This work was supported by the Fundamental Research Funds for the Central Universities (2017ZY21) and the National Natural Science Foundation of China (21607038).

1. Leaf Collection, Elution and Mass Measurement of PM
<ol>
	<li>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 (w...

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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,&#160;Fig...

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 method1,2, the membrane filter method3,4,5, and the elution-weighing method coupled with particle size analysis6 have been applied to quantitatively estimate the mass of PM2.5 (diameter &#8804; 2.5 &#181;m), PM10 (diameter &#8804; 10 &#181;m) 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 brushing5,8,9. However, some studies10,11 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 PM2.5, PM10, or TSP on unit leaf area has often been utilized to evaluate the abilities of different tree species to remove PM from ambient air14,15. 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 trees5,17. 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&#176; N, 116.36&#176; 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, PM2.5, PM5, PM10, and TSP.

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	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:349px;">MSA2258-1CE-DU ten-thousandth scale</td>
			<td style="width:328px;">Sartorius Scientific Instruments (Beijing) Co., Ltd.</td>
			<td style="width:153px;">MSA2258-1CE-DU</td>
			<td style="width:688px;">precision: 0.01 mg</td>
		</tr>
		<tr height="22">
			<td height="22" style="height:22px;">The IS13320 laser granularity instrument</td>
			<td>Beckman Coulter, Brea, USA</td>
			<td>IS13320</td>
			<td>working conditions: liquid/power samples; particle size range of measurement: 0.017-2000 &#956;m</td>
		</tr>
		<tr height="23">
			<td height="23" style="height:23px;">Epson Twain Pro high-quality scanner</td>
			<td>Seiko Epson, Nagano, Japan</td>
			<td>expression1680</td>
			<td></td>
		</tr>
		<tr height="23">
			<td height="23" style="height:23px;">Automatic image analysis software WinRHIZO</td>
			<td>Regent Instruments Inc., Quebec, Canada</td>
			<td>WinRHIZO Pro 2013a</td>
			<td></td>
		</tr>
	</tbody>
</table>

assessing the particulate matter removal abilities of tree leaves

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 (AEleaf) 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.

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.</p...

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Assessing the Particulate Matter Removal Abilities of Tree Leaves

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.

Based on the conventional cleaning methods (water cleaning (WC) + brush cleaning (BC)), this study evaluated the influence of ultrasonic cleaning (UC) on ...

This method can help answer key questions in the relationship between urban forests and particulate matter pollution field, such as how to select tree species with high particulate matter removal abilities. The main advantage of this technique is that the PM removal abilities of trees can be assessed accurately and quantitatively by collecting the PM retained on leaf surface completely. To begin, select five healthy individual trees with similar diameter at breast height of each tree species.Then, randomly collect four larger branches from four directions of the out canopy in the middle canopy layer, and cut off all intact leaves. Place the leaves in labeled valve bags before transporting the bags to the laboratory and storing them in the fridge. Next, wash and dry beakers in an oven set to 80 degrees Celsius.Equilibrate the beakers to room temperature and humidity before weighing them. After this, place randomly selected leaves into one of the large beakers. Then, add 270 milliliters of DI water to beaker A, and immerse the leaves completely.Stir the water for 60 seconds in one direction with a glass rod. Then, evenly pour the eluent into three smaller beakers. Next, add 270 milliliters of DI water to a different large beaker, and immerse the leaves in water once more.Using a nylon brush, scrub the leaf surfaces with the DI water, taking care to avoid destroying the leaf microstructure. Next, wash the leaves in DI water with a squeezable bottle with a fine tip, and transfer them to a new large beaker. After this, pour the eluent into a clean set of three smaller beakers.Add 270 milliliters of DI water to the large beaker, and immerse the leaves in water again. Then, place the beaker in an ultrasonic cleaning machine for three to 10 minutes, and stir the leaves in one direction with a glass rod. Then, transfer the leaves to a new large beaker.Next, pour the eluent into a new set of three small beakers. Cover each of the sets of three small beakers with clean filter paper, and dry the beakers in the drying oven for five days or until the mass of the beakers becomes constant. After this, put the beakers on a laboratory bench for 30 minutes, and measure the mass of each beaker.First, add 50 milliliters of DI water to each of the small beakers. Then, place the beakers in the ultrasonic cleaning machine for 30 minutes until the PM disperses in the liquid. Add the supernatant from the beakers to a laser granularity instrument, and measure the size distribution of the PM eluted in the various cleaning steps.Next, spread the leaves on a plastic board, and scan the leaves with a scanner. Finally, use image analysis software to estimate the surface area and projected area of the leaves. In this study, PM was eluted from the leaves of five species of trees via different cleaning methods.A large number of different-sized PM were eluted from leaf surfaces after supplement ultrasonic cleaning to water cleaning plus brush cleaning. The average eluted proportions of various-sized PM of the five tree species by ultrasonic cleaning were 41%and 36%under the short dust retention and long dust retention periods, respectively. While attempting this procedure, it's important to remember to control the ultrasonic pulse and ultrasonic time strictly.Following this procedure, other methods like collecting the PM retained in waxy layer of leaves can be performed in order to answer additional questions, like the production of the mass of PM on surface and in waxy layer of leaves. After its development, this technique paved the way for research in the field of forest and particulate matter pollutions to explore the particulate matter retained on leaves in selecting tree species with how particulate matter removal abilities.

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6.7K visualizações.  Beijing Forestry University. Este método pode ajudar a responder a perguntas-chave na relação entre florestas urbanas e campo de poluição de matérias particuladas, como como selecionar espécies de árvores com altas habilidades de remoção de material particulado. A principal vantagem dessa técnica é que as habilidades de remoção de PM das árvores podem ser avaliadas de forma precisa e quantitativa, coletando a PM retida na superfície da folha completamente. Para começar, selecione cinco árvores individua...