Published on 01/10/2024
Every year millions of people around the world breathe in polluted air, endangering their health. Concentrations of fine particles, nitrogen dioxide (NO2) and ozone (O3) are particularly high in cities and other urban areas. Trees offer a natural solution to improving everyone’s quality of life.
There are many benefits to trees: they reduce heat islands, regulate the water cycle and promote biodiversity in urban spaces. But beyond these well-established roles in regulating urban microclimates, what impact do trees have on urban air quality?
In this article, we look at the “filtration” and release mechanisms of trees, and the importance of choosing the right trees to plant in urban areas.
Trees help reduce high-pollution episodes in urban areas in two ways:
The relationship between atmospheric compounds and trees varies immensely from one species to the other. For example, conifers such as Scots pine (Pinus sylvestris) capture more fine particles than broadleaf trees due to their complex structure and adherent, wind-resistant needles. Another advantage is that, like most conifers, they do not lose their needles in the winter.
The health and size of a tree also affects its ability to reduce air pollution; a large, healthy tree can remove up to 60 times more pollution than a small one.
Coniferous vs deciduous trees
Although trees absorb pollutants, they also emit what are known as biogenic volatile organic compounds. BVOCs function as a means of communication between trees and between trees and insects. They are also produced in response to stresses caused by heat, drought or disease.
BVOCs pose no immediate risk to human health. It is only when they get converted over time that they become a source of pollution.
As BVOCs oxidize in the atmosphere, they form secondary organic aerosols, which are known to be a component of fine particulate matter (PM2.5).
For example, the impact of BVOCs on the concentrations of fine particles and ozone was studied in Paris in June and July 2022. The study found that the incidence remained localized and minor, with the concentration of PM2.5 and O3 increasing on average by just 0.6% and 1%, respectively. However, these concentrations doubled during heat waves, when trees in the city produced BVOCs (PM2.5: +1.3%, O3: +2.4%).
It has also been observed that fine particles increase more when the chemical composition of BVOCs consists of more terpenes (monoterpenes and sesquiterpenes), particularly from conifers. On the other hand, terpenes seem to have little effect on ozone formation.
That is why cities must take into account the conversion of BVOCs and the increased frequency of heat waves when planting conifers. Low-terpene trees, such as arborvitae (thuja), help reduce the formation of secondary fine particles.
Does this mean that just one species should be planted throughout a city or neighborhood? On the contrary, a diversity of species is crucial to increasing the resilience of trees to climate change and disease.
Watering an urban tree to avoid moisture stress in the summer:
Planting the right type of tree in an urban environment is strategic.
As we’ve seen, trees take up gases and trap fine particles suspended in the air. But another point worth noting is that trees have an influence on the dispersion of pollutants. For example, a dense tree canopy extending over a narrow, highly polluted street can prevent air circulation and allow pollutants to linger and accumulate.
Urban layouts and the pollutants present should be considered to ensure pollution is captured rather than allowed to accumulate.
Planners in cities with high local pollution and narrow streets cut off from the wind should prioritize the planting of low-height plants with large pore surface areas, such as dense hedges or ground cover, to promote air dispersion and reduce local pollution.
Conversely, trees are more effective in wide-open spaces and broad streets. In addition, they reduce background pollution levels.
In conclusion, trees are key allies in the fight against urban air pollution. Their ability to absorb pollutants, reduce heat islands and promote biodiversity makes them essential assets to the world’s cities. However, the choice and planting of species must be carefully considered to maximize their benefits and minimize their negative effects, particularly when stressors occur.
Having environmental data on a particular urban area before starting a planting project is vital. Ecomesure’s solutions enable cities to quantify the pollutants present in a given urban environment and assess the effectiveness of urban planning and landscaping on air quality. By combining these data with tools such as France's Municipal Biodiversity Atlas, cities can develop customized planting strategies and monitor their impact over time.
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