Impact of Roadside Tree Lines on Indoor Concentrations of Traffic-Derived Particulate Matter

By Barbara A. Maher, Imad A. M. Ahmed, Brian Davison, Vassil Karloukovski, and Robert Clarke

Lancaster, UK (November 11, 2013) – New research shows trees planted along a city street screen residents from tiny particles that pollute urban air. Tree leaves can capture more than 50% of the particulate matter that’s a prime component of urban pollution and a trigger for disease.

In urban settings, particulates come primarily from car exhaust, brake pad wear, and road dust and can contain metals, such as iron and lead. Exposure to airborne particulate pollution is associated with premature mortality and a range of inflammatory illnesses, linked to toxic components within the particulate matter assemblage. These particles are tiny enough for people to inhale and can exacerbate heart disease, asthma, and other health conditions.

Researchers want to understand how trees capture particles so that urban planners might eventually take advantage of these natural tools for mitigating pollution. However, modeling this process is challenging because air flow and particle movement on a street follow complex fluid dynamics. Models have shown wildly varying results for just how much particulate matter trees can trap, with some as low as 1% and others as high as 60%.

Researchers at Lancaster University, in the U.K., wanted to get some numbers from a real-life situation that they could feed into the models, so they decided to study a row of eight houses without a tree screen on a busy street in Lancaster. First, they tracked the amount of dust and particulate matter entering two of the houses as controls. Using automated dust monitoring devices, they collected data on the sizes and concentrations of particles every 10 minutes for five days. They also used simple wet wipes to gather dust from LED or plasma television screens inside four houses and then analyzed the samples with magnetic remanence, a technique that provides information on concentrations of iron-bearing particles.

Then the team placed a screen of 30 young silver birch trees in wooden planters in front of four of the houses, including one of the control houses, for 13 days. Wipes from all eight houses showed that ones with the tree screens had 52 to 65% lower concentrations of metallic particles. A comparison of all of the dust monitoring data from the two original control houses confirmed that drop, showing a 50% reduction in PM1, PM2.5, and PM10 in the house with the trees in front.

By examining the silver birch leaves with a scanning electron microscope, the researchers confirmed that the hairy surfaces of the leaves trapped metallic particles. Like the particles measured inside the houses, these metallic particles are most likely the product of combustion and brake wear from vehicles passing by.

While the swipe method for monitoring particulate matter deposition is novel, the new research doesn’t show the actual deposition rate of the trees’ leaves, a crucial missing piece of the models. Still, researchers concluded that the efficacy of roadside trees for mitigation of PM health hazard might be seriously underestimated in some current atmospheric models.

Impact of Roadside Tree Lines on Indoor Concentrations of Traffic-Derived Particulate Matter (Environmental Science & Technology)
Trees Capture Particulate Matter From Road Exhaust (Chemical & Engineering News)
Can roadside trees screen houses from pollution? (Conservation Magazine)

Related Research:
Type of trees planted a factor in reducing emissions (Eco-Business)