The effectiveness of urban trees in reducing airborne particulate matter by dry deposition in Tehran, Iran

Abstract

Deposition of atmospheric pollution as particulate matter (PM) has become a serious issue in many urban areas. This study measured and estimated the amount of atmospheric PM deposition onto oriental plane (Platanus orientalis L.) trees located in Tehran Megapolis, Iran. PM deposited on the leaves of urban trees during spring and summer was estimated using leaf wash measurements. In addition to direct measurements, the dry deposition velocity and the yearly whole-tree PM deposition were estimated using both field measurements and a theoretical model of deposition flux. We estimated air quality improvement as a result of the trees at respiratory height (1.5 m), tree height (10 m), and boundary layer height (1719 m). Foliar PM deposition during spring and summer was estimated to average 0.05 g/leaf and 41.39 g/tree using direct measurements. The annual PM deposited on the leaves, trunk, and branches of an average urban tree was calculated to be 78.60 g/tree. Trees were estimated to improve air quality at 1.5 m, 10 m, and 1719 m from ground level by 25.8%, 5.8%, and 0.1%, respectively. Hence, oriental plane trees substantially reduce PM at respiratory height.

Appendices

Appendix 1

Paraboloid outer surface area equation (S; m²) was used to calculate the outer or lateral area of tree’s trunk (Vcalc, 2018):

$$S=\pi {b}^{2}+\frac{\pi b}{{6h}^{2}}\left[{\left({b}^{2}+{4h}^{2}\right)}^\frac{3}{2}-{b}^{3}\right]$$

(15)

where b and h are respectively the diameters (m) at the tree collar and the tree height.

Appendix 2

McGowan and Clark (2008) estimated the relationship between height and PM₁₀ concentration in Queensland, Western Australia as below equation:

$$H=4.27e+05\times {e}^{\left(-0.089\times P{M}_{10}\right)}$$

(16)

The above equation was rewritten as follows:

$$P{{M}_{10}}_{\left({H}_{i}\right)}=\frac{Ln\left({H}_{i}\right)-12.96}{0.089}$$

(17)

In the next step, the integral of Eq. 17 was used to calculate the cumulative PM₁₀ values (CPM₁₀) in each height range (Hi) (Eq. 18). This equation was calculated for four height ranges (Hi) including the respiratory height (1.5 m), the average height of the tree (10 m), and the atmospheric boundary layer height (m).

$$CP{M}_{10}={\int }_{0}^{{H}_{i}}\frac{\mathit{ln}\left({H}_{i}\right)-12.96}{0.089}$$

(18)

The average value of PM₁₀ in each height range was obtained by dividing the integral value (Eq. 18) by the corresponding height (Hi). Average PM₁₀ concentration in each height range (Hi) of the study site was calculated (µg m⁻³) using the PM₁₀ concentration (measured at standard pollution measurement height of 4 m that equal to 130.04 µg m⁻³) at the study site and the ratio between the average PM₁₀ value in each height range and the PM₁₀ concentration in McGown and Clark (2008).