Urban tree species selection impacts ground-level ozone levels

Research indicates that certain tree species used in urban planting can inadvertently contribute to ground-level ozone formation. While trees provide essential benefits such as shade and CO2 absorption, some species release biogenic volatile organic compounds (VOCs), specifically isoprene, which react with sunlight and vehicle exhaust to create harmful ozone.

A study conducted in Beijing utilized real-time air monitors to track chemical signals, revealing that although trees produced only about 10% of all VOCs in the air, these emissions accounted for nearly half of the chemical reactivity driving ozone formation. The impact of these natural emissions intensifies with heat; as temperatures rose from 20°C to 35°C, the reactivity of tree-emitted VOCs increased seven to eight times, whereas human-made VOCs increased by only 40%. On extremely hot days, trees were responsible for up to 74% of reactive chemicals in the air.

Common high-emitting species include willows, poplars, oaks, and eucalyptus. Experts suggest that to mitigate air quality issues, urban planners should prioritize low-emitting species when selecting trees for city environments, particularly in hot, high-traffic regions.