Cities worldwide are increasingly turning to urban greening to combat urban overheating, but new research reveals a potential downside: while urban vegetation provides substantial cooling benefits, it can also complicate ozone control. Led by Professors Xin Huang and Mengmeng Li from Nanjing University, the new study uncovers how urban greenery shapes compound heat and ozone pollution across major metropolitan regions in the Northern Hemisphere, offering new insights for more sustainable urban planning.
As global warming intensifies, heatwaves have become more frequent and intense across the Northern Hemisphere. Extreme heat does not just push urban temperatures higher—it also triggers sharp spikes in ground-level ozone concentration. The co-occurrence of extreme heat and ozone pollution has therefore emerged as a major environmental and public health threat to cities. To mitigate urban overheating, municipal governments globally have prioritized green infrastructure as a go-to cooling solution. “We know urban trees and green spaces effectively cool overheated cities, but their overall effects on compound heat and ozone pollution hazards have remained poorly understood,” the research team explained. To address this gap, they combined in situ observations, reanalysis data, and coupled urban meteorology-chemistry simulations to evaluated the environmental effects of urban greenery across five major metropolitan regions hardest hit by the record-breaking 2022 summer heatwaves, including cities in California, the U.S. Atlantic Coast, western Europe, the Yangtze River Delta, and the Tokyo–Pacific Coast.
The results show that urban greenery provides substantial cooling benefits, although the magnitude of cooling varies considerably with vegetation coverage and local climate. Across the five metropolitan regions, greenery reduced summer-average daily maximum air temperatures by 0.26 to 1.08 °C. The cooling effect became even stronger during heatwaves, helping reduce population exposure to extreme heat.
Urban greenery, however, exerts a complex double-edged influence on ozone pollution. Greenery-induced cooling stabilizes the urban boundary layer and weakens atmospheric mixing and pollutant dispersion. In VOC-limited urban centers, these meteorological changes can suppress photochemical ozone production, whereas ozone production in suburban areas may be less affected or even enhanced. Cooling can also weaken urban heat island-driven circulation, altering the transport and spatial distribution of ozone between urban centers and surrounding areas. At the same time, urban vegetation releases biogenic volatile organic compounds, or BVOCs, that act as ozone precursors. Although urban BVOC emissions are generally lower than those from natural forests, their overlap with dense urban NOx emissions can give them a disproportionate influence on ozone chemistry. “Urban greenery provides clear cooling benefits, but plant-emitted BVOCs can partly offset these gains by enhancing ozone formation, creating a trade-off between heat mitigation and ozone control,” the team noted.
This study clarifies the coupled meteorological and biochemical mechanisms behind greenery’s dual effects on compound heat-ozone pollution. Rather than assuming that more greenery will improve both thermal conditions and air quality, the findings highlight the need for locally tailored, climate-adaptive greening strategies that account for vegetation characteristics, local meteorology, and ozone chemistry. Such integrated planning could help maximize cooling benefits while minimizing unintended air-quality trade-offs, supporting healthier and more climate-resilient cities.
National Science Review
Computational simulation/modeling