Spatially coupled effects of urban vegetation on air pollution and heat islands in coastal cities across the globe

Urban vegetation is considered an optimal mitigation strategy to combat urban air pollution and heat islands. However, the spatially coupled effects of vegetation landscape morphologies and canopy traits on air pollution and heat islands remain unclear, especially for coastal cities vulnerable to climate change. We investigated the spatial patterns of air pollution intensity (API) and surface urban heat islands (SUHI) in major coastal cities across the globe by utilizing multi-source remote sensing data, and examined the coupled effects of multiple urban vegetation factors (UVFs). We found strong spatial variation in API and SUHI among major coastal cities across the globe, with spatial averages of 0.51 ± 0.18 and 1.15 ± 2.31 ℃, respectively; API was higher in the tropical climate zone than other climate zones (0.53 ± 0.18), while SUHI was the greatest in temperate climate zones (1.48 ± 2.29 ℃). The direction or magnitude of effects of UVFs on API and SUHI varied with climate zones, demonstrating the strong climate dependency. Vegetation landscape morphologies, including fragmentation index (FI), canopy height (TH), and fractional vegetation cover (FVC), have a more direct and positive impact on API and SUHI reduction, compared to canopy traits. Vegetation landscape morphologies exhibited stronger synergistic effects on API and SUHI than vegetation canopy traits, suggesting that the former has great potential for co-mitigation of air pollutants and heat stress; notably, they also showed a strong Lose-lose coupled scenario in regulating API and SUHI, accounting for over 35% of total paired observations. Besides, the tallest canopy height (6.83 ± 1.43 m) occurred in high SUHI and high API spatial clusters. This study provides valuable insights for vegetation landscape optimization to mitigate urban pollution and heat islands.