Fast-growing city trees found to boost ozone pollution in megacities

Scientists sampled air in Beijing during the summer of 2021 and found that roughly 35% of the city's trees, notably willows and poplars, emit large quantities of isoprene, a volatile organic compound. While traffic and industry contribute the bulk of total VOC emissions, the plant-based share accounts for about 52% of the chemical reactions that generate ozone, especially under sunny, hot conditions. Laboratory estimates indicate that at 35 °C the reactivity of vegetation-derived VOCs is seven times higher than at 20 °C, raising the vegetation contribution from 21% to 74%.

The authors extended their analysis to 24 megacities and projected that cities such as Sydney could see even higher isoprene emissions, with about 65% of trees acting as emitters. The findings highlight a paradox where green infrastructure intended to improve air quality may inadvertently worsen ozone pollution as global temperatures rise.