sunny morning in a forest

What a Breath of Fresh Air!

We now know yet another way that trees help combat air pollution, and this knowledge could play a big part in environmental policy.


The fresh air from the forest goes beyond trees cleaning up carbon dioxide. It turns out that the shade from forest canopies provides darkness that reduces light-triggered photolytic chemistry, and this leads to reduced formation of ozone pollutants in the air we breathe.

Ozone, most widely known for its critical role in filtering out ultraviolet radiation in the high-altitude ozone layer, is actually classified as one of the five major air pollutants at ground level. It is a key component of smog, and it triggers a vast number of serious respiratory conditions, like chronic bronchitis and asthma. Surface ozone can also damage crops and trees.

Paul Makar, senior research scientist at Environment and Climate Change Canada, says the reduction of ozone formation under forest shade can cut surface ozone pollution in half.

Recently published in Nature Communications, this new understanding solves the mystery of why ozone pollution is so often overestimated by our best computer models, and paves the way for more accurate air quality forecasting.

Field measurements confirm that ozone levels drop below the forest canopy. Combined with satellite image data that describe the density of forest foliage, Makar found that there were also varying levels of impact depending on the level of shade provided. He notes that this also means that the effect is seasonal: more dense summertime foliage leads to a greater reduction in ground-level ozone.

Another important factor that was missing from previous models was the stillness of forest air. The trees block the wind, isolating the air under the forest canopy from the rest of the atmosphere.

It took a year and a half, but Makar and his team have finally gone from their initial observations to an updated mathematical description to Environment Canada’s air quality model.

Beyond prediction, Makar’s work also presents implications for policy: clearing forested land immediately impacts air quality in more ways than were previously recognized. This adds another layer to the importance of conservation of our local forests and parklands.

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Karyn Ho is a science animator and engineer who thrives at the interface between science, engineering, medicine, and art. She earned her MScBMC (biomedical communications) and PhD (chemical engineering and biomedical engineering) at the University of Toronto. Karyn is passionate about using cutting edge discoveries to create dynamic stories as a way of supporting innovation, collaboration, education, and informed decision making. By translating knowledge into narratives, her vision is to captivate people, spark their curiosity, and motivate them to share what they learned.