 2). Assuming that the particles are spherical, the sedimentation speed by gravity is affected by the diameter of the particles, and thus heavier particles subside more quickly, whereas lighter particles float relatively longer in the air. The chamber with plants showed greater reductions than the empty chamber. While the fine particles float in the chamber, some are sedimented and others are adsorbed on the plant leaves, thereby reducing PM concentrations within the chamber. This is because the plant takes up a certain volume of the chamber and thus reduces the distance between PM particles and the surface that can adsorb them (
Ryu et al., 2019). This result is the same as the study by
Lohr and Pearson-Mims (1996) which explained the effective decrease of PM when there are plants compared to the control plot without plants.
Since each plant has a different size, we calculated reductions based on 1m
2 of leaf area to compare PM reduction efficiency among the plants. There was a difference in reductions of PM2.5 after 2 hours and 4 hours depending on the plant, and this difference was also statistically significant (
Table 2).
Pachira aquatica and
Dieffenbachia amoena showed reductions of 182.42 and 188.95 μg m
−2 leaf area respectively after 2 hours, showing remarkably higher levels of reductions than other plants. On the other hand,
Pittosporum tobira,
Anthurium andraeanum,
Nephrolepis exaltata ‘Bostoniensis’, and
Ficus elastica showed a reduction of less than 50 μg·m
−2 leaf area, which is relatively lower. Reductions after 4 hours were 163.89 for
Pachira aquatica, 161.45 for
Philodendron selloum, and 153.48 for
Dieffenbachia amoena, which were high, and similar to after 2 hours,
Pittosporum tobira,
Pelargonium hortorum ‘Golden Ears’,
Anthurium andraeanum,
Nephrolepis exaltata ‘Bostoniensis’, and
Ficus elastica tended to show low levels of reductions. Plants with medium-level reductions were
Epipremnum aureum, Philodendron ‘Congo’,
Platycerium bifurcatum, and
Rhododendron brachycarpum. There was a certain difference in the ranking of reductions, but the reduction of each plant tended to be similar after 2 hours and 4 hours.
Correlation between leaf characteristics and reduction effect
To determine how the characteristics of leaves, which are an important part of plants, affect PM reduction efficiency, we classified the leaf characteristics into size, thickness, shape, and area to analyze the reduction efficiency and determine statistical significance. A correlation analysis with leaf size, thickness, area, and reduction as the variables showed that leaf size had a positive correlation with PM2.5 reductions in 2 hours and 4 hours, while total leaf area had a negative correlation (
Table 3). Leaf thickness was not significant within 5% but had a negative correlation with reduction in 2 hours within 10%. Previous studies also explained that leaf characteristics such as leaf stoma, unit leaf area, and wax layer affect the level of deposition and dispersion of PM (Quit et al., 2009;
Räsänen et al., 2013;
Janhäll, 2015).
We analyzed the difference in reductions by classifying leaf size into five stages. Leaf size was classified based on the area of one leaf into 0–10cm2 (Podocarpus macrophyllus, Schefflera arboricola ‘Hong Kong’, Pelargonium hortorum ‘Golden Ears’, Pittosporum tobira), 20 – 40 cm2 (Ficus benjamina, Ardisia japonica, Epipremnum aureum, Rhododendron brachycarpum), 41 – 100 cm2 (Philodendron selloum, Pachira aquatica, Nephrolepis exaltata ‘Bostoniensis’, Asplenium nidus), 100 – 200 cm2 (Asplenium antiquum, Anthurium andraeanum, Ficus elastica, Philodendron ‘Congo’, Platycerium bifurcatum), and over 200 cm2 (Dieffenbachia amoena).
There was a statistically significant difference in PM2.5 reductions by group of leaf size.
Dieffenbachia amoena with the biggest leaf size showed relatively more reductions after 2 hours and 4 hours than other plants. Except
Dieffenbachia amoena, there was no statistically significant difference in reductions after 2 hours by leaf size among different plants. As for reductions after 4 hours, there was statistical significance between the 41 – 100 cm
2 group and the over 200 cm
2 group (
Table 4). As the PM exposure time increased, medium-sized leaves relatively showed higher PM reduction efficiency than plants with big or small leaves.
Weerakkody et al. (2018) also explained that PM reductions vary depending on leaf size, and small leaves showed greater PM reductions than big leaves.
Leaf thickness was classified into four stages for analysis: 0–0.20 mm (Nephrolepis exaltata ‘Bostoniensis’, Ardisia japonica, Ficus benjamina, Pachira aquatica), 0.21–0.35 mm (Philodendron selloum, Dieffenbachia amoena, Pelargonium hortorum ‘Golden Ears’, Asplenium antiquum), 0.36–0.50 mm (Rhododendron brachycarpum, Pittosporum tobira, Philodendron ‘Congo’, Schefflera arboricola ‘Hong Kong’, Epipremnum aureum, Anthurium andraeanum, Podocarpus macrophyllus), and 0.60–0.80 mm (Asplenium nidus, Platycerium bifurcatum, Ficus elastica).
PM2.5 reductions by leaf thickness were high in the 0.21–0.35 mm group at 117.52 and 101.59 after 2 hours and 4 hours, but there was no statistically significant difference from other groups (
Table 5). Although there is no statistical significance, PM reduction levels were higher when the leaves were thinner than thicker. Leaf thickness is related to the weight of the wax layer, which is a plant characteristic that affects PM reduction; thus, an analysis must be conducted on the wax layer and PM reduction.
Leaf shape was divided into three types: round shape with no split edge or serra, vertically long and linear shape (
Podocarpus macrophyllus), and irregular or lobed shape (
Pelargonium hortorum ‘Golden Ears’,
Nephrolepis exaltata ‘Bostoniensis’,
Philodendron selloum,
Platycerium bifurcatum). There is a statistically significant difference in PM2.5 reductions depending on leaf shape. Reduction levels after 2 hours and 4 hours were relatively higher in the linear shape, while other groups showed no statistical difference (
Table 6). Round-leaf plants showed relatively greater reduction effect than lobed-leaf plants.
Leonard et al. (2016) also showed similar results, proving that linear and lobed leaves retained more PM than oval leaves. Leaf shape is related to fluttering, which may separate the adsorbed PM and thus affect the amount.
Alll leaf area of plants used in the experiment was classified into 1300 cm
2 and below (
Platycerium bifurcatum,
Philodendron selloum,
Philodendron ‘Congo’,
Podocarpus macrophyllus,
Rhododendron brachycarpum), 1301–1600 cm
2 (
Pachira aquatica,
Dieffenbachia amoena,
Schefflera arboricola ‘Hong Kong’,
Asplenium nidus,
Ardisia japonica), 1900–2500 cm
2 (
Pelargonium hortorum ‘Golden Ears’,
Anthurium andraeanum,
Ficus elastica,
Pittosporum tobira), 2600–3000 cm
2 (
Asplenium antiquum,
Epipremnum aureum,
Ficus benjamina), and 3500 cm
2 and above (
Nephrolepis exaltata ‘Bostoniensis’). The groups with narrow leaf areas showed relatively greater PM2.5 reductions after 2 hours and 4 hours, while broader leaf areas led to relatively lower levels of reduction (
Table 7). There was a statistically significant difference among the leaf area groups. Since plants with broader leaves have relatively more area to adsorb PM, we may assume that they would have higher PM reduction efficiency. However, broader leaf area in a similar volume indicates that there are many overlapping leaves. When leaves overlap, the air flow is not smooth and thus have relative advantage in PM adsorption. Since there is a positive correlation between the leaf area index (LAI) of leaves and PM sedimentation (
Liu et al., 2015), it is necessary to analyze LAI instead of leaf area to determine the relationship with PM reductions.
To determine the leaf characteristics that affect PM2.5 reductions, we conducted a regression analysis with reduction as the dependent variable, and leaf area, leaf size, leaf thickness, and leaf number as the independent variables. Three regression analysis formulas were derived after entering the variables by selection of stage (
Table 8). Total leaf area was the characteristic that affected all three models. In other words, leaf area is an important variable in PM reduction by plants and has a negative correlation, and thus it must be taken into consideration. The model with high explanatory power had the adjusted R
2 of 0.408 and selected total leaf area, leaf thickness, and leaf size as the variables. Leaf area and thickness had a negative effect, and leaf size had a positive effect. In other words, leaf characteristics turned out to be important factors affecting PM reduction.
We analyzed through the regression analysis the factors of leaf characteristics that affect PM reduction and came up with scatter plots and second-order linear regression model to determine the relationship between each leaf characteristic and PM reduction (
Fig. 3). The variable with high explanatory power was total leaf area, and PM2.5 reduction after 4 hours was the lowest when the leaf area was 2,682 cm
2, showing a U-shape. In other words, adequate leaf area is required in PM reduction by indoor plants. Leaf number, although it does not have high explanatory power, also showed a U-shape. This similar result is due to the positive correlation between leaf number and leaf area. Medium-sized leaves had a low reduction effect, with one leaf size at 74cm
2, showing a U-shape. Previous studies also showed conflicting results in the reduction effect depending on leaf characteristics, which, compared to the results of this study, can be explained that there is high PM reduction effect when the leaves are either extremely big or small.