Local air quality benefits of street-level foliage much greater than previously thought

By Bill Holloway

In a recently released study, researchers in the UK have found that street-level plantings can reduce two of the dominant pollutants—particulate matter (PM10) and nitrogen dioxide (NO2)—by 60 and 40 percent, respectively, in urban street “canyons.” Previous city-scale studies had estimated that vegetation could only reduce levels of pollution by less than 5 percent. However, these previous estimates had not specifically investigated urban street canyons—streets where tall buildings on either side limit circulation. While in many environments the concentration of auto emissions is reduced by wind-driven dispersion, in urban street canyons emissions tend to linger, raising health risks.

Street-level plants reduce this pollution through dry deposition. While pollution can be reduced through dry deposition on other surfaces, such as buildings, plants attract much greater levels of deposition than other surfaces due to their metabolic uptake, the “stickiness” of the leaf surface, their large surface area, and their aerodynamic properties. Given the other benefits of greening urban areas—reducing storm water run-off, reducing the heat island effect, and improving aesthetics, this new research should bolster efforts to green cities, particularly along the highest density corridors.

Although trees have been thought of as contributing to the aesthetics and cooling of cities for many years, the new study found that shrubs, green walls, creeping plants, and ivy also provide environmental benefits. Because urban street trees can experience up to a 30% mortality rate, require larger planting areas, and are slow to mature, these smaller plantings also represent options that can be more quickly and inexpensively added to the streetscape. However the researchers emphasized the need to protect even smaller plantings from heat stress and lack of water in an urban environment.

Bill Holloway is a Transportation Policy Analyst at SSTI.