The University of Surrey’s Professor Prashant Kumar and partners in the EU and U.S. have published a new study in the journal Atmospheric Environment that argues that cities need low hedges as well as trees in order to combat the adverse effects of air pollution. According to the study, hedges are more effective than trees at absorbing air pollution from vehicle exhaust pipes, making them ideal for placement along roads in cities and the built-up environment. The study comes under the umbrella of the H2020 funded project iSCAPE: Improving Smart Control of Air Pollution in Europe.
According to the WHO, seven million premature deaths are linked to air pollution annually, with traffic emissions being one of the main sources of air pollution. Other than finding that low-level hedges improve air quality in street canyons, the researchers noted that green infrastructure can play a significant role in mitigating urban air pollution. The study also found that green walls and roofs are effective at reducing pollution in streets and open roads.
The study found that the only case where higher trees are more efficient at reducing air pollution is in areas that are more open and less densely populated by taller buildings. However, the study found that trees actually have a negative impact on air quality when placed in concrete jungle environments, where high buildings and skyscrapers are fixed together on one side of the street. Instead, low-level hedges and bushes work better in these areas, as they reduce air pollution exposure.
Since road traffic creates many harmful pollutants, including particulate matter, ultrafine particles and gaseous pollutants such as nitrogen oxide, carbon monoxide and sulphur dioxide, it is in areas like these that urban vegetation is most needed. Urban vegetation barriers, such as low-level hedges, remove gaseous pollutants by absorption through leaf stomata or plant surfaces. Thus, in open road conditions, thick, dense and tall vegetation barriers restrict the fresh vehicle emissions from reaching roadsides, where people walk, cycle or simply live.
The researchers also studied the weathering impact of air pollution on building materials such as limestone, sandstone and carbon steel, which are used in many heritage buildings and built infrastructure, and concluded that urban planners and architects need to protect buildings as well as humans in cities by placing hedges, trees and other green infrastructure in strategic places to act as an air pollution control measure in cities.
Kumar, who chairs Air Quality & Health at the University of Surrey, expressed the urgency of having future urban planners who consider designing and implementing more suitable green infrastructure, in the built environment to contribute to a healthier urban lifestyle. This “green infrastructure” includes street trees, vegetation barriers, green walls, and green roofs. These green solutions act as porous bodies, which if placed correctly, block local dispersion of pollution and aid the deposition and removal of airborne pollutants, making the air cleaner and therefore the city healthier.