Name: assessing the particulate matter removal abilities of tree leaves
Uploaded: 2018-10-07T15:00:00
Description: Watch this Scientific Journal Video about Assessing the Particulate Matter Removal Abilities of Tree Leaves at JoVE.com

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...

<ol>
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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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		<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...

Watch this Scientific Journal Video about Assessing the Particulate Matter Removal Abilities of Tree Leaves at JoVE.com

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 ...

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