Beijing's Trees Are Secretly Making the City's Smog Worse

Scientists studying Beijing's air pollution made a surprising discovery: the city's beautiful poplar and willow trees are major producers of ozone smog. An international team from Jinan University found these trees release large amounts of isoprene, a natural chemical. When isoprene mixes with nitrogen oxides from cars or factories in hot sunlight, it forms ground-level ozone — the main ingredient in smog.

This matters because Beijing has spent years trying to clean its air, including switching many cars to electric. But ozone levels stayed puzzlingly high. The trees, which were planted decades ago to provide shade and green space, are now seen as contributors to the problem. Ozone is a serious health risk, particularly for children, the elderly, and people with respiratory issues like asthma.

The science is simple: isoprene itself is harmless. But in a city full of traffic and industry, it reacts with pollution and becomes a toxic gas. The researchers were trying to measure human-made ozone, but discovered the trees emit far more isoprene than expected. In fact, they say the city's hundreds of thousands of willow and poplar trees are the most likely culprit behind the persistent smog.

The takeaway isn't that we should cut down trees. Instead, it shows that urban planning needs to think more carefully about which trees we plant. Some species emit more isoprene than others. Cities around the world may need to choose low-emission trees while continuing to reduce vehicle and factory pollution. After all, the goal is clean air and green cities — and it turns out we can have both, but only with smarter choices.