Assessing the Particulate Matter Removal Abilities of Tree Leaves

Summary

The ultrasonic cleaning method was applied to elute the particulate matter (PM) retained on leaf surfaces after PM was eluted by the conventional cleaning methods (water cleaning only or water cleaning plus brush cleaning). The methodology can help to improve the estimation accuracy for PM retention capacity of leaves.

Abstract

Based on the conventional cleaning methods (water cleaning (WC) + brush cleaning (BC)), this study evaluated the influence of ultrasonic cleaning (UC) on collecting various sized particulate matter (PM) retained on leaf surfaces. We further characterized the retention efficiency of leaves to various sized PM, which will help to assess the abilities of urban trees to remove PM from ambient air quantitatively.

Taking three broadleaf tree species (Ginkgo biloba, Sophora japonica, and Salix babylonica) and two needleleaf tree species (Pinus tabuliformis and Sabina chinensis) as the research objects, leaf samples were collected 4 days (short PM retention period) and 14 days (long PM retention period) after the latest rainfall. PM retained on the leaf surfaces was collected by means of WC, BC, and UC in sequence. Then, retention efficiencies of leaves (AE_leaf) to three types of the various sized PM, including easily removable PM (ERP), difficult-to-remove PM (DRP), and totally removable PM (TRP), were calculated. Only around 23%-45% of the total PM retained on leaves could be cleaned off and collected by WC. When the leaves were cleaned through WC+BC, the underestimation of the PM retention capacity of different tree species was in the range of 29%-46% for various sized PM. Almost all PM retained on leaves could be removed if UC was supplemented to WC+BC.

In conclusion, if the UC was complemented after the conventional cleaning methods, more PM on leaf surfaces could be eluted and collected. The procedure developed in this study can be used for assessing the PM removal abilities of different tree species.

Introduction

The abilities of different tree species to remove PM from ambient air can be assessed through quantifying the mass of PM retained on leaf surfaces. To achieve this objective, the subtraction method^1,2, the membrane filter method^3,4,5, and the elution-weighing method coupled with particle size analysis⁶ have been applied to quantitatively estimate the mass of PM_2.5 (diameter ≤ 2.5 µm), PM₁₀ (diameter ≤ 10 µm) or total suspended particulate (TSP) retained on leaves. However, the accuracy of these methods basically depends on their performance in collecting PM retained on the leaf surfaces. At present, the conventional leaf cleaning method used in related studies often includes one or two steps, namely only water washing (soak and rinse leaves using deionized water)^3,7 or plus brushing^5,8,9. However, some studies^10,11 have demonstrated that PM on leaf surfaces could not be completely eluted by the conventional cleaning method. As ultrasonic cleaning has the advantages of high speed, high quality, and little damage to the surface of the object, it has great potential to be used to collect the PM retained on leaf surfaces with complex microstructures. At present, ultrasonic cleaning has been applied in some studies to collect PM retained on leaf surfaces (i.e., put the leaves into deionized water, and use the ultrasonic cleaner to elute PM)^12,13. However, this method is only used as a supplement to a leaf cleaning method, while it is not known whether the ultrasonic cleaning has a positive effect on collecting PM from leaf surfaces and its optimum operating parameters are also not clear. Our previous research has shown that the PM retained on Ginkgo biloba leaf surface could be completely eluted without destroying the leaf surfaces, if a proper ultrasonic cleaning procedure was supplemented to the conventional cleaning method¹¹. However, the stability and general applicability of the ultrasonic cleaning parameters (ultrasonic power, time, and other information) to different plant species experiencing different dust retention periods are still not clear.

Currently, the mass of PM_2.5, PM₁₀, or TSP on unit leaf area has often been utilized to evaluate the abilities of different tree species to remove PM from ambient air^14,15. Under the natural condition, the PM retained on leaf surfaces can be classified into two parts: the first part is the PM that can fall off leaves due to the effects of wind and rainfall, while the other part is the PM that is tightly adhered to leaf surfaces and cannot be easily washed off by rainfall. However, few studies have focused on the mass of both types of PM on leaf surfaces. In addition, the PM retention periods of leaves in different studies differ enormously. Thus, the comparability of the results of these studies will be poor, if the mass of PM retained on unit leaf area is adopted to assess the PM removal abilities of trees¹⁶. Consequently, the PM retention efficiency (the mass of PM retained on unit leaf area per unit time), as an alternative, was proposed to evaluate the PM purification effects of urban trees^5,17. In general, there is still a lack of research in this aspect. It is extremely necessary to carry out relevant studies for different tree species to provide methodological basic and data support for assessing the PM removal abilities of different tree species accurately.

Here, three broadleaf tree species (G. biloba, Sophora japonica, and Salix babylonica) and two needleleaf tree species (Pinus tabuliformis and Sabina chinensis) were selected to evaluate their PM removal abilities under two PM retention periods. The leaf sampling site was in Xitucheng Park (39.97° N, 116.36° E), located in an area with heavy pollution in Beijing. The three specific objectives of this study were: (1) to assess the efficiency of different leaf cleaning methods (water cleaning (WC), brush cleaning (BC), and ultrasonic cleaning (UC)) in eluting the PM on leaves, (2) to verify the effect of ultrasonic cleaning on eluting PM, and (3) to assess the retention efficiency of different tree species to PM₁, PM_2.5, PM₅, PM₁₀, and TSP.

Protocol

1. Leaf Collection, Elution and Mass Measurement of PM

1. Select five healthy individual trees (i.e., five replicates) of each tree species with similar diameter at breast height. Collect four larger branches randomly from four directions of the outer canopy in the middle canopy layer and cut off all intact leaves.
   NOTE: All plants for leaf sampling should be located closely in a greening strip with length and width of about 250 and 60 m, respectively, to ensure that the environment conditions (wind, light, and rain) of these trees are similar. The leaves used in the protocol were collected on October 15th (short dust retention (SDR) period) and October 25th (long dust retention (LDR) period) in 2014, which were 4 and 14 days after the latest rainfall (> 15 mm), respectively. The average levels of PM under the short and long dust retention period (i.e., the duration between the last rainfall and the leaf sampling time) in our experiment were 26 (PM_2.5), 57 (PM₁₀), and 111 (PM_2.5), 160 µg/m³ (PM₁₀), respectively.
   1. Place the sampled leaves in labeled valve bags and transport the bags to the laboratory immediately. Store the leaf samples in the fridge.
2. Wash and dry the beakers in the 80 °C oven. Equilibrate the beakers to room temperature and humidity and weigh the empty beakers (W₁).
3. Randomly select a certain amount of leaves from leaf samples and put the leaves in a 1000 mL beaker (Beaker A).
   NOTE: The leaf area is about 2000 cm², which can guarantee all the leaves can be immersed in the water completely and the eluted dust has sufficient weight to be weighed accurately.
4. Add 270 mL of deionized water to the Beaker A and immerse the leaves in water completely.
   1. Stir the water for 60 s with a glass rod in one direction (frequency: 2 seconds for one rotation). Afterwards, pour the eluent into three 100 mL small beakers (Beaker a) evenly.
   2. Wash the leaves using a fine tipped squeeze bottle with 30 mL of deionized water and transfer the washed leaves to a 1000 mL beaker (Beaker B). Pour the eluent into three 100 mL small beaker (Beaker a) evenly.
5. Add 270 mL of deionized water to Beaker B and immerse the leaves in water again. Then use a nylon brush to scrub the leaf surface (placing on flat thin plastic plate) with deionized water and avoid destroying the microstructure of leaf surface. Pour the eluent into three 100 mL small beakers (Beaker b).
   1. Wash the leaves using the squeezable bottle with fine tip with 30 mL of deionized water and transfer the leaves into a 1000 mL beaker (Beaker C). Pour the eluent into three 100 mL small beaker (Beaker b).
6. Add 270 mL of deionized water to Beaker C and immerse the leaves in water again.
   1. Put the glass container into the ultrasonic cleaning machine. Using an ultrasonic power of 500 W, clean for 3 min and 10 min for leaves of the broadleaf and needleleaf tree species, respectively. Stir the leaves with a glass rod in one direction (frequency: 2 seconds for one circle) simultaneously.
   2. Wash the leaves using the squeezable bottle with fine tip with 30 mL of deionized water and pour the eluent into three 100 mL small beakers (beaker c).
7. Cover a piece of clean filter paper (diameter = 11 cm, area = 94.99 cm²) on each beaker (a, b, c) and dry the beakers in the 80 °C oven for approximately 5 days until the mass of the beakers becomes constant.
   1. Put the beakers in a balance chamber to equilibrate the temperature and humidity for 30 min, and weigh the mass of each 100 mL beakers (W₂). Calculate the mass of PM eluted by each cleaning step by W₂-W₁.

2. Measurement of PM Size Distribution and Leaf Area

1. Add 50 mL of deionized water to each weighed beaker (a, b, c) mentioned above and place these beakers in an ultrasonic cleaning machine for 30 min until the PM disperses in deionized water.
2. Add the supernatant in beaker (a, b, c) to the laser granularity instrument and measure the size distribution of PM eluted by different cleaning steps.
   1. Assume the measured volume percentages to be mass percentages (Q) of different-sized particles. Calculate the proportion of different-sized particles eluted by each cleaning step by equation (1):
         (1)
      where P_i,j represents the mass proportion (%) of the particles within the j diameter class eluted from the leaf surfaces by the cleaning step i; W_i represents the total mass (g) of all sized particles eluted by the cleaning step i; Q_i,j represents the mass percentage (%) of the particles within the j diameter class in the total PM mass eluted by the cleaning step i; i is the cleaning step (i.e., WC, BC, and UC); and j is the diameter class, which was set to d ≤ 1 µm (PM₁), 1 < d ≤ 2.5 µm (PM_1-2.5), 2.5 < d ≤ 5 µm (PM_2.5-5), 5 < d ≤ 10 µm (PM_5-10), d > 10 µm (PM_>10) in the present study.
3. Spread leaves on the plastic board and scan the leaves with a high-quality scanner. Use automatic image analysis software to estimate the surface area and projected area of leaves.
   NOTE: The protocol can be paused here.

3. Data Presentation and Analysis

1. Calculate the total removable particulate matter (TRP) as the sum of the ERP and DRP that can be eluted by WC + BC + UC.
2. Under different dust retention periods, calculate the total mass of the PM within a specificdiameter class retained on leaves as the sum of the mass of the PM within corresponding diameter class eluted by the different cleaning steps (i.e., WC, BC, and UC).
   1. Using these data and the leaf area data, calculate the retention efficiency (AE_leaf) of the various sized particles on unit leaf surface area using equation (2):
           (2)
      where LZ_j and SZ_j are the mass (g) of the particles within the j diameter class retained on unit leaf area under the periods of LDR and SDR, respectively; LT and ST are the numbers of days in the periods of LDR and SDR, respectively.
3. Conduct all the statistical analyses with SPSS software.
   1. Use the Kolmogorov-Smirnov test and the Levene test to verify the ANOVA assumptions of normality and the homogeneity of variances, respectively, for the elution percentages of the different-sized particles and the PM retention capacity data.
   2. Apply one-way ANOVA to investigate the effects of the different cleaning steps on the elution percentages of the different-sized particles under various dust retention periods. Use Duncan's test (P = 0.05) to detect the significant differences among different cleaning steps.

Results

The PM retained on leaf surfaces had two types under natural conditions. The PM falls off easily by rainfall and wind under natural conditions is defined as the easily removable particulate matter (ERP). This type of PM was represented by the PM eluted by WC in this study. The PM that tightly adheres to leaf surfaces and cannot be easily washed off by BC and UC is defined as the difficult-to-remove particulate matter (DRP). This kind of PM cannot be eluted by natural rainfall and wind.

Discussion

Accurate and proper collection of the PM retained on leaf surfaces is the basis for assessing the PM removal abilities of different tree species. However, the conventional cleaning method (WC or plus BC) cannot completely remove the dust on leaf surfaces, which has been confirmed by scanning electron microscopy¹⁰. This was further demonstrated clearly by the present study (Figure 1, Figure 2, Fig...

Materials

Name	Company	Catalog Number	Comments
MSA2258-1CE-DU ten-thousandth scale	Sartorius Scientific Instruments (Beijing) Co., Ltd.	MSA2258-1CE-DU	precision: 0.01 mg
The IS13320 laser granularity instrument	Beckman Coulter, Brea, USA	IS13320	working conditions: liquid/power samples; particle size range of measurement: 0.017-2000 μm
Epson Twain Pro high-quality scanner	Seiko Epson, Nagano, Japan	expression1680
Automatic image analysis software WinRHIZO	Regent Instruments Inc., Quebec, Canada	WinRHIZO Pro 2013a