Urban Trees That Help Reduce Air Pollution

Since the industrial revolution, cities worldwide have undergone an extensive and neverending transformation. Creating more residential areas, business venues, and roads for the increasing population has put trees in the background. The lack of trees and overall vegetation in urban areas, along with a growing concentration of people and their daily activities, has generated higher levels of air pollution. For example, Beijing - The capital of China - frequently reports PM levels above 125 micrograms per cubic meter. According to the World Health Organization, the recommended threshold is 10 micrograms per cubic meter.

Thankfully, cities are now shifting to more eco-friendly urban landscaping and including more and more greenery every day, and for a good reason. Vegetation creates better habitats for wildlife and people, but it also helps clean the air and reduce pollution. In 2019, London announced that 7,000 trees would be planted by the end of 2020. Paris has the fantastic project of creating an urban forest next to iconic landmarks and wants to cover 50% of the city with planted areas by 2030.
These ideas sound fascinating, but their science is not as simple as just planting random trees. Cities cannot plant certain trees in some areas because they could endanger certain wildlife species or create an unstable environment.
Trees can improve air quality by shading surfaces and reducing temperature. By shading buildings and houses, trees reduce the need for air conditioning, thus reducing greenhouse emissions. Another way trees help is by directly removing pollutants from the air. Not only can they absorb carbon dioxide and emit oxygen, but trees can also filter pollutants like sulfur dioxide and nitrogen dioxide through their leaves. But, not all trees are as effective in reducing air pollution.
Generally speaking, trees effectively remove particulate matter from the environment, which can cause severe problems to human health, but not all trees can filter it in the same way. This difference widely depends on the canopy and leaf size and structure. Bigger canopies can trap smaller particles, and larger leaves trap more pollutants than smaller ones. Moreover, leaves with rough and rugged surfaces have higher efficiency when filtering PM. Research findings suggest that tiny hairs on the leaves have an important role in trapping PM made of solid and liquid particles.

Which trees are best to plant?

In a 2015 research done by Jun Yang, an Urban ecologist at the Center for Earth System Science at Tsinghua University in Beijing, he found that the best pollution filters were the pines and cypresses. The London plane, silver maple, and honey locust also ranked above average, as the canopy structure of the conifers allows them to trap pollutants in a highly efficient way.

Furthermore, conifers are an evergreen species and can act as a filter all year round. Even though these species are highly effective, Jun Yang also highlights the importance of biodiversity and context. Just because one tree is exceptionally efficient in absorbing pollutants doesn't mean it should or can be planted everywhere. Factors like maintenance and lifespan should also be considered when planting a tree.

In the article "Designing vegetation barriers for urban air pollution abatement: a practical review for appropriate plant species selection." written by scientists Yendle Barwise and Prashant Kumar, you can find a guide with 61 species with 12 traits suited for air-pollution reduction. You can find and select the species that best suit your context and reduce roadside pollution. The guide also points at species with the opposite effect, like VOCs, pollen generators, and high-maintenance trees. So now you know! Have a look at the guide and plant some trees. Remember to monitor your indoor and outdoor air with your PurpleAir sensor.