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News Article

Cities should “green up” to help combat air pollution

Hedges found to absorb pollution better than trees in “street canyons” 

The impact of urban air pollution could be alleviated by strategically planting low hedges, instead of tall trees along roadsides in built up areas, according to new research.  The study found that low hedges reduced the impact of vehicle exhaust pollution in areas where there were large buildings close to roads, much more effectively than taller trees. It was also noted that in some areas, trees were found to actually make the pollution more concentrated, depending on the configuration of the built up area and prevailing wind conditions.

Air quality in urban areas is a significant health concern as the majority of the world’s population live in urban areas (54% in 2014) and this is estimated to increase to 66% by 2050. This is combined with the fact that one of the primary sources of air pollution in cities is traffic emissions. Increasing green infrastructure in urban areas has been considered as a method to help combat air pollution in cities, yet the impact of these different vegetation types used in various built-up environments has yet to be fully examined.

The study is a collaborative effort by researchers from the UK, USA and Europe, led by Professor Prashant Kumar from the University of Surrey and is part of the H2020 funded project iSCAPE: Improving Smart Control of Air Pollution in Europe.

According to Prof. Kumar, in order to create a healthier urban lifestyle, future urban planning needs to consider including more green infrastructure in the built environment. Green solutions could include vegetation barriers such as hedges, street trees, living walls and green roofs.  These act as porous surfaces which can influence local dispersion of pollution as well as aid the deposition and removal of airborne pollutants, thereby making the air cleaner. 

“We all know air pollution is a major factor of everyday urban life.  This comprehensive review highlights that trees and hedges, as well as other infrastructure, must be used strategically to help create healthier, less polluted cities that are also more pleasant for everyone to live and work in.

"Our other research study into London's air quality, published this week in Atmospheric Environment, investigated the underlining factors responsible for the air pollution exposure in urban environments. The other aspects, such as time of day and wind speed, emerged as important predictors of air pollution exposure for the above-ground modes (car, bus, walk) compared with openable/non-openable windows for the underground trains.

"Our earlier study showed the weathering impact of air pollution on the building materials such as limestone, sandstone and carbon steel, used in many heritage buildings and built infrastructure. This is why we need to protect buildings as well as humans in cities in future urban planning, so the strategic placing of hedges, trees and other green infrastructure can have a direct benefit as an air pollution control measure in cities."

The research also pointed out that green infrastructure has advantages and disadvantages on air quality at the street level, depending on its vegetation characteristics and location within an urban area.  In street canyons, where tall buildings are located close together on each side of the street, high-level infrastructure, including trees, generally have a negative impact on air quality.  In these areas, low-level hedges can lessen air pollution exposure.  Green roofs and walls can also work in a similar way, acting as a sink to reduce pollution.

However, it was also highlighted that in more open road areas, thick, dense and tall vegetation barriers prevent vehicle emissions from reaching roadsides in high concentrations, where people walk, cycle or live nearby.

As well as air pollution reduction measures, there are other benefits that can arise from implementing green infrastructure.  These include urban heat island mitigation, better stormwater management, potential reduction in energy consumption and climate change mitigation.

“Under the framework of the iSCAPE project, we are currently performing targeted field investigations to quantify the effects of different types of green barriers along the busy open-road sides.  This will help to develop evidence-based guidelines to support future urban planning and the public to make informed choices to “green up” their surrounding environments,” said Kumar.

Further information on green infrastructure and urban air pollution is available to subscribers of the Forest Science database.  For example, using the search string ("street trees" OR hedges) AND "air pollution" yields 355 results, while "air pollution" AND "roadside plants" returns 93 records. A selection of these is provided in the further reading section below.

Journal reference

K.V. Abhijith, Prashant Kumar, John Gallagher, Aonghus McNabola, Richard Baldauf, Francesco Pilla, Brian Broderick, Silvana Di Sabatino, Beatrice Pulvirenti. Air pollution abatement performances of green infrastructure in open road and built-up street canyon environments – A reviewAtmospheric Environment, 2017; DOI: 10.1016/j.atmosenv.2017.05.014

Article details

  • Author(s)
  • Stephanie Cole
  • Date
  • 17 May 2017
  • Source
  • University of Surrey
  • Subject(s)
  • Arboriculture