Bounce-Back Effect - The Complex Relationship Between Trees and Pollution

This article was originally published in August 2019 in WSP’s magazine The Possible

“An avenue of trees in a city street might look beautiful but, as far as local air pollution is concerned, they may be doing more harm than good.”

John Gallagher, assistant professor in environmental systems at Trinity College Dublin, is being deliberately provocative, and immediately explains himself: “Of course, the presence of trees in cities is overwhelmingly positive. They provide shade, ameliorate the heat-island effect, encourage biodiversity, improve mental health and are aesthetically pleasing.” And yes, he adds, they can play a very useful role in mediating pollution too — as long as they are positioned and managed carefully.

“I look at trees as physical structures and assess the effect they have on the dispersion of pollutants,” he says. “If tall trees with dense canopies are planted close together in a narrow street canyon, then the canopy may act as a green roof. It can potentially trap vehicle emissions at street level and restrict natural air flow, preventing pollution from escaping.”

With the right approach, however, results can be positive and impressive. “Recent research has identified that tall, narrow tree canopies are more suitable as they allow adequate ventilation, and that we should prioritize low hedgerows along the roadside,” says Gallagher. “These intercept particulate matter (PM) and reduce exposure for people walking on the pavement, and can lead to a minimum of 14% less PM locally.” This is because the plants act as a baffle, altering airflows, either deflecting polluted air away from potential recipients such as pedestrians, or by slowing currents and thereby encouraging PM to settle out. Pollutants can also deposit themselves on the large surface area presented by the leaves, and plants absorb small amounts of nitrogen dioxide (NO₂) and other undesirable gases by breathing them in through their stomata (pores), though the overall effect of this is small.

To get the best from these useful qualities, it is vital to choose both the plant and its location carefully. “For example, a dense hedge can create a bounce-back effect, trapping pollution on the road, stopping it dispersing. That might make things worse for cyclists.”

As well as vegetation, Gallagher has started to look at the impact of low walls and also parked cars. “Walls and cars can actually work well: they create a solid barrier between traffic and pedestrians, with cars providing an additional space, although gaps between the cars allow pollutants to disperse.” 

As always, says Gallagher, any consideration of airflow-altering objects must take into account local conditions such as the prevailing winds, the height of surrounding buildings and what the intervention is precisely meant to achieve: “Would you prefer a baffle or a barrier? Do you want to prevent or encourage the dispersion of PM?”

Gallagher is part of an international group of scientists studying how trees and other objects or street features such as shop awnings and cafe barriers passively affect air pollution dispersion and deposition. “The difficulty for planners is that they cannot analyze every situation in depth, so we are developing a centralized resource that can be used to determine what kind of baffles or barriers — vegetation or otherwise — will meet air pollution mitigation requirements.” It’s a slow process to collate a reliable set of evidence, but Gallagher invites planners to reach out to his group in the meantime. This year, the group is also hosting a webinar series on passive air pollution mitigation: tcd.ie/civileng/air-pollution-webinar-series

 This article appeared in The Possible issue 05, as part of a longer feature on improving air quality in cities