Unraveling the potential of urban greenery for extreme heat and ozone mitigation in Northern Hemisphere cities.

Escalating exposure to extreme heat and ozone (O3) pollution has emerged as a principal challenge for cities worldwide. While urban greening is widely advocated for mitigating extreme heat, its impacts on compound heat-O3 pollution remain poorly understood due to intricate urban meteorology-chemistry coupling. By integrating observations, reanalysis data, and urban-land-atmosphere modeling, we demonstrate that urban greenery provides a notable yet uneven cooling effect of -0.26°C to -1.08°C in Northern Hemisphere's metropolises, shaped by greenery fraction and local climate. Furthermore, the nonlinearly weakened O3 photochemical production within a greenery-stabilized urban boundary layer is capable of decreasing O3 in populated city centers. Despite the dual positive effects in alleviating heat and O3 exposures meteorologically, greenery-boosted biogenic emissions could chemically raise urban O3 via supplementing precursors, totally by up to 7.6 µg m-3. These findings underscore the need to incorporate fully coupled urban meteorology-chemistry interactions in urban greening strategies to achieve climatic and environmental co-benefits.