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    • 01 January 2020

      The Connection Between Urban Trees and a Healthier Breeze

      With urban density comes airborne pollutants. How can architects and city planners help design healthier cities in the future? The answer, it would seem, is blowing in the wind.

    • If a city has a pulse, so too must it breathe. The flow of air through city streets is a product of infrastructure and environment: Wind patterns, temperature shifts, solar radiation, humidity factors and building facades all influence urban ventilation. But with these breezes come a steady flow of urban pollutants. Designing healthy cities means acknowledging this challenge, and creating solutions that encourage clean air for clean living.

      Urban vegetation has a long-recognized ability to improve local air quality. Growing trees capture and contain carbon dioxide from the air and absorb nitrogen dioxide with their leaves, while also helping the soil around them capture significant amounts of carbon. Larger trees can absorb up to 20 kilograms of carbon dioxide each day, or seven tonnes each year – The same as the average Dane’s annual carbon dioxide usage. However, in the pursuit of absorbing pollutants, not all plants are created equal. Finer, more complex structured foliage is most effective in capturing particulates, as these offer greater surface area for absorbing pollutants. In this regard, conifers are particularly effective not only because of their fine structure of hairy needles, but also because they are evergreens, retaining their function throughout the winter. Common ivy, also an evergreen plant, is especially good at capturing very fine particulates. Understanding the most common pollutants in a given area is an important prerequisite to developing a planting strategy.

      Just as important as the plant type is the positioning of the plants themselves. The closer to the pollutant source the plant is, the more air it is able to filter (Vos et al., 2013; Pugh et al., 2012). For a city street, planting a dense green base of herbaceous plants and bushes at the same height as the exhaust pipes creates a natural sponge for exhaust-borne pollutants. At the same time, too much plant density can effectively concentrate pollutants within the city. Planting strategies must consider large-scale airflow, ensuring staggered or semi-permeable plant barriers in order to encourage a constant circulation of fresh air.

      The shape of our streets – From building facades to macro-scale urban plans – Is a key factor in shaping healthy ventilation. Studies addressing pollutant dispersion in urban geometry suggests that proper ventilation is dependent on the street layout’s orientation relative to prevailing wind directions. It may help to think of the street as a canyon, with street-facing buildings forming the canyon wall: If pollutants tend to settle in this canyon, as occurs with heavy vehicle traffic, ventilation strategies should seek to encourage greater windflow through this street. If few pollutants settle inside the canyon, perhaps for a pedestrian street or narrow park, it is more favorable to shelter this area from surroundings winds, which may carry greater concentrations of pollutants. Planting strategies carry utility in this regard for their ability to direct the wind – While solid walls of dense shrubs can block breezes entirely, height-ascending plantings of semi-permeable vegetation can encourage wind to pass up and over rooftops, out of the urban canyon.

      Solutions to healthy urban ventilation range from the natural to the cutting-edge. Computational Fluid Dynamics (CFD) simulations are an emerging technology that can find critical use in this application, allowing city planners and architects to predict how urban geometry will influence windflow. As urban density increases, the importance of this commitment only grows, requiring city planners, architects and designers to draw on all possible options to create healthy communities.