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.
