Trees have long been viewed as a positive contributor to urban air quality. A recent study from EPFL highlights the positive and negative impact trees have on air quality in the Geneva area.
The researcher utilized data from a tree inventory in Geneva Canton. This inventory had data on about 240,000 “isolated” trees, which are trees outside of forests. These trees can be along streets or in city parks.
In isolation
These isolated trees make up about 25% of all trees in the canton. The inventory includes details about the tree’s type, where it’s located, how tall and wide the trunk is, and how wide the top part of the tree is.
“I used these data to generate maps of the trees’ total leaf area, which in turn gives an indication of their ability to filter out particulate matter from the air,” they explain.
Trees naturally release certain compounds called biogenic volatile organic compounds (BVOCs). How much they release depends on factors like the type of tree, the weather (temperature and humidity), how much sunlight they get, and if they’ve been damaged or stressed.
These BVOCs then mix with other substances in the air that come from human activities. This mix, when exposed to sunlight, turns into ozone, which is known to be bad for both our health and the environment.
Negative impact
When measuring the ozone-forming potential of trees, the researcher surprisingly found that, under certain conditions, trees can have a negative impact on air quality.
The maps show that trees in urban areas help remove about 25% of the tiny particles created by human activities in Geneva Canton, as per the 2014 assessment of emissions in that region. The researcher also discovered that these trees produce around 10 times more substances that create ozone compared to what they can remove, and they release 130 metric tons of certain compounds (BVOCs) each year. This is almost 18% of the total amount of similar compounds released by cars each year.
These findings highlight how human activities release enough pollutants (nitrogen oxides) that lead to specific chemical reactions. This means that we could potentially reduce the ozone produced by trees by lessening human emissions of nitrogen oxides. This all depends on the exact mix of the tree compounds and nitrogen oxides in the environment.
No miracle cure
The researcher believes this should remind us that trees shouldn’t be regarded as a miracle cure, and that more research is needed to ascertain when they can be useful and when they are not.
“There are still some question marks surrounding our estimates, and I’m working to make them more robust. Also we did not take into account the formation potential of particulate matter from BVOCs,” they conclude.
“But for now, our findings show that even though trees can make a major contribution to improving urban air quality, they’re not a miracle cure in all conditions.”
