Urban microclimatic is a local set of atmospheric conditions (wind, sun, humidity etc.) that has significant influence on the concentration of air pollutants. Therefore, to actively reduce poor air quality in cities, we need to understand and simulate the urban climate before we design our urban space. Here are three steps to design for better air quality, based on the scientific work we have done.
Wind flows, temperature gradient, solar radiation and humidity levels influence pollutant concentrations and dispersion. The urban wind flow determines the level of urban ventilation, which transport pollutants by advection and mixes it up with cleaner air by turbulence. In general, low wind speeds creates a more favorable environment for the chemical reactions necessary to create ozone and particle pollutions, which is why wind sheltered, in some cases, allow pollutants to build up (EPA, 2007). Therefore, it is necessary to create ventilated corridors where the city can “breathe”.
If trees are not blocking the wind, they are a great help in reducing air pollution. Trees absorbs the greenhouse gas carbon dioxide and leafs beneficially captures, and contains, nitrogen dioxide. Not all plant species are as effective, however. Finer, more complex structured foliage is most effective in capturing particulates. Conifers are effective because of their fine structure of hairy needles, but also because they are evergreen and therefore retain their function in the winter. Common ivy – also an evergreen plant - is also good at capturing particulates, especially the finer fraction PM2.5 (Ottelé et al., 2010). Knowing the collection of dominant pollution types for the specific location is valuable for informing the vegetation strategy.
The difference in building height also influence the mixing of pollutants with clean air. Studies addressing pollutant dispersion in relation to urban geometry shows that urban ventilation is depending on how the urban street grid is orientated to the prevailing wind directions. As a rule of thumb, urban ventilation is beneficial if there are pollutants emitted inside the canyon, for instance from traffic, as shown in the illustration below. In cases of no pollution sources inside the canyon, it can be favorable to shelter from the background-polluted winds. To know the consequences of urban building design is necessary to be able to protect the city from high pollution concentration. Computational Fluid Dynamics (CFD) simulations can be used to analyze how the geometry influence the wind flow around buildings.