Across Beijing, millions of weeping willow and poplar trees turn the megacity into a green metropolis — but compounds released by these species are now a major driver of ozone air pollution, according to a study published in Science Advances on August 20.
The researchers collected air samples across Beijing and found that vegetation accounted for about 10% of volatile organic compound (VOC) emissions between May and July 2021. However, because VOCs from plants react far more readily with atmospheric chemicals, vegetation drove 52% of the chemical reactions that produce ozone — with isoprene, released by willows and poplars during photosynthesis, as the dominant contributor.
Roughly 35% of Beijing's trees are isoprene emitters, the team found. And the problem is poised to intensify: at 35°C, the rate at which vegetation-derived VOCs react with hydroxyls in the atmosphere was seven times higher than at 20°C. As global temperatures continue to rise, vegetation's role in ozone formation will only grow.
The study also examined 24 megacities worldwide and found that isoprene emissions in cities such as Sydney and Melbourne were predicted to be even higher than in Beijing — with about 65% of Sydney's trees classified as isoprene emitters.
Ian Jamie, an environmental chemist at Macquarie University in Sydney, noted that vegetation is becoming a proportionally larger source of VOCs because emissions from vehicles and industry have declined substantially over recent decades. As human-caused emissions fall, the relative contribution of urban trees rises.
The findings have immediate implications for urban planning and public health: cities choosing quick-growing, climate-resilient tree species may be inadvertently worsening the air quality problems they are trying to solve.
