Green streets can cut pollution, says study
The creation of "green walls" in urban areas could cut pollution by up to 30%, scientists have suggested.
UK researchers say more trees and other vegetation at street level would clean air in areas that are normally exposed to higher pollution levels.
Plants in towns and cities have been shown to remove nitrogen dioxide (NO2) and particulate matter (PM), both of which are harmful to human health.
"Up until now, every initiative around reducing pollution has taken a top-down approach, [such as] scrapping old cars, adding catalytic converters, bringing in the congestion charge - some of which have not had the desired effect," said co-author Rob MacKenzie from the University of Birmingham.
"The benefit of green walls is that they clean up the air coming into and staying in the street canyon," Prof MacKenzie observed.
"Planting more [green walls] in a strategic way could be a relatively easy way to take control of our local pollution problems."
Street canyons refer to the effect created by high buildings lining a street, preventing much of the pollution escaping.
Previous studies have shown that greening urban spaces can cut pollution, but only by about 5%. This study suggests that strategic placement of vegetation in street canyons can cut air pollution by up to 30%.
Green walls, consisting of climbing plants such as ivy, built on billboard-like structures could act as air pollution filters, the team said.
Nicola Cheetham, head of environment (surface transport) for Transport for London (TfL), welcomed the findings.
"Our own research, conducted by Imperial College London, shows the ability of different plants to trap particulate matter," she said.
Ms Cheetham added that TfL had just installed its second green wall in the capital to help mitigate the pollution associated with heavy flows of urban traffic.
The team reached their findings about the effectiveness of green walls by using a computer model that showed the effect of street canyons trapping air at street level and the accumulation of pollution.
The model also showed that street trees were effective filters, but only in less polluted streets and provided the trees' canopies did not result in the pollution being trapped at ground level.
Co-author Tom Pugh, from Lancaster University, said one of the challenges of greening urban areas was ensuring the plants were able to survive in the projected change in conditions.
"More care needs to be taken as to how and where we plant vegetation," Dr Pugh said.
"[We need to make sure] that it does not suffer from drought, become heat stressed or vandalised."
Anne Jaluzot from the co-ordinating group Trees and Design Action Group told BBC News that councils were planting too many small trees that did nothing for biodiversity, flood prevention or pollution control. She said they should concentrate on finding space for a smaller number of very big trees.
She also said money was being wasted on designer green walls - vertical planting systems. "These green walls often look great, but they're unsustainable because of the high maintenance costs and need for fertilisers.
"Councils and developers would often be better to simply cover a wall with ivy and other creepers," she said.