Green Canyons – which plants best fight roadside air pollution

Hedges are best for combating vehicle pollutionOxford, 02/04/2020. Choosing the right plants for specific areas can make a huge difference to reducing local air pollution from traffic. This is the point of a new Surrey University guide, which suggests what tree and hedge species are best for combating roadside air pollution, and how best to plant green barriers to reduce the problem.

In a paper published in npj Climate and Atmospheric Science, scientists from the university’s Global Centre for Clean Air Research (GCARE) reviewed the literature on green infrastructure (trees and hedges) and air pollution. They found there is ample evidence that such ‘green canyons’ can divert and dilute pollutant plumes or reduce outdoor concentrations of pollutants by direct capture.

As part of their review, the authors identified a gap in the information to help people – including urban planners, landscape architects and garden designers – make informed decisions on which species of vegetation to use and, crucially, what factors to consider when designing a green barrier.

They identified 12 traits from 61 tree species that make them potentially effective barriers against pollution. The important plant properties include small leaf size, high foliage density, long in-leaf periods (e.g. evergreen or semi-evergreen), and micro-characteristics such as leaf hairiness.

In the paper, the team emphasise that the effectiveness of a plant is determined by its environmental context – whether for example it is used in a ‘deep canyon’ (e.g. a city commercial centre) or a ‘shallow canyon’, typical of a residential road or an open road environment. To help planners with complex problems, such as which trees are best outside a school in a medium-sized street, the team developed a plant-selection framework to aid in the decision-making.

Professor Prashant Kumar, Founding Director of GCARE at the University of Surrey, says, “We are all waking up to the fact that air pollution and its impact on human health and the health of our planet is a defining issue for our time. Air pollution is responsible for one in every nine deaths each year and this could be intensified by projected population growth.”

“The use of green infrastructure as a physical barrier between ourselves and pollutants originating from our roads is a promising way to protect ourselves from the long-term impact of air pollution. We hope that our detailed guide to vegetation species and on how to plant and use green infrastructure will help everyone looking to combat air pollution.”

In a related study published in Atmospheric Environment, the group looked at how three different types of roadside green infrastructure – trees, hedges, and a combination of trees with hedges and shrubs – affect air-pollution concentration levels. The study used six roadside locations in Guildford, UK, as test sites where the green infrastructure was between one to two metres away from the road.

The researchers found that roadsides with hedges only were the most effective at reducing pollution exposure, cutting black carbon by up to 63 percent. Ultrafine and sub-micron particles followed this reduction trend, while fine particles (less than 2.5 micrometres in diameter) showed the least reduction among the measured pollutants.

The maximum reduction in pollution was observed when the winds were parallel to the road, due to a kind of sweeping effect. The hedges only – and a combination of hedges and trees – emerged as the most effective green infrastructure in improving air quality behind them under different wind directions.

Some communities are built with ‘green barriers’ separating road traffic from residential housing from the outset. This is an established suburb on the outskirts of Eindhoven, Netherlands. Picture © Nigel Saych, 2020.

Roadsides with trees only showed no positive effect on pollution reduction at breathing height (usually between 1.5 and 1.7m), as the tree canopy was too high to provide a barrier/filtering effect for road-level tailpipe emissions.

“This study extended our previous work,” says Professor Kumar, “and provided new evidence to show the important role strategically placed roadside hedges can play in reducing pollution exposure for pedestrians, cyclists and people who live close to roads. Urban planners should consider planting denser hedges, and a combination of trees with hedges, in open-road environments.”

“Many local authorities have, with the best of intentions, put a great emphasis on urban greening in recent years,” he adds. “However, the dominant focus has been on roadside trees, while there are many miles of fences in urban areas that could be readily complemented with hedges, with an appreciable air pollution exposure dividend. Urban vegetation is important given the broad role it can play in urban ecosystems – and this could be about much more than just trees on wide urban roads.”

This study was carried out under the umbrella of the iSCAPE (Improving Smart Control of Air Pollution in Europe) project, part of the European Union’s Horizon 2020 research programme.

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