Mitigating urban heat and air pollution considering green and transportation infrastructure

Global urbanization is causing an increase in population and transportation along with a reduction of green spaces. This phenomenon is leading to the development of Urban Heat Island (UHI) and Urban Pollution Island (UPI) effects, negatively impacting the urban environment. Numerous mitigation strategies have been proposed to reduce heat and air pollution. However, the combined impact of green and transportation infrastructure and their potential to mitigate heat and air pollution remains partially understood. This review aims to contribute by systematically investigating studies on mitigation strategies for urban green and transportation infrastructure from 2003 to 2023. These strategies are classified into two types: green infrastructurerelated (greenery, green roofs, and green walls) and transportation-related (transportation fuel, transportation networks, and transportation organization systems). The findings are presented under five aspects: (1) Most research on transportation infrastructure adopts qualitative research methods, whereas research on green infrastructure tends to adopt quantitative methods; (2) Developed countries prioritize upgrading their transportation fuel, while developing countries remain inclined towards developing urban green spaces; (3) In subtropical and temperate zones, green roofs and green walls may cool the local ambient (within 1 m above the surface) by up to 10 degrees C, whereas the cooling effect in other climate zones is within 5 degrees C; (4) More focus was put on reducing the surface temperature rather than the air temperature. Green roofs and green walls are found to cool the former 2-3 times more than the latter; (5) Green infrastructure in the subtropical zone is 1.2 to 2 times more effective at adsorbing gaseous pollutants than in the temperate zone. This review can assist policymakers, urban planners and researchers in effectively mitigating urban heat and air pollution, and guide future research endeavors.