Urban trees and the air we breathe

“Fog everywhere. Fog up the river, where it flows among green aits and meadows; fog down the river, where it rolls deified among the tiers of shipping and the waterside pollutions of a great (and dirty) city”. Charles Dickens, Bleak house, 1853.

On cold, blustery days, whether walking the dog, sitting with friends or doing nothing more than enjoying a casual stroll through the park, opportunity allows us to take respite from the pressures of life.

Sometimes admiring large, overarching trees, it is easy to forget how integral they are to our existence. As Fedrick Law Olmstead once observed “trees provide the lungs of the city”. As with the East End’s Victoria Park a ‘vital lung’ for those living in the slums of the 19th century.

Pollution in “Coketown”, a phrase coined by Charles Dickens in his novel Hard Times, referred to the polluted and degraded urban environments seen in cities such as Manchester and London, a glum consequence of an early, industrial Britain. For decades government policies and their actions have sought to reduce emissions and pollutant exposure, from the urban reform movement in the last quarter of the 19th century  to the Clean Air Act 1956, aimed at reducing the infamous smog known as “pea-souper” that choked London on a regular basis. As long as there have been cities, there has been pollution and our ever-evolving attempts to deal with it

With a constant growth in population and an influx of people moving closer to cities, the rise of fossil fuel consumption continues to make air pollution a major concern. Air pollution is a considerable threat to billions of people’s health and is detrimental to the environment. Within the UK, poor outdoor air quality is linked to 50,000 deaths each year. A study done on the 25th July 2018, ranked the most polluted cities in the world, finding that London ranked 23 on the list, Manchester at 28 and Liverpool 33. Improving urban air quality is a key factor in ensuring that our cities remain places where it is safe and enjoyable to live.

Over the past decades, open spaces and green infrastructure have shown to be great at combating air pollution, improving urban air quality, with the additional benefit of providing critical habitat for animals and plant species. Trees are essential in purifying the air that we breathe by capturing CO2 and releasing Oxygen through photosynthesis. Trees also release moisture in the air though plant respiration; heavy water molecules that drop out of the air capture molecules reducing pollen content, dust particles, and other airborne pollutants. In the urban environment where there are heat reflecting buildings and heat absorbing surfaces such as roads trees are needed to mitigate the urban heat island effect; they can provide shade and evapotranspiration, by cooling and lowering the air and surface temperatures, reducing the formation of ozone and other temperature-dependent pollutants.

London’s urban trees remove 2.4 Million tons of airborne toxins from the air per year. The leaf and plant structure uptake common air pollutants like Carbon monoxide, carbon dioxide, and Nitrogen oxide,  Sulfur dioxide, volatile organic compounds (VOC), ground-level ozone, and particulate matter of 10 microns PM10, (referring to the size of the particle that can be inhaled and potential to enter the human respiratory system). The City of London is trying to reduce the level of air pollutants occurring in the first places by introducing an Ultra Low Emission Zone (ULEZ) which forces vehicles exceeding set-out standards to pay a daily charge when travelling within the ULEZ. Increases in the number of ebikes and bike couriers across the city supports this policy.

With good urban design, we can reduce air pollutants, provide spaces that can encourage active functions and reconnect people with nature. It is essential to give consideration to species and position of trees; as greater tree foliage density such as evergreens are able to work as better screens, mechanical filters, reducing the flow and slow the air that passes through them. As the air is slowed it drops out heavier particles removing more air pollutants than less dense trees such as deciduous species. However many broadleaf species can be more efficient at capturing pollutants as the leaf surface area is greater. The size of the tree can also affect its ability to remove pollutants from the air, a study showed that trees with a larger tree canopy can remove 60 -70 times more pollutants from the air, then smaller ones (Nowak, Crane, Stevens. 2006) though younger trees can be more effective due to there denser foliage.

A simple and effective way to think about air quality management is to Reduce, Extend, Protect (Trees & Design action Group). So, by increasing the distance from the source allowing the pollutants to be intercepted by a tree or another form of green infrastructure, provides pollutants to be broken down and reduced, protecting people at a street level. Using different tree forms, densities, textures, and at different heights creates a rougher surface for air to bounce off and mix. Thus, cleaning the air and extending the distance between the polluted source and human receptor. With this same scenario, it is important to consider where you want pedestrian movement to be, by creating safer, healthier and enjoyable spaces for users to move through. One of the inner-city schemes where Exterior Architecture has put these ideas into practice is Kampus. On this project in central Manchester the client aspiration is for a green oasis (open to the public) providing respite and a green lunge for the densely built cityspere. 

Improving urban air quality at Kampus, Manchester

But caution is advised to avoid what are referred to as street canyons – this is where dense tree canopies can enclose streets and polluted air can get trapped within causing fumigation, forming ground-level ozone. Street canyons can be dangerous in trapping polluted air, but if the tree canopy separated from the receptor, the tree canopy can protect from outside pollutants, providing cleaner air for users and more functional space.

When planning large scale urban tree planting, it is essential to note that some trees emit natural Biogenic volatile organic compounds (BVOCs) more than others. These are harmless until they react with nitrogen oxides that are emitted through car exhausts or other forms of fossil fuels, and form ground-level ozone. Therefore, a low BVOC species of tree can be used to decrease this risk. An urban tree air quality score (UTAQS) has been developed that can be found on the woodland trust website (Smith, 2012), this shows the classification of 30 of the most common UK urban tree species listed from the best with the highest UTAQS and then the poorest with a Low UTAQS. Species such as Acer campestre, Pinus nigra, Betula pendula, and Acer platanoides have some of the highest UTAQS score; then Malus spp.; Tilia x europaea; and Prunus avium with a medium score; then finally with species such as Quercus rubra and Salix fragilis as the poorest with a low UTAQS score. Although the overall benefits provided by trees outweigh any negative effects or impacts of biogenic emissions, it’s also important to consider the positioning and heights of the trees within the space and where the source of the pollutant is coming from and where the human receptor is.

If we want to make a difference to air pollution within our cities and densely populated areas, it is essential that politically, socially and creatively all disciplines collectively convey the importance of providing well-designed spaces, incorporating green infrastructure and consider air quality at all stages of the design process. It has proven in the past that dense tree canopies and large areas of green infrastructure can provide considerable benefits to air quality by providing the “Lungs of City”, but they also provide healthier environments for people to live and connect. With this in mind, we can look to a future where cities and nature can be entwined.