3 August 2012
Have you ever stepped out into a narrow city street and began to cough? You may have been a victim of the way that 'street canyons' trap polluted air, keeping harmful substances like nitrogen dioxide and microscopic particular matter at exactly the right heights for people to breathe them it.
However, scientists at the Universities of Birmingham and Lancaster argue that by ‘greening up’ our streets a massive 30% reduction in pollution could be achieved, according to research published in the journal Environmental Science and Technology.
Plants growing in the concrete-and-glass urban canyons of cities can deliver cleaner air at the roadside, where most of us are exposed to the highest pollution levels, and can be implemented street-by-street without the need for large-scale and expensive initiatives.
So Transport for London has now constructed its first green walls to help combat the Capital's air pollution. The latest, installed just in time for the Olympics, is at The Mermaid event centre in Blackfriars (shown in the photo, right). This 200 square-metre wall is made up of 15 varieties of
emission-trapping plants which help reduce locally generated pollution,
particularly from nearby busy roads. Best of all, it looks gorgeous, as well as improving health.
Carefully selected varieties of plants remove pollutants including nitrogen dioxide and microscopic particulate matter, both of which are 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 Organization’s outdoor air quality database puts the figure at more than 1 million worldwide.
Originally, researchers thought that planting green walls would have only a small effect on urban pollution, but the Birmingham and Lancaster teams have reversed this theory. 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%, 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.