They discover that some species favor the formation of ozone, harmful to health and the environment
Not all diseases have a cure. Nor those suffered by the Earth. Even trees don’t always work as a miracle therapy for urban air pollution. In fact, Under certain conditions, some trees can have a detrimental effect on air quality.
This is the conclusion of the research carried out by Donato Kofel, a graduate in Environmental Science and Engineering from the Federal Polytechnic School of Lausanne (EPFL), for his master’s project. Kofel quantified the positive and negative effects of trees on air quality exterior in the canton of Geneva, in Switzerland.
To do this, he delved into the world of geographic information systems (GIS), a type of advanced mapping software. “These maps convey a lot of information in a single image, in a way that allows people to capture it all at once,” he says.
The researcher developed a new way to use the GIS application to study how trees are affecting air quality of the canton of Geneva. Kofel relied on an inventory containing around 240,000 ‘isolated’ trees; that is, located outside of forested areas.
They are, for example, trees that line a boulevard or planted in a city park. The isolated trees of the Geneva inventory represent around 25% of the total of the canton. The inventory lists various tree characteristics, such as species, location, trunk height, trunk diameter, and crown diameter.
The analyst used these data to generate maps of the total leaf area of the trees, which in turn gives an indication of their ability to filter particles from the air. In parallel, he also studied another important process: the role of trees in the formation and deposition of ozone.
The oaks, in the spotlight
The positive aspects of trees are well known; among others, help lower the temperature in urban areas, filter fine particles of the atmosphere, capture CO2 and release oxygen.
But they also have some negative aspects. For example, naturally emit biogenic volatile organic compounds (BVOC) at a rate that depends on factors such as the tree species, air temperature and humidity, amount of sunlight, whether the trees have been damaged or are stressed.
Oak trees emit compounds that convert to ozone, which can be harmful to people
These BVOCs then they turn into ozone by photochemical oxidation with other compounds in the air that are emitted by human activities, and it has been shown that ozone negatively affects health and the environment.
The environmental engineer calculated the ozone formation potential of the emissions from the trees and was surprised to find that, under certain conditions, trees can also have a detrimental effect on air quality.
Their first task was to collect literature on the 51 most common tree species in the canton of Geneva and use this information to calculate their hourly BVOC emission rates.
During the analysis, it was found that some oak species have some of the highest BVOC emission rates, and therefore the greatest potential for ozone formation. And the oak is the tree that is most frequently found in the streets and parks of the Geneva canton.
With the help of two other scientists, Romana Paganini and Ilann Bourgeois, he estimated how much particulate matter and ozone urban trees filter each year to emphasize their positive effect. The elaborated maps suggest that lUrban trees removed about 25% of the particles produced by anthropogenic activity in the canton of Geneva.
additional air pollution
But the research also found that the ozone-forming potential of these trees is about 10 times their removal potential, and that emit 130 metric tons of BVOCs per yearwhich is equivalent to around 18% of the VOCs emitted annually by road traffic.
The results show that anthropogenic activities emit enough nitrogen oxides for the proper chemical reactions to form ozone to take place.
“The potential exists to reduce ozone formation from trees by reducing human emissions of nitrogen oxidesdepending on the actual mix of BVOC and those oxides,” explains Sandrine Perroud, EPFL communicator.
Translated: there is no clear answer on how good urban trees are for air quality because the combination with anthropogenic emissions can lead, under certain conditions, to additional air pollution.
Kofel points out that more in-depth studies are needed to clear up all the questions. For now, though, his findings show that while trees can make an important contribution to improving urban air quality, they are not a miracle cure in all conditions.
“The problem of air pollution must be addressed at its source, taking into account both road traffic and other sources of emissions,” adds Kofel. In any case, urban planners can use the method of this environmental engineer to design their large-scale planting programs more effectively.
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