# The Dew Particle Interception Abilities of Typical Plants in Northeast China Plant Leaves Capture Particles in Dew

Authors: Yingying Xu; Yingbo Dou; Yan Yi; Xu Yang
Journal: Advances in Meteorology
 (2022)

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## Abstract

The dew condensation frequency is high, and the dew amount is heavy in urban ecosystems. During the condensation process, particulate matter acts as a condensation core, playing an important role in purifying the air. At night, dew mainly condenses on plant leaf surfaces, the plant leaves settle the particles in the dew, and some of the particles are resuspended into the atmosphere in the process of dew evaporation after sunrise. This paper monitored the condensation and evaporation processes of dew on four common plants in Changchun city from June to September 2020. By analyzing the mass and size of particles on different leaves after dew condensation and evaporation, the ability of different plants to retain particles in dew was analyzed. The results showed that there was no significant difference in the TSP capture ability during dew condensation betweenBuxus sinica (Rehd. et Wils.) Cheng subsp. sinica var. parvifolia M. Cheng, Syringa oblata Lindl., Hemiptelea davidii (Hance) Planch., and Pinus tabuliformis Carrière, with a TSP content of 0.21 ± 0.06 μg/cm2. Coarse particulate matter is the main type of deposit in the dew condensation stage. Particulate deposition varied according to species, leaf shape, and microstructure. The proportion of TSP remaining on leaves after dew evaporation from Pinus tabuliformis Carrière, Hemiptelea davidii (Hance) Planch., Buxus sinica (Rehd. et Wils.) Cheng subsp. sinica var. parvifolia M. Cheng, and Syringa oblata Lindl. tree was 89.7 ± 3.9%, 80.6 ± 3.6%, 75.9 ± 4.5%, and 71.4 ± 3.7%, respectively. The ability of the leaves to trap fine particles was significantly higher than that for coarse particles (P<0.05) after dew evaporation. The highest amount of particle captured by Syringa oblata Lindl. individual was 15.17 g/y during dew condensation, and the amount of remaining particles after dew evaporation was 10.83 g/y. This paper provides a theoretical basis for the selection of tree species for urban greening.


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## Body

## 1. Introduction

Atmospheric particulate matter poses a threat to human health, especially to the human respiratory system and immune function [1]. The urban plants are considered to play the role of natural filter in the interception and removal of particulate matter [2]. In Beijing, 772 t of PM10 was captured by leaves in one year [3]. The dust retaining capacity of leaves was 8012.89 t/y in Guangzhou [4]. The urban canopy of the Greater London Authority is currently estimated to remove between 852 and 2121 t of PM10 annually, representing between 0.7% and 1.4% of PM10 from the urban boundary layer [5]. Dew condensation is a common weather phenomenon, and it is also an effective natural atmospheric purification process with the particles in the air serving as a condensation core [6, 7]. Particles in the atmosphere, as condensation cores, experience the process of moisture absorption and homogeneous-heterogeneous phase transition, finally turning to dew. During this condensation stage, particles migrate and settle toward the surface continuously with the condensation of dew, and the dew becomes the sink of near-surface particles (PM2.5, PM10, etc.) at night [8, 9]. This leads to the concentration of particulate matter in dew being significantly higher than that in rainwater [10, 11], especially when the concentrations of total dissolved solids (TDS) are high [12]. The average TDS in the dew of Changchun (China) [10] and Delhi (India) [13] can reach 154 and 135 mg/L, respectively.Due to the large-scale hardening of underlying surfaces and the heat island effect in urban ecosystems, urban areas have become a “landscape unit” with frequent dew condensation, which not only occurs at a high frequency but also in large amount [8, 14]. In Changchun, for example, the annual dew amount in 2017 was 35 mm, and the mass concentrations of TDS, PM2.5, and PM10 in the dew were 175.31, 24.39, and 49.65 mg/L, respectively. In normal weather, the removal rates of TSP, PM10, and PM2.5 by dew were 28.2%, 25.9%, and 21.5%, respectively [15]. The annual removal of particulate matter by dew was 3.11–4.73 kg/ha.However, atmospheric particulate matter temporarily settles on the leaf surfaces of plants and returns to the air via resuspension [16, 17]. Dew is effective at removing particulate matter from the air near surfaces in urban ecosystems. As an intermediate product forming and dissipating by day and night, the dew evaporates after sunrise, and the particulate matter condensed at night is transferred from the sink to the source during the evaporation process. The PM2.5 concentration is highest in the morning but lower in the afternoon and evening in the urban forest of Beijing [18]. Generally, the concentrations of PM10 and PM2.5 near the surface are minimal from 22 : 00 to 3 : 00 and peak from 7 : 00 to 9 : 00 [18, 19], coinciding with the dew evaporation period [20]. Dew condensation occurs between 20 : 00 and 5 : 00, and the particles can be reduced by wet deposition of dew [21, 22]. The low concentration of particulate matter in the atmosphere at night is closely related to the condensation of dew. However, whether the peak concentration of particulate matter in the atmosphere is related to dew evaporation has not yet been determined.Because of the hygroscopic behavior of aerosol particles [23, 24], humidity can affect the particle shape and size [25, 26]. The particle size in the atmosphere will change due to aerosol hygroscopicity or volatility whether in the process of dew condensation or evaporation, and it is very complicated and hard to measure. There are some advanced techniques such as ultraviolet aerodynamic particle sizer (UV-APS) fluorescence techniques or fluorescence in situ hybridization (FISH) to measure biological aerosol [27–29]. In this research, the particles deposited on dew condensation and evaporation nodes were observed. The sediment particles and the leaf microstructures were examined by scanning electron microscopy (SEM) in this study. In addition, studies on the interception of particulate matter in plant leaves focus on the interception of dry deposits over time [30]. Some studies identify the source of aerosols by analyzing the chemical composition and backward trajectories [31, 32]. In this study, more attention was paid to the retention capacity of leaves. There is a large and frequent amount of dew water on the leaves of plants in the urban ecosystem, and the concentration of particulate matter is high. Therefore, it is urgent to discuss the amount of particulate matter on leaves in dew deposited at night and evaporated in the morning. The particles monitored in this study are the sediment particles intercepted by leaves. The purpose of this paper is to investigate the capacity of common plants to retain particles of different sizes during the dew condensation and evaporation processes and to discuss the main influencing factors of particles trapped by leaves in dew. The particles measured in this study were naturally captured by leaves, and it will improve the understanding of the ability of dew material captured by different leaves in the urban ecosystems.


## 2. Materials and Methods

### 2.1. Experimental Site

Dew and leaf samples were collected at Jilin Jianzhu University, which is located in the southeastern part of Changchun in Jilin Province, China. The continental monsoon climate of the area produces average annual temperatures that vary from −15°C in January to 25°C in August. Precipitation ranges from 522–615 mm/a. The research area is located at the edge of the city, with no industrial and traffic pollution. The dew intensity in summer and autumn is relatively high. For example, in 2014 and 2015, the amount of dew in July, August, and September in the Changchun green area was 46.29 mm and 72.24 mm, respectively, accounting for 22.52% and 23.61% of the precipitation in the same period [15]. Therefore, samples were collected during the peak dew period of summer and fall (June–September) in 2020.


### 2.2. Plant Selection

Four plants commonly found in northeast China were selected for this study, (Buxus sinica (Rehd. et Wils.) Cheng subsp. sinica var. parvifolia M. Cheng (Buxus); Syringa oblata Lindl. (Syringa); Hemiptelea davidii (Hance) Planch. (Hemiptelea); and Pinus tabuliformis Carrière (Pinus)). 10–15 plants of each species were randomly selected to actually measure the main characteristics, which are shown in Table 1. To avoid differences in particle interception under different pollution conditions, the selected tree species were all in relatively concentrated areas (see Figure 1).Table 1
Leaf or leaflet traits of the 4 selected plant species.
SpeciesHeight (m)Leaf size (cm2)TextureTrichomesShapeMarginNumber of leaves (×104)Leaf morphologyBuxus1.8–3.22 ± 0.5Smooth, waxyNoneEllipticalSmooth1.5–2.3Syringa2.5–3.551 ± 12.7SmoothNoneCompoundLobate0.9–2.1Hemiptelea2.2–3.115 ± 3.8Rough with multiple veinsLarge and few (adaxial); short and numerous (abaxial)EllipticalSerrate2.5–3.5Pinus1.8–3.50.4 ± 0.1Smooth, water repellentNoneAcicularSmooth25–28Figure 1
Location of the sample sites.


### 2.3. Time of Dew Evaporation and Condensation Periods

A tray (diameter of 30 cm) with turf covering both sides was placed on an electronic balance (range of 0.01 to 3000 g). The balance was placed in a waterproof box (with small holes to discharge rain water), and the box was placed on an observation shelf. The observation shelf was equipped with an arm that adjusts in height according to the growth of plants so that the tray and the plant canopy remained at the same height. The weight change of the turf can be continuously monitored, and it is the objective indicator as dew condensation or evaporation. The initial value was recorded half an hour after sunset; when the peak value was reached (approximately half an hour before sunrise), the weight gain value was taken as the dew condensation amount, and the corresponding period was considered the dew condensation period. When the value dropped to a constant weight (the value remained unchanged for half an hour), the dew was considered to have evaporated completely, and the corresponding period was defined as the dew evaporation period.


### 2.4. Particulate Matter Mass in Dew

Four plant leaves were collected from four directions of the branches at random points in the tree canopy. Thirty to fifty leaves were collected, sealed to avoid vibration, and immediately brought back to the laboratory. Cotton balls moistened with alcohol and deionized water were used to thoroughly wash and wipe the surfaces of leaves, to remove dust from the leaves, and to reach an initial state of zero dust accumulation. The leaves were divided into two groups. The leaves in each group were evenly divided into two parts, with half adaxial surface up and half abaxial surface down. The area of leaves (cm2) was recorded. The leaves were fixed on a polystyrene foam board. The polystyrene board was kept parallel to the surface and was placed at the height of the plant canopy at the beginning of dew condensation.At the point node when dew condensation turns to evaporation, a piece of foam board was sealed and brought back to the laboratory. The 10.00, 2.50, and 0.22μm membrane filters were dried (65°C) to constant weight (Wjf, g). The surface of the leaves was washed using the ultrasonic cleaner with deionized water for 1 hour, and leaching water was filtered through the filters. The filters were weighed again after dried (65°C), and the weight of filters Wjc (g) was recorded. The TSP, PM2.5, and PM10 values of the dew were calculated based on the weight differences. After the dew evaporated completely, the other foam plate were sealed and brought back to the laboratory. The above steps for weighing leaves (We, g) and determining particle residue weight (Wje, g) were repeated. All particles were naturally intercepted by leaves, and it was passive sampling.PM2.5/PM10/TSP retention by individual plants during dew condensation and after evaporation was calculated as follows:(1)Fj=2×Wjc−Wjfn×N,Pj=2×Wje−Wjfn×N,where j is the type of PM2.5, PM10, or TSP; Fj is the weight of particles during dew condensation for a single plant (mg/plant); Wjc is the weight of particulate matter in deionized water (washed off the leaves) when the dew condenses to the evaporation node (g); Wjf is the initial weight of the filter paper (g); n is the number of collected leaves on one polystyrene foam; N is the number of leaves on one plant; 2 is the transfer coefficient; Pj is the weight of particles after dew evaporation for a single plant (mg/plant); and Wje is the weight of particulate matter in deionized water (washed off the leaves) after dew evaporation (g).


### 2.5. Particle Diameter Analysis

Particle diameters was measured using a model JL-1166 Laser Particle Analyzer (JL-1166, Chengdu Jingxin Powder Analysis Instrument Co., Ltd., Chengdu, Sichuan, China).


### 2.6. Meteorological Factor Monitoring

Daily meteorological data, including air temperature (°C),RH (%) and wind speed (m/s) at 1 m height, were measured at hourly intervals during the condensation period using a Milos 520 automatic weather station (Vaisala, Finland) at Jilin Jianzhu University.


### 2.7. Microstructural Analysis of Leaves

The microstructures of the leaves was examined using scanning electron microscopy (SEM). The drop contact angle (DCA) was determined using a Canon EOS550D camera attached to a macrolens (MP-E 65 MM 1 : 2.8) [33]. A 6 μL droplet (for broadleaves) and a 2 μL droplet (for needles) of distilled water were placed on the leaf sample.Thirty leaves were randomly selected with 3 replicates for each species. Total leaf area was recorded (S). The weighing bottle weight was recorded with a 0.0001 g analytical balance (W0). Leaf blades were held with tweezers and sprayed with chloroform onto the front or back surfaces for 60 seconds. The extraction solution was then transferred into the weighing bottle. The weighing bottle was weighed on a balance again when the chloroform was completely volatilized in a fume cupboard (W1). The twice difference (W1−W0) of the wax content was measured. The wax content per unit leaf area (W, g/m2) was determined based on the following formula:(2)W=W1−W0S.


## 2.1. Experimental Site

Dew and leaf samples were collected at Jilin Jianzhu University, which is located in the southeastern part of Changchun in Jilin Province, China. The continental monsoon climate of the area produces average annual temperatures that vary from −15°C in January to 25°C in August. Precipitation ranges from 522–615 mm/a. The research area is located at the edge of the city, with no industrial and traffic pollution. The dew intensity in summer and autumn is relatively high. For example, in 2014 and 2015, the amount of dew in July, August, and September in the Changchun green area was 46.29 mm and 72.24 mm, respectively, accounting for 22.52% and 23.61% of the precipitation in the same period [15]. Therefore, samples were collected during the peak dew period of summer and fall (June–September) in 2020.


## 2.2. Plant Selection

Four plants commonly found in northeast China were selected for this study, (Buxus sinica (Rehd. et Wils.) Cheng subsp. sinica var. parvifolia M. Cheng (Buxus); Syringa oblata Lindl. (Syringa); Hemiptelea davidii (Hance) Planch. (Hemiptelea); and Pinus tabuliformis Carrière (Pinus)). 10–15 plants of each species were randomly selected to actually measure the main characteristics, which are shown in Table 1. To avoid differences in particle interception under different pollution conditions, the selected tree species were all in relatively concentrated areas (see Figure 1).Table 1
Leaf or leaflet traits of the 4 selected plant species.
SpeciesHeight (m)Leaf size (cm2)TextureTrichomesShapeMarginNumber of leaves (×104)Leaf morphologyBuxus1.8–3.22 ± 0.5Smooth, waxyNoneEllipticalSmooth1.5–2.3Syringa2.5–3.551 ± 12.7SmoothNoneCompoundLobate0.9–2.1Hemiptelea2.2–3.115 ± 3.8Rough with multiple veinsLarge and few (adaxial); short and numerous (abaxial)EllipticalSerrate2.5–3.5Pinus1.8–3.50.4 ± 0.1Smooth, water repellentNoneAcicularSmooth25–28Figure 1
Location of the sample sites.


## 2.3. Time of Dew Evaporation and Condensation Periods

A tray (diameter of 30 cm) with turf covering both sides was placed on an electronic balance (range of 0.01 to 3000 g). The balance was placed in a waterproof box (with small holes to discharge rain water), and the box was placed on an observation shelf. The observation shelf was equipped with an arm that adjusts in height according to the growth of plants so that the tray and the plant canopy remained at the same height. The weight change of the turf can be continuously monitored, and it is the objective indicator as dew condensation or evaporation. The initial value was recorded half an hour after sunset; when the peak value was reached (approximately half an hour before sunrise), the weight gain value was taken as the dew condensation amount, and the corresponding period was considered the dew condensation period. When the value dropped to a constant weight (the value remained unchanged for half an hour), the dew was considered to have evaporated completely, and the corresponding period was defined as the dew evaporation period.


## 2.4. Particulate Matter Mass in Dew

Four plant leaves were collected from four directions of the branches at random points in the tree canopy. Thirty to fifty leaves were collected, sealed to avoid vibration, and immediately brought back to the laboratory. Cotton balls moistened with alcohol and deionized water were used to thoroughly wash and wipe the surfaces of leaves, to remove dust from the leaves, and to reach an initial state of zero dust accumulation. The leaves were divided into two groups. The leaves in each group were evenly divided into two parts, with half adaxial surface up and half abaxial surface down. The area of leaves (cm2) was recorded. The leaves were fixed on a polystyrene foam board. The polystyrene board was kept parallel to the surface and was placed at the height of the plant canopy at the beginning of dew condensation.At the point node when dew condensation turns to evaporation, a piece of foam board was sealed and brought back to the laboratory. The 10.00, 2.50, and 0.22μm membrane filters were dried (65°C) to constant weight (Wjf, g). The surface of the leaves was washed using the ultrasonic cleaner with deionized water for 1 hour, and leaching water was filtered through the filters. The filters were weighed again after dried (65°C), and the weight of filters Wjc (g) was recorded. The TSP, PM2.5, and PM10 values of the dew were calculated based on the weight differences. After the dew evaporated completely, the other foam plate were sealed and brought back to the laboratory. The above steps for weighing leaves (We, g) and determining particle residue weight (Wje, g) were repeated. All particles were naturally intercepted by leaves, and it was passive sampling.PM2.5/PM10/TSP retention by individual plants during dew condensation and after evaporation was calculated as follows:(1)Fj=2×Wjc−Wjfn×N,Pj=2×Wje−Wjfn×N,where j is the type of PM2.5, PM10, or TSP; Fj is the weight of particles during dew condensation for a single plant (mg/plant); Wjc is the weight of particulate matter in deionized water (washed off the leaves) when the dew condenses to the evaporation node (g); Wjf is the initial weight of the filter paper (g); n is the number of collected leaves on one polystyrene foam; N is the number of leaves on one plant; 2 is the transfer coefficient; Pj is the weight of particles after dew evaporation for a single plant (mg/plant); and Wje is the weight of particulate matter in deionized water (washed off the leaves) after dew evaporation (g).


## 2.5. Particle Diameter Analysis

Particle diameters was measured using a model JL-1166 Laser Particle Analyzer (JL-1166, Chengdu Jingxin Powder Analysis Instrument Co., Ltd., Chengdu, Sichuan, China).


## 2.6. Meteorological Factor Monitoring

Daily meteorological data, including air temperature (°C),RH (%) and wind speed (m/s) at 1 m height, were measured at hourly intervals during the condensation period using a Milos 520 automatic weather station (Vaisala, Finland) at Jilin Jianzhu University.


## 2.7. Microstructural Analysis of Leaves

The microstructures of the leaves was examined using scanning electron microscopy (SEM). The drop contact angle (DCA) was determined using a Canon EOS550D camera attached to a macrolens (MP-E 65 MM 1 : 2.8) [33]. A 6 μL droplet (for broadleaves) and a 2 μL droplet (for needles) of distilled water were placed on the leaf sample.Thirty leaves were randomly selected with 3 replicates for each species. Total leaf area was recorded (S). The weighing bottle weight was recorded with a 0.0001 g analytical balance (W0). Leaf blades were held with tweezers and sprayed with chloroform onto the front or back surfaces for 60 seconds. The extraction solution was then transferred into the weighing bottle. The weighing bottle was weighed on a balance again when the chloroform was completely volatilized in a fume cupboard (W1). The twice difference (W1−W0) of the wax content was measured. The wax content per unit leaf area (W, g/m2) was determined based on the following formula:(2)W=W1−W0S.


## 3. Results and Discussion

### 3.1. Particles Settled by Leaves in the Dew Condensation Stage

As shown in Figure2, the TSP contents settled by Pinus, Hemiptelea, Buxus, and Syringa leaves in the dew condensation stage were 0.21 ± 0.07, 0.21 ± 0.06, 0.22 ± 0.06, and 0.21 ± 0.06 μg/cm2, respectively. There was no significant difference in TSPs per unit area among leaves of each tree.Figure 2
The amount of particles per unit area of different leaves in the dew deposition stage.Coarse particulate matter is the main type of deposit captured by leaves in the dew condensation stage. As shown in Figure3, the four kinds of trees mainly removed coarse particles (>PM10) in the dew deposition stage. The proportions of PM2.5, PM2.5–10, and >PM10 were 9.2%–16.0%, 26.3%–32.2%, and 56.7%–62.4%%, respectively. As evergreen tree (Pinus), the interception of PM2.5 proportion was significantly higher compared to other deciduous trees (Hemiptelea, Buxus, and Syringa) (P<0.05), but there was no significant difference in the PM2.5–10 and >PM10 proportion (P>0.05). It indicated that evergreen plants were better at trapping fine particles. This is similar to the results of other studies. More than 80% of the particles intercepted were PM10 of several common green plants in Qingdao [30]. PM larger than 10 μm comprised 73% of PM deposited on leaves in Beijing, with the coarse and fine particle size fractions comprising 16% and 11% of the deposited PM, respectively [22].Figure 3
Particle size distribution during dew condensation and after dew evaporation among different plant species.


### 3.2. Particles Settled by Leaves after Dew Evaporation

After dew evaporation, the ability of the four kinds of trees to retain particulate matter was obviously different. The proportions of intercepted TSP settled byPinus, Hemiptelea, Buxus, and Syringa leaves were 89.7 ± 3.9%, 80.6 ± 3.6%, 75.9 ± 4.5%, and 71.4 ± 3.7%, respectively (see Figure 4). The ability of Pinus (evergreen) to trap particles after dew evaporation was significantly higher compared to several other deciduous tree species. The ability of the four kinds of trees to trap fine particles on their leaves was significantly higher than that for coarse particles (P<0.05). Taking Syringa as an example, the leaf interception ratios of PM10, PM2.5–10, and PM2.5 after dew evaporation were 65.1 ± 3.9%, 83.7 ± 5.5%, and 88.5 ± 7.3%, respectively. After the dew evaporation process, some of the particles were still trapped on the surface of the leaves, and some of them left. The proportion of fine particles trapped in the leaves was higher. After dew evaporation, the particles trapped on the leaves were still mainly coarse particles, but the proportion of PM2.5–10 increased to 30.7%–33.7%, and the proportion of PM2.5 increased to 11.6%–17.6%. Both the dew condensation and the secondary suspension process were dominated by coarse particles.Figure 4
The amount of particles by different leaves and the removal rate of particles of different particle sizes.There was no significant difference in the mass of particulate matter captured by leaves of different tree species per unit area during dew condensation, but the deposited PM loads varied significantly among individual tree species due to the leaf area (see Table1). Syringa showed the highest PM deposition, followed by Hemiptelea, Pinus, and Buxus. According to the calculation of 130 dew days in one year, the settlement of TSP by one Syringa, Hemiptelea, Pinus, and Buxus individual was 15.17 g/y, 12.87 g/y, 2.97 g/y, and 1.07 g/y, respectively, during dew condensation. The remaining TSP trapped by these leaves was 10.83 g/y, 10.35 g/y, 2.67 g/y, and 0.81 g/y, respectively, after dew evaporation. Evergreen or deciduous plants cannot differentiate the intercept of the TSP amount. As the leaves’ area increases for each species, the TSP values should be slightly higher than the actual TSP considering the leaves inclination and mutual shadowing with respect to the vertical direction.


### 3.3. Factors Affecting Leaf Interception of Particles

The relationship between precipitation and particles captured by leaves of the four plants during the dew deposition period was close. The mass of particles on leaves decreased after precipitation (see Figures2 and 5). For example, the cumulative precipitation reached 53.3 mm from August 13 to 16, and on August 18, the deposition of dew particles in the four plants showed a significant declining trend. This may have been due to purification by precipitation, i.e., the dew captured few particles and resulted in a reduction in interception [10]. With the increase in pollutants in the atmosphere after rain, samples were taken continuously on August 19 and 20, and the deposition of particles in dew gradually increased.Figure 5
The rainfall from June to September and the meteorological factors during dew evaporation in 2020.As demonstrated in Table2, there was no relationship between wind speed and particulate matter interception amount of four plants (P>0.05). Some researchers found the correlation was positive or negative depending on the local wind speed values, and there was a threshold in the correlation between wind speed and particle concentration [34, 35]. In this study, neither mean wind speed (0.83 m/s) nor 1 m/s wind speed was related to particulate interception (P>0.05). The wind speed had no significant effect on the particulate matter during the dew evaporation period. This is because dew formation events always occur at wind speeds under 2 m/s, and high wind speed conditions are not conducive to dew formation. In addition, the dew evaporation period lasted 3 to 4 hours, and wind has little effect on particulate matter in a short time. The relative humidity during the dew evaporation period was positively correlated with the interception amount (P<0.05) because the particulate matter was not easily dispersed with high air humidity. Air temperature or dew point temperature had no correlation with the interception amount (P>0.05).Table 2
Correlation coefficients between wind speed and particulate matter interception amount.
PinusHemipteleaBuxusSyringaWind speed−0.1830.1640.032−0.208>Mean wind speed0.2210.3430.2970.047<Mean wind speed−0.2280.1210.038−0.189>1.0 m/s wind speed0.3220.0470.2430.210<1.0 m/s wind speed0.1750.1630.0040.153The four kinds of trees showed little difference in terms of trapping particulate matter in the dew deposition stage, but there were significant differences in their ability to trap particles after the evaporation process (see Figure4). The ability of plants to capture particle matter is related to multiple factors, including leaf shape and the leaf surface microstructure [36]. According to their leaf morphology, conifers have the strongest ability to trap particles in dew. Conifers exhibited higher PM2.5 adsorption amounts than broadleaf trees [37]. The oil secreted by Pinus leaves has strong particle adsorption properties (see Figure 6(c)), which also makes it difficult for particles to fall off. The leaves of Hemiptelea have serrated edges (see Figure 7(c)), which can also make them more strongly adsorb particles in dew.Figure 6
Microstructures of different species with SEM. (a–c) Leaf surface ofPinus (×500; ×600; ×1000); (d) adaxial leaf surface of Hemiptelea (×1000); (e) stomata of the abaxial leaf surface of Hemiptelea (×2000); (f) the base of one of a dense mat of glochid trichomes of Hemiptelea (×4000); (g) adaxial leaf surface of Buxus (×2000); (h) abaxial leaf surface of Buxus (×3000); (i) one stomata of the abaxial leaf surface of Buxus (×4000); (j) adaxial leaf surface of Syringa (×400); (k) abaxial leaf surface of Syringa (×1000); (l) one stomata of the abaxial leaf surface of Syringa (×8000). The structure in the the red rectangle is more inclined to capture particulate matter.
(a)(b)(c)(d)(e)(f)(g)(h)(i)(j)(k)(l)Figure 7
Drop contact angle (DCA) and leaf morphology of different species. (a) DCA of theHemiptelea adaxial surface; (b) DCA of the Hemiptelea abaxial surface; (c) photograph of the Hemiptelea leaf; (d) DCA of the Buxus adaxial surface; (e) DCA of the Buxus abaxial surface; (f) photograph of the Buxus leaf; (g) DCA of the Syringa adaxial surface; (h) DCA of the Syringa abaxial surface; (i) photograph of the Syringa leaf).
(a)(b)(c)(d)(e)(f)(g)(h)(i)From the aspect of microstructure, the trichome density and length ofHemiptelea leaves was significantly higher compared to Syringa and Buxus leaves (see Figure 6). The trichome surface structures were helpful in enhancing the effective surface area for PM adsorption. Species with trichomes accumulated significantly more particulate matter than other species [38]. Particulate matter adsorption can be observed at the tip and base of the trichomes of Hemiptelea (see Figures 6(d), 6(g), and 6(j)). The difference between the removal efficiencies of T. usneoides with and without trichome structures was approximately 7% for PM2.5 and 2% for PM10 [39]. The effect of trichome structure is particularly notable for the adsorption of PM2.5. Trichome density has a significantly positive correlation with the maximum particle size (P<0.05), and when the wind force is weak, stomatal density and trichome density have a significant effect on particulate matter resuspension [17]. Therefore, Hemiptelea can capture more PM (see Figure 6(f)) than Syringa and Buxus.As Figure6 shows, many fine and coarse particles accumulated on the adaxial leaf surfaces, while some large particles attached to the abaxial leaf surfaces, and few particles became attached in the vicinity of the stomata. Grooves are the main parts of a blade that absorb PM2.5. Deep grooves can intercept more particles (see Figure 6(k)); additionally, deeper grooves make the release of PM less likely. Ridge ultrastructures at 1-2 μm scale are the most efficient at capturing PM. The stomatal densities of Syringa, Hemiptelea, and Buxus were 213.0 ± 20.5/mm2, 92.5 ± 7.3/mm2, and 84.5 ± 18.6/mm2, respectively. Both stomata and trichomes are adjunct structures that are conducive to the adsorption of particles. Different shapes of leaves captured particles in dew with different ways. Evergreen tree with acicular leaves such as Pinus adhered to particulate matter by the secreted oil. While deciduous plants trapped to particulate matter around its unique microstructure, deep grooves or dense trichome such as the elliptical leave (Hemiptelea) used dense trichome to trap particulate matter (see Figure 6(f)) and the compound leave (Syringa) used deep grooves (see Figure 6(k)).Otherwise, the content of wax on the surface ofSyringa (0.79 ± 0.09 g/m2) was higher than that of Buxus (0.38 ± 0.15 g/m2) and Hemiptelea (0.30 ± 0.11 g/m2), and the DCA of Syringa (adaxial surface 143.2° ± 6.5° and abaxial surface 123.4° ± 10.7°) was also higher than that of Buxus (adaxial surface 107.5° ± 16.5° and abaxial surface 95.3° ± 12.7°) and Hemiptelea (adaxial surface 80.24° ± 11.7° and abaxial surface 44.2° ± 7.8°) (see Figure 7). 10° < DCA < 90° indicates hydrophilic leaves, and 90° < DCA < 150° indicates hydrophobic leaves. Syringa leaves have a smoother surface and stronger hydrophobicity than Buxus and Hemiptelea davidii leaves. Leaves with strong hydrophobicity do not absorb particles easily.


## 3.1. Particles Settled by Leaves in the Dew Condensation Stage

As shown in Figure2, the TSP contents settled by Pinus, Hemiptelea, Buxus, and Syringa leaves in the dew condensation stage were 0.21 ± 0.07, 0.21 ± 0.06, 0.22 ± 0.06, and 0.21 ± 0.06 μg/cm2, respectively. There was no significant difference in TSPs per unit area among leaves of each tree.Figure 2
The amount of particles per unit area of different leaves in the dew deposition stage.Coarse particulate matter is the main type of deposit captured by leaves in the dew condensation stage. As shown in Figure3, the four kinds of trees mainly removed coarse particles (>PM10) in the dew deposition stage. The proportions of PM2.5, PM2.5–10, and >PM10 were 9.2%–16.0%, 26.3%–32.2%, and 56.7%–62.4%%, respectively. As evergreen tree (Pinus), the interception of PM2.5 proportion was significantly higher compared to other deciduous trees (Hemiptelea, Buxus, and Syringa) (P<0.05), but there was no significant difference in the PM2.5–10 and >PM10 proportion (P>0.05). It indicated that evergreen plants were better at trapping fine particles. This is similar to the results of other studies. More than 80% of the particles intercepted were PM10 of several common green plants in Qingdao [30]. PM larger than 10 μm comprised 73% of PM deposited on leaves in Beijing, with the coarse and fine particle size fractions comprising 16% and 11% of the deposited PM, respectively [22].Figure 3
Particle size distribution during dew condensation and after dew evaporation among different plant species.


## 3.2. Particles Settled by Leaves after Dew Evaporation

After dew evaporation, the ability of the four kinds of trees to retain particulate matter was obviously different. The proportions of intercepted TSP settled byPinus, Hemiptelea, Buxus, and Syringa leaves were 89.7 ± 3.9%, 80.6 ± 3.6%, 75.9 ± 4.5%, and 71.4 ± 3.7%, respectively (see Figure 4). The ability of Pinus (evergreen) to trap particles after dew evaporation was significantly higher compared to several other deciduous tree species. The ability of the four kinds of trees to trap fine particles on their leaves was significantly higher than that for coarse particles (P<0.05). Taking Syringa as an example, the leaf interception ratios of PM10, PM2.5–10, and PM2.5 after dew evaporation were 65.1 ± 3.9%, 83.7 ± 5.5%, and 88.5 ± 7.3%, respectively. After the dew evaporation process, some of the particles were still trapped on the surface of the leaves, and some of them left. The proportion of fine particles trapped in the leaves was higher. After dew evaporation, the particles trapped on the leaves were still mainly coarse particles, but the proportion of PM2.5–10 increased to 30.7%–33.7%, and the proportion of PM2.5 increased to 11.6%–17.6%. Both the dew condensation and the secondary suspension process were dominated by coarse particles.Figure 4
The amount of particles by different leaves and the removal rate of particles of different particle sizes.There was no significant difference in the mass of particulate matter captured by leaves of different tree species per unit area during dew condensation, but the deposited PM loads varied significantly among individual tree species due to the leaf area (see Table1). Syringa showed the highest PM deposition, followed by Hemiptelea, Pinus, and Buxus. According to the calculation of 130 dew days in one year, the settlement of TSP by one Syringa, Hemiptelea, Pinus, and Buxus individual was 15.17 g/y, 12.87 g/y, 2.97 g/y, and 1.07 g/y, respectively, during dew condensation. The remaining TSP trapped by these leaves was 10.83 g/y, 10.35 g/y, 2.67 g/y, and 0.81 g/y, respectively, after dew evaporation. Evergreen or deciduous plants cannot differentiate the intercept of the TSP amount. As the leaves’ area increases for each species, the TSP values should be slightly higher than the actual TSP considering the leaves inclination and mutual shadowing with respect to the vertical direction.


## 3.3. Factors Affecting Leaf Interception of Particles

The relationship between precipitation and particles captured by leaves of the four plants during the dew deposition period was close. The mass of particles on leaves decreased after precipitation (see Figures2 and 5). For example, the cumulative precipitation reached 53.3 mm from August 13 to 16, and on August 18, the deposition of dew particles in the four plants showed a significant declining trend. This may have been due to purification by precipitation, i.e., the dew captured few particles and resulted in a reduction in interception [10]. With the increase in pollutants in the atmosphere after rain, samples were taken continuously on August 19 and 20, and the deposition of particles in dew gradually increased.Figure 5
The rainfall from June to September and the meteorological factors during dew evaporation in 2020.As demonstrated in Table2, there was no relationship between wind speed and particulate matter interception amount of four plants (P>0.05). Some researchers found the correlation was positive or negative depending on the local wind speed values, and there was a threshold in the correlation between wind speed and particle concentration [34, 35]. In this study, neither mean wind speed (0.83 m/s) nor 1 m/s wind speed was related to particulate interception (P>0.05). The wind speed had no significant effect on the particulate matter during the dew evaporation period. This is because dew formation events always occur at wind speeds under 2 m/s, and high wind speed conditions are not conducive to dew formation. In addition, the dew evaporation period lasted 3 to 4 hours, and wind has little effect on particulate matter in a short time. The relative humidity during the dew evaporation period was positively correlated with the interception amount (P<0.05) because the particulate matter was not easily dispersed with high air humidity. Air temperature or dew point temperature had no correlation with the interception amount (P>0.05).Table 2
Correlation coefficients between wind speed and particulate matter interception amount.
PinusHemipteleaBuxusSyringaWind speed−0.1830.1640.032−0.208>Mean wind speed0.2210.3430.2970.047<Mean wind speed−0.2280.1210.038−0.189>1.0 m/s wind speed0.3220.0470.2430.210<1.0 m/s wind speed0.1750.1630.0040.153The four kinds of trees showed little difference in terms of trapping particulate matter in the dew deposition stage, but there were significant differences in their ability to trap particles after the evaporation process (see Figure4). The ability of plants to capture particle matter is related to multiple factors, including leaf shape and the leaf surface microstructure [36]. According to their leaf morphology, conifers have the strongest ability to trap particles in dew. Conifers exhibited higher PM2.5 adsorption amounts than broadleaf trees [37]. The oil secreted by Pinus leaves has strong particle adsorption properties (see Figure 6(c)), which also makes it difficult for particles to fall off. The leaves of Hemiptelea have serrated edges (see Figure 7(c)), which can also make them more strongly adsorb particles in dew.Figure 6
Microstructures of different species with SEM. (a–c) Leaf surface ofPinus (×500; ×600; ×1000); (d) adaxial leaf surface of Hemiptelea (×1000); (e) stomata of the abaxial leaf surface of Hemiptelea (×2000); (f) the base of one of a dense mat of glochid trichomes of Hemiptelea (×4000); (g) adaxial leaf surface of Buxus (×2000); (h) abaxial leaf surface of Buxus (×3000); (i) one stomata of the abaxial leaf surface of Buxus (×4000); (j) adaxial leaf surface of Syringa (×400); (k) abaxial leaf surface of Syringa (×1000); (l) one stomata of the abaxial leaf surface of Syringa (×8000). The structure in the the red rectangle is more inclined to capture particulate matter.
(a)(b)(c)(d)(e)(f)(g)(h)(i)(j)(k)(l)Figure 7
Drop contact angle (DCA) and leaf morphology of different species. (a) DCA of theHemiptelea adaxial surface; (b) DCA of the Hemiptelea abaxial surface; (c) photograph of the Hemiptelea leaf; (d) DCA of the Buxus adaxial surface; (e) DCA of the Buxus abaxial surface; (f) photograph of the Buxus leaf; (g) DCA of the Syringa adaxial surface; (h) DCA of the Syringa abaxial surface; (i) photograph of the Syringa leaf).
(a)(b)(c)(d)(e)(f)(g)(h)(i)From the aspect of microstructure, the trichome density and length ofHemiptelea leaves was significantly higher compared to Syringa and Buxus leaves (see Figure 6). The trichome surface structures were helpful in enhancing the effective surface area for PM adsorption. Species with trichomes accumulated significantly more particulate matter than other species [38]. Particulate matter adsorption can be observed at the tip and base of the trichomes of Hemiptelea (see Figures 6(d), 6(g), and 6(j)). The difference between the removal efficiencies of T. usneoides with and without trichome structures was approximately 7% for PM2.5 and 2% for PM10 [39]. The effect of trichome structure is particularly notable for the adsorption of PM2.5. Trichome density has a significantly positive correlation with the maximum particle size (P<0.05), and when the wind force is weak, stomatal density and trichome density have a significant effect on particulate matter resuspension [17]. Therefore, Hemiptelea can capture more PM (see Figure 6(f)) than Syringa and Buxus.As Figure6 shows, many fine and coarse particles accumulated on the adaxial leaf surfaces, while some large particles attached to the abaxial leaf surfaces, and few particles became attached in the vicinity of the stomata. Grooves are the main parts of a blade that absorb PM2.5. Deep grooves can intercept more particles (see Figure 6(k)); additionally, deeper grooves make the release of PM less likely. Ridge ultrastructures at 1-2 μm scale are the most efficient at capturing PM. The stomatal densities of Syringa, Hemiptelea, and Buxus were 213.0 ± 20.5/mm2, 92.5 ± 7.3/mm2, and 84.5 ± 18.6/mm2, respectively. Both stomata and trichomes are adjunct structures that are conducive to the adsorption of particles. Different shapes of leaves captured particles in dew with different ways. Evergreen tree with acicular leaves such as Pinus adhered to particulate matter by the secreted oil. While deciduous plants trapped to particulate matter around its unique microstructure, deep grooves or dense trichome such as the elliptical leave (Hemiptelea) used dense trichome to trap particulate matter (see Figure 6(f)) and the compound leave (Syringa) used deep grooves (see Figure 6(k)).Otherwise, the content of wax on the surface ofSyringa (0.79 ± 0.09 g/m2) was higher than that of Buxus (0.38 ± 0.15 g/m2) and Hemiptelea (0.30 ± 0.11 g/m2), and the DCA of Syringa (adaxial surface 143.2° ± 6.5° and abaxial surface 123.4° ± 10.7°) was also higher than that of Buxus (adaxial surface 107.5° ± 16.5° and abaxial surface 95.3° ± 12.7°) and Hemiptelea (adaxial surface 80.24° ± 11.7° and abaxial surface 44.2° ± 7.8°) (see Figure 7). 10° < DCA < 90° indicates hydrophilic leaves, and 90° < DCA < 150° indicates hydrophobic leaves. Syringa leaves have a smoother surface and stronger hydrophobicity than Buxus and Hemiptelea davidii leaves. Leaves with strong hydrophobicity do not absorb particles easily.


## 4. Conclusions

Through the monitoring of leaf particle deposition during the dew condensation and evaporation periods of four common urban plants, it was found that the amounts of TSPs settled byPinus, Hemiptelea, Buxus, and Syringa trees in the dew condensation period were 0.21 ± 0.07, 0.21 ± 0.06, 0.22 ± 0.06, and 0.21 ± 0.06 μg/cm2, respectively. There was no significant difference in the TSP amounts in the dew condensation period by leaves per unit area among tree species (P>0.05). Coarse particulate matter (>PM10) accounted for 56.7%–62.4% of PM and was the main type of deposit in the dew condensation stage. According to the difference in leaf shape (conifer and broadleaf), microstructure (stomata and trichomes), and hydrophobicity, the proportion of the TSP trapped by Pinus, Hemiptelea, Buxus, and Syringa was 89.7 ± 3.9%, 80.6 ± 3.6%, 75.9 ± 4.5%, and 71.4 ± 3.7%, respectively, after dew evaporation. The fine particle interception ability of leaves accounted for 88.5 ± 7.3% of the condensed fine particles for the four kinds of trees and was significantly higher than that for coarse particles (65.1 ± 3.9%) (P<0.05) during the dew evaporation process. Temperature and wind speed had no significant effect on the secondary suspension of particles in the dew evaporation period (P>0.05). More particles were trapped by leaves at higher relative humidity in the morning than at other times. Based on 130 dew days per year, the settlement of TSP by one Syringa, Hemiptelea, Pinus, and Buxus individual was 15.17 g/y, 12.87 g/y, 2.97 g/y, and 1.07 g/y, respectively, during dew condensation, and the corresponding values were 10.83 g/y, 10.35 g/y, 2.67 g/y, and 0.81 g/y after dew evaporation. Identifying sources of particles with different particle sizes can help to understand the trajectory of particles and reveal the role of plants in trapping particles. So, the source analysis of particulate matter should be further discussed.



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*Source: 7157012-2022-08-18.xml*