A comprehensive study by He et al. (2026) reveals that the choice of tree species in urban greening programs plays a critical role in shaping urban air quality, particularly ground-level ozone pollution.
Key Findings
- Dominance of Biogenic Reactivity: Using direct eddy covariance flux measurements and source apportionment in Beijing, researchers found that urban vegetation contributes 52% of the total organic reactivity (OHRflux) driving ozone production. This makes biogenic emissions the leading driver of reactive organic carbon in the city, surpassing vehicle exhaust (17%) and volatile chemical products (21%).
- Temperature and Heat Waves: Biogenic volatile organic compound (BVOC) emissions, particularly isoprene, exhibit strong temperature dependence. During hot summer days, biogenic emissions account for up to 74% of organic reactivity, directly driving peak ozone formation during heatwaves.
- The Isoprene Emitter Factor: Beijing’s surprisingly high biogenic emissions are primarily driven by the urban planting of high-emitting native tree species, such as poplars (Populus spp.) and willows (Salix spp.).
- Global Implications: By extrapolating these findings across 24 global megacities, the study shows that cities in Asia and Oceania face similar or greater ozone challenges due to high proportions of high-emitting trees (e.g., Quercus, Salix, or Eucalyptus).
Urban Planning Takeaway
Urban greening strategies must go beyond simply increasing tree coverage or focusing solely on native species diversity. Replacing high-emitting trees with low-emitting species can substantially reduce urban biogenic emissions. For example, swapping just 10% of Beijing’s high-isoprene emitting trees with non-emitters could reduce urban isoprene emissions by nearly 30%. Targeted tree selection offers urban planners a powerful, cost-effective tool to mitigate ozone pollution alongside concurrent NOx controls.
Reference: Tree selection in urban greening shapes air quality for global cities | Science Advances