Hedges along the side of roads are the best for reducing air pollution from traffic as they act as a green barrier, a new study found.
But while tree lined avenues may look appealing they are the least effective in removing black soot from diesel engines and other smog particles from exhausts.
This was because the tree canopy was too high to provide a barrier or filtering effect for roadside exhaust emissions.
Hedges on the other hand reduced diesel soot known as black carbon by 63 per cent while ultrafine and sub-micron particles followed this reduction trend.
This was because hedges are normally at breathing height of usually between 1.5 and 1.7m.
Fine particles (less than 2.5 micrometres in diameter) showing the least reduction among all the measured pollutants.
There was also an appreciable reduction in harmful heavy metals behind the vegetation.
A combination of hedges and trees were the next best option, scientists at the University of Surrey found.
And the direction of the wind also played a role in reducing the amount of traffic pollution breathed in.
The maximum reduction in concentrations was observed when the winds were parallel to the road due to a sweeping effect, followed by winds across the road.
Professor Prashant Kumar, director of the Global Centre for Clean Air Research said: “Many millions of people across the world live in urban areas where the pollution levels are also the highest.
“The best way to tackle pollution is to control it at the source.
“However, reducing exposure to traffic emissions in near-road environments has a big part to play in improving health and well-being for city-dwellers.
“This study, which extends our previous work, provides 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.
“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 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.”
The study looked at how trees, hedges, and a combination of trees with hedges and shrubs affected the concentration levels of air pollution.
It used six roadside locations in Guildford as test sites where the “green infrastructure “was less than a metre away from the road or over two metres away from the road.
It looked out how tress and scrubs influenced concentrations of particulate matter ≤10 μm (PM10), ≤2.5 μm (PM2.5), ≤1 μm (PM1), black carbon (BC) and particle number concentrations (PNC).
For ten hours each day over three months pollution concentrations were recorded simultaneously behind and in front of the green infrastructure or adjacent clear area, capturing both morning and evening traffic peaks
It found for a green infrastructure over two metres away, all three configurations showed reductions behind the green barrier for all pollutants.
The ‘hedges only’ configuration showed higher pollutant reductions than the other two configurations, with maximum reductions of up to 63 per cent shown for BC.
But for trees and hedges planted closer to the road the results were mixed.
The ‘trees only’ configuration reported increases in most of the pollutant concentrations, whereas the combination of trees and hedges resulted in reduced pollutant concentrations behind the green infrastructure.
Among all pollutants, the highest relative changes in concentration were observed for BC (up to 63%) and lowest for PM2.5 (14%).
Categorising the data based on wind directions showed the highest reduction during along-road wind conditions (i.e., parallel to the road).
This was expected due to the sweeping of emissions by the wind and the wake of road vehicles whilst the barrier effect of green infrastructure enhanced this cleansing, limiting lateral diffusion of the pollutants.
However, cross-road winds that took vehicular emissions to pass through the green infrastructure allowed us to assess their influence, showing up to 52, 15, 17, 31 and 30 per cent reduction for BC, PM10, PM2.5, PM1 and PNC, respectively.
Senior author Prof Kumar concluded: “The largest reductions were consistently noted for the mixed ‘trees and hedges’ configuration in close-road conditions and the ‘hedge only’ configuration in away-road conditions.
“The assessment of various fractions of PM showed that ‘hedges only’ and a combination of trees and hedges lowered fine particles behind green infrastructure.”
Almost two thirds of the population in the European Union live in urban areas where, according to the European Environmental Agency, air pollution levels in many cities are above permissible levels, making air pollution a primary environmental health risk.
The study was published in the journal Atmospheric Environment.
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