A parametric study on the effects of green roofs, green walls and trees on air quality, temperature and velocity

Rapid increase in urbanisation and population growth living in urban areas leads to major problems including increased rate of air pollution and global warming. Assessing the impact of buildings on wind flow, air temperature and pollution dispersion on people at the pedestrian level is therefore of crucial importance for urban design. In this study, the effect of different forms of urban vegetation including green roofs, green walls and trees on velocity, air temperature and air quality is assessed using Computational Fluid Dynamics (CFD) for a selected area of the East Village. This study indicates that adding a building increases air temperature, pollution concentration, and velocity at the pedestrian level. A parametric analysis is conducted to assess the impact of various key parameters on air temperature, pollution, and velocity at the pedestrian level. The variables under consideration include wind speed which, ranging from 4-8 m/s at a reference height of 10m, and vegetation cooling intensity which, ranging from 250-500 Wm-3. Three scenarios are tested in which the streets have no bottom heating, 2 °C bottom heating, and 10 °C bottom heating. Pollution is simulated as a form of passive scalar with an emission rate of 100 ppb s-1, considering NO2 as the pollutant. In all cases, vegetation is found to reduce air velocity, pollutant concentration and temperature. However, the presence of vegetation in various forms alter the pattern of pollution dispersion differently. More specifically, the results indicate that planting trees (e.g. birch trees) close to the edge of buildings can decrease the air temperature by up to 2-3 °C at the pedestrian level. Increasing the cooling intensity of the vegetation from 250 to 500 W m-3 results in significantly lower air temperature, whereas lower wind speeds result in higher concentration of pollutants at pedestrian level. A combination of green walls and trees is found to be the most effective strategy to improve thermal environment and air quality.