Urban trees play an important role in reducing air pollution. They remove gaseous pollutants– including SO2, O3, and NO2– via uptake through stomata. These pollutants diffuse into the leaf, and may form acids or participate in reactions inside the leaf. Some air pollution is also removed by the surface of the tree. Trees can reduce airborne pollutants via the interception of particles.
Trees do emit volatile organic compounds (VOCs) that play a role in the formation of ozone and carbon monoxide. However, low VOC emitting urban trees in particular are suitable candidates for the reduction of ozone levels.
Per acre of tree cover, the economic benefits of air pollution reduction are said to be worth around $300 dollars. Computer simulations suggest that in 2010, 17.4 million tonnes of air pollutants were removed by trees in the US. In Strasbourg, from 2012 to 2013, public trees helped remove an estimated 88 tonnes of pollutants.
Air pollution is seen to negatively impact human health, affecting cardiac, neurological, and pulmonary systems. By helping reduce pollutants in the air, urban trees play an important role in improving air quality and human health.
Air pollution affects tree growth: ozone and particulate matter can negatively impact the leaves' gas exchange system and therefore the development of the tree. One study found that the tree Tipuana tipu was negatively impacted by the airborne pollutants Al, Ba, and Zn, and by particulate matter. The study noted that particulate matter accumulates on leaf surfaces, altering the properties of the leaves and preventing photosynthesis.
Works Cited
Baldocchi, Dennis D., et al. “A Canopy Stomatal Resistance Model for Gaseous Deposition to Vegetated Surfaces.” Atmospheric Environment (1967), vol. 21, no. 1, Jan. 1987, pp. 91–101, https://doi.org/10.1016/0004-6981(87)90274-5.
Locosselli, Giuliano Maselli, et al. “The Role of Air Pollution and Climate on the Growth of Urban Trees.” Science of the Total Environment, vol. 666, May 2019, pp. 652–661, www.sciencedirect.com/science/article/pii/S0048969719307892, https://doi.org/10.1016/j.scitotenv.2019.02.291. Accessed 13 Nov. 2019.
Nowak, David J., et al. “Tree and Forest Effects on Air Quality and Human Health in the United States.” Environmental Pollution, vol. 193, no. 193, Oct. 2014, pp. 119–129, www.sciencedirect.com/science/article/pii/S0269749114002395, https://doi.org/10.1016/j.envpol.2014.05.028.
Nowak, David, and Gordon Heisler. “Air Quality Effects of Urban Trees and Parks.” Research Series Monograph. Ashburn, VA: National Recreation and Parks Association Research Series Monograph. 44 P., 2010, pp. 1–44, www.fs.usda.gov/research/treesearch/52881.
Selmi, Wissal, et al. “Air Pollution Removal by Trees in Public Green Spaces in Strasbourg City, France.” Urban Forestry & Urban Greening, vol. 17, June 2016, pp. 192–201, https://doi.org/10.1016/j.ufug.2016.04.010.
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