Street canyons--narrow streets lined by buildings hundreds of feet tall--are unique to the urban landscape. But while their structure undoubtedly adds to the allure of cities, it also effectively traps pollutants emitted by vehicular traffic traversing the canyon floor, resulting in poor air quality. Fortunately, like canyons in nature, street canyons can be made healthier through the presence of plants--the ability of which to improve air quality in urban environments was found recently
to have a far greater impact than earlier reports had estimated.
According to the new study, published in the journal Environmental Science & Technology, plants can reduce levels of nitrogen dioxide and particulate matter, which are major urban pollutants, by as much as 40 and 60 percent, respectively. These new figures far exceed the 1 to 5 percent reductions estimated by previous analyses. Furthermore, the pollution-reducing benefits occur not only at the level of single street canyons but also on the scale of city-sized areas, indicating that plants play a significant role in taking up pollutants across the urban landscape.
Of all pollutants, particulate matter, which consists of dust, metals, and organic chemicals and acids, has the greatest impact on human health, with exposure linked to breathing difficulties, asthma attacks, and nonfatal heart attacks. This risk comes with the very small size (10 µm or less) of pollutant particles, which allows them to penetrate deep into the lungs; fine particles (2.5 µm) may even enter the bloodstream. Nitrogen dioxide and other gaseous pollutants (carbon monoxide, sulfur dioxide, and ozone) can also cause respiratory illness and exacerbate symptoms in persons with existing respiratory conditions.
The new study highlights the influence of urban structure on pollutant levels and consequent risks to human health. In wide streets lined by low buildings, air travels relatively smoothly over rooftops and streets, and flow at both levels occurs generally in the same direction, with relatively minor eddies forming at the windward and leeward sides of buildings. In contrast, as streets become narrower and the buildings lining them taller, air has a greater tendency to skim along the top, such that only some air circulates down into the canyon, and once there, it enters into a stable vortex. As a result, air that circulates upward within a street canyon tends to cycle back down toward the street, and thus pollutants have little opportunity to escape.
Plants clear gaseous pollutants from the air through their stomata, mouth-like structures in leaves that open and close in response to environmental conditions. When open, stomata facilitate the exchange of gases across the leaf surface, taking up, for instance, carbon dioxide and nitrogen dioxide and releasing oxygen. The removal of particulate matter from the air by plants occurs through a process known as deposition, in which particles accumulate on the sticky surfaces of leaves.
The large surface area provided by vegetation is key to enabling the deposition of large quantities of particles. For this reason, plants like climbing ivy, which grows up along the walls of street canyons, tall grasses, and plants with broad leaves can be highly effective in improving air quality in urban environments. But in order for plants to thrive and clear pollutants from the air effectively in urban settings, careful attention must be paid to their distribution and spacing. For example, a wind-tunnel study
based on small-scale modeling found that the canopies of trees planted too close together down the middle of the street in an urban canyon could hinder airflow and potentially trap vehicle exhaust fumes at street level.
Plants are central elements of green infrastructure, which has been recognized increasingly as an important means of improving the health of urban environments. Because pollutants can stunt plant growth and cause discoloration and premature death of leaves, potentially impairing photosynthesis, it is important that we take additional measures as well to keep urban air clean. For city dwellers, riding a bicycle or taking public transportation when possible are among the simplest solutions.