A headline claiming that trees “make smog” is attractive and misleading. Plants naturally release volatile organic compounds such as isoprene. On a hot, sunny day these compounds can react in the presence of nitrogen oxides and contribute to ground-level ozone. Yet urban nitrogen oxides come primarily from combustion and transport. Without that human contribution, the chemical equation looks very different.
Long-term research has examined emissions from poplars and willows under Beijing conditions. Its value is not that it identifies trees as culprits, but that it improves models of what happens during a heatwave. A city can successfully reduce PM2.5 while still facing summer ozone because the pollutants are formed through different processes. A policy focused on a single indicator would miss this transition.
Ground-level ozone is not the protective ozone layer high in the atmosphere. In the stratosphere ozone blocks ultraviolet radiation; near the ground it irritates airways, worsens asthma and damages plants. Peak levels need not occur directly beside an exhaust pipe, because pollutants change chemically while travelling across the city and region. A suburban park can under certain conditions record more ozone than a congested junction.
The answer is not to remove urban greenery. Trees provide shade, reduce heat stress, retain water, buffer noise and create habitat. Some species also intercept particulate matter. Good urban planning weighs these benefits against allergenic potential, water demand and volatile-compound emissions. Selecting a species is a technical decision, not a referendum for or against trees.
Ozone chemistry is nonlinear. Reducing nitrogen oxides or organic compounds may have different effects in different parts of a city and may not immediately produce the expected decline. Models therefore need ground measurements, satellite observations, meteorology and information about vegetation. Research on tree emissions supplies one missing layer; it is not a complete municipal instruction manual.
The issue matters as heatwaves become more frequent. Ozone formation accelerates in sunlight, while cities plant thousands of trees without always considering species, location and long-term care. “More greenery” is not a technical specification. Crown size, available soil and water, species diversity and simultaneous reduction of traffic emissions determine whether a planting programme works.
Visitors should follow ozone separately from particulate pollution during hot weather. A respirator designed for particles does not offer the same protection against gaseous ozone. Reducing intense outdoor activity in the afternoon, when levels can peak, is more useful. An early walk may be a better choice than a run along a sun-baked avenue after lunch.
The correct conclusion is not “trees are harmful” but “a city is a reactor”. Temperature, sunlight, exhaust gases, industrial emissions and biological processes interact. Beijing has greatly reduced classic particle smog; the next stage requires more precise management of ozone and climate. That complexity is the study’s most valuable message for cities everywhere.
