Freshening up our cities
19 July 2012
Scientists are claiming massive environmental benefits from installing 'green walls' in the streets and urban canyons of our cities.
With all the rain we have had lately, it's not surprising to see the unseasonally lush pastures and lawns we normally associate with the spring, as yet not parched by the mid-summer sun.
Town and city parks - our 'urban lungs' - are no doubt doing a great job tackling traffic- and building-generated pollution. But scientists at the Universities of Birmingham and Lancaster want to go one step further, arguing that by ‘greening up’ our streets as well, pollution could be massively reduced.
Trees, bushes and other greenery growing in the concrete-and-glass urban canyons of cities would deliver cleaner air at the roadside where most of us are exposed to the highest pollution levels, and could be implemented street-by-street without the need for large-scale and expensive initiatives, say the researchers, whose work was published (July 18) in the journal Environmental Science and Technology.
Plants in cities clean the air by removing nitrogen dioxide and microscopic particulate matter, both of which are harmful to human health. These pollutants are significant problems in cities in developed and developing countries. UK government Environmental Audit Committee estimates are that outdoor air pollution causes 35,000-50,000 premature deaths per year in the UK, while the World Health Organisation’s outdoor air quality database puts the figure at more than one million worldwide.
The researchers have found that, because pollution cannot easily escape street canyons, ‘green walls’ of grass, climbing ivy and other plants have a better opportunity than previously thought to act as an air pollution filter. Instead of reducing pollution by 1 or 2 percent, reductions of more than ten times this magnitude could be achieved, according to this study.
Using a computer model that captures the trapping of air in street canyons, as well as the hundreds of chemical reactions that can affect pollution concentrations, the research team could distinguish the effects of plants in canyons from those of plants in parks or on roofs. Green walls emerged as clear winners in terms of pollutant removal. Street trees were also effective, but only in less polluted streets where the tree crowns did not cause pollution to be trapped at ground level.
The researchers even suggest building plant-covered ‘green billboards’ in these urban canyons to increase the amount of foliage.
For the first time, the study predicts that a significant effect on pollution could be achieved on a street-by-street basis. Professor Rob MacKenzie, from the University of Birmingham’s School of Geography, Earth and Environmental Sciences, says that up until now, every initiative around reducing pollution has taken a top-down approach – scrapping old cars, adding catalytic converters to cars, and bringing in the congestion charge – some of which have not had the desired effect.
“The benefit of green walls is that they clean up the air coming into and staying in the street canyon,” he says. Planting more of these in a strategic way, could be a relatively easy way to take control of our local pollution problems.”
Of course, all this depends on the plants and their ability to withstand all that today’s cities can throw at them. Dr Tom Pugh, from Lancaster University suggests that more care needs to be taken as to how and where we plant vegetation in our towns and cities, so that it does not suffer from drought, become heat stressed, vandalised, or interact negatively with other aspects of our urban areas, while carrying out the very important job of filtering our air.
Transport for London’s (TfL’s) Nicola Cheetham, welcomes this new research. No stranger to placing green walls in transport corridors, TfL has completed a second green wall at The Mermaid, Blackfriars, installed as part of the Clean Air Fund programme. “Our own research, conducted by Imperial College London, shows the ability of different plants to trap particulate matter,” she says. “The bringing together of these various strands of research will help to inform the planners, designers and green infrastructure professionals who are responsible for the provision and management of green infrastructure in our towns and cities.”
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