Can urban green infrastructure influence the urban heat island effect and air quality nexus?

Global warming and urban densification are leading to intensified urban heat island effects (UHIE) and degraded air quality (AQ), both with significant public health consequences. Urban green infrastructure (UGI), including ground-level plantings, vertical greening systems and green roofs, has been increasingly deployed to address both challenges, while delivering co-benefits such as habitat restoration and aesthetic appeal. Yet, despite growing evidence of UGI's efficacy, its thermal and AQ benefits have rarely been assessed together. This comprehensive review explores the two-fold role of UGI in mitigating the urban heat island effect and air quality nexus (UHIE-AQ). The interactions between thermal and AQ-mitigative traits are discussed: shading and evapotranspiration lowered leaf surface temperatures, promotion of thermophoresis and deposition of particulate matter, the foliar sequestration of gaseous pollutants, and the inhibition of secondary aerosol formation. Mechanistic and qualitative evidence on the dual-mitigation potential across UGI types are synthesised, indicating that well-maintained urban greenspaces offer comparatively greater potential for managing the UHIE-AQ nexus, while engineered UGI tend to deliver more localised effects. Limitations of UGI are discussed, such as biogenic volatile organic compound emissions, small effect sizes, species selection, climate zone variability and tree canopy-induced night-time warming. Recommendations are made for the combination of different forms of UGI, which may be necessary to simultaneously purify and cool urban air. A proposal is made for high-performance UGI that considers local and regional climate, vegetation type and barrier size, to maximise the potential of UGI across metropolitan regions.