How can greenery space mitigate urban heat island? An analysis of cooling effect, carbon sequestration, and nurturing cost at the street scale

Rapid urbanization has contributed to urban heat islands, which can potentially lead to increased energy consumption and carbon emissions, further worsening global warming. The U-shaped street canyon is one of the leading causes of urban heat islands, which may block air circulation and lead to urban heat accumulation. The canyon heat issues can be usually mitigated by nature-based solutions, such as street trees. It is important to increase the greenery space benefits (e.g., cooling effect of trees) with limited canyon space. However, there is an absence of refined greenery space design strategy in various street canyons. This work explored the quantitative design of greenery space (e.g., tree spacing) in different street canyons with complex morphological characteristics, in order to effectively improve co-benefits of trees and mitigate urban heat islands. Eighteen morphological types were considered, including symmetrical & asymmetrical shallow, ideal, and deep street canyons. Co-benefit considering cost of different tree spacings were analyzed, to maximize the benefits of cooling effect and carbon sequestration at minimal nurturing cost. Compared with street canyons without trees, ideal street canyon with tree spacing of 0.2W (W is canyon width) achieved the maximum temperature reduction of 6 °C. The positive correlation between tree spacing and co-benefits was found. The maximum co-benefits of street canyon trees occurred at tree spacing of less than 0.7W, which was largely increased by about 14% compared with 0.2W. This work can provide the guideline for efficient greenery space design, which is crucial for mitigating urban heat islands by nature-based solutions.