Urban Street Canyons – Pollution
“There is mounting evidence that local concentrations of air pollutants are greatly affected by the form of the city, but there has been relatively little attempt to enhance street-level air quality through the manipulation of urban form.”—Anne Whiston Spirn, 1986
The shape of the urban street canyon can greatly affect heat and wind. In turn, heat and wind can help or hinder the dispersion of pollutants such as carbon monoxide, hydrocarbons, and ozone. Illustrations from Spirn’s air quality report show some possible interactions between wind, pollutants, and pedestrians. Note that the skimming flow can sweep pollutants from above buildings or along traffic corridors into pedestrians’ path.
An example of design's power to create the urban climate, limiting building height or stepping buildings back from the street can greatly affect the amount of pollution at street-level. These approaches allow more sunlight to reach the canyon’s bottom. As Harvey Bryan describes in his chapter "Natural Light and the Urban Environment" in Anne Vernez Moudon’s 1991 book Public Streets for Public Use, these are old strategies, employed even by the Ancient Greeks. This drive for more light also led a Boston architect named William Atkinson to push for new city zoning legislation that focused on “sky exposure” in 1912 (as illustrated below). Atkinson’s ideas would later find their way into other cities’ zoning regulations, including one for New York in 1916.
As it stops skimming flow, stepping buildings back also helps wind dispersion of pollutants by avoiding temperature inversions. Usually the air temperature increases the further measured from the warm ground. When high buildings keep any sunlight from entering the canyon until very late in the day, the air in the canyon’s base remains cooler than the air above it. The inversion keeps cold air at the canyon’s bottom, holding with it any pollutants emitted by traffic.
Christof Gromke’s (University of Karlsruhe, Germany) 2008 study of tree plantings’ effects on canyon pollution is a recent example of work on this topic. Though past researchers often ignored the complexities of vegetation in their models, Gromke hoped to add this level of detail in a series of wind tunnel tests of trees with varying canopy density in street canyons. Though his model proved insufficient for distinguishing between varying canopy densities, the presence of a single row of trees hindered airflow in certain areas, leading to increased pollution levels (represented by tracer gas in the wind tunnel) compared to the tree-free case.
Though trees increasing pollution levels may seem counterintuitive, Gromke’s study actually confirms older beliefs in the field about the distinction between single-row, avenue plantings and larger tree groves. Larger areas devoted to diverse plantings with layered canopies, as discussed in Sprin’s report, can have a significant impact on heat, wind, and pollution: acting as both a pollution filter and a heat sink.
In 2007, Xiaomin Xie at Shanghai Jiao Tong University in China, notably in the school of mechanical engineering, published in Atmospheric Environment another study that applies findings from urban climatology to determine the dispersion of pollutants in street canyons. Specifically, Xie looked at the impact of various building facades and ground heating on pollution concentrations.
Using a computer simulation of air flow, Xie tested canyons in each of the wind regimes mentioned in the previous section: street canyons with aspect ratios varying from 0.1 to 2 which constituted skimming flow (ratio was 1 or 2), wake interference flow (ratio was 0.5), and isolated roughness flow (ratio was 0.1).
The canyon with isolated roughness flow, the geometry with the widest streets and shortest buildings, allowed dispersion of pollutants quickly. The other canyons often created strong vortices that trapped pollutants at pedestrian level, depending on surface heating and height of the buildings. Some had one primary vortex in the center of the canyon; this might be visualized as a sideways corkscrew. Others had multiple vortices driven by the prevailing wind, the convection currents from surface heating, and each other--almost as interweaved gears.
Boundary conditions in computer simulation. From Xie, Xiaomin et al. "Impact of building facades and ground heating on wind flow and pollutant transpot in street canyons ." Atmospheric Environment. 41 (2007).
Left picture shows wind pattern (skimming flow with one vortex, aspect ratio is 1). Right picture shows pollutant concentrations. Xie, Xiaomin et al. "Impact of building facades and ground heating on wind flow and pollutant transpot in street canyons ." Atmospheric Environment. 41 (2007).
Left picture shows wind pattern (skimming flow with two vortices, aspect ratio is 2). Right picture shows pollutant concentrations. Note how winds can trap pollutants near the canyon's bottom or where the sidewalks would be. From Xie, Xiaomin et al. "Impact of building facades and ground heating on wind flow and pollutant transpot in street canyons ." Atmospheric Environment. 41 (2007).
Left picture shows wind pattern (skimming flow with two vortices, aspect ratio is 0.5). Right picture shows pollutant concentrations. From Xie, Xiaomin et al. "Impact of building facades and ground heating on wind flow and pollutant transpot in street canyons ." Atmospheric Environment. 41 (2007).
Left picture shows wind pattern (isolated roughness flow, aspect ratio is 0.1). Note low pollution concentrations in right picture. From Xie, Xiaomin et al. "Impact of building facades and ground heating on wind flow and pollutant transpot in street canyons ." Atmospheric Environment. 41 (2007).
Describing, in their conclusion, their paper as a “stepping stone” to further research, Xie’s team confirmed that the geometry of the canyon is a crucial factor that determines wind flow, but that surface heating can also lead to counterintuitive results–for example certain cases where convection currents caused by surface heating actually concentrated pollutants in the windward side of the canyon, the place where the constant current of wind should disperse pollutants. Again Xie’s study shows the importance of understanding the interactions of heat and wind in determining street design.
Breakdown of various vortex formation boundaries dependent on aspect ratio. From Xie, Xiaomin et al. "Impact of building facades and ground heating on wind flow and pollutant transpot in street canyons ." Atmospheric Environment. 41 (2007).
