Emma Ferranti, James Levine & Rob MacKenzie
April 2019

Role of trees & other green infrastructure in urban air quality

environmental SCIENTIST | Right Tree • Right Place | March 2019

Emma Ferranti, James Levine and Rob MacKenzie explain the role of vegetation for air quality management in our cities. 

‘Green infrastructure’ (let’s shorten that to ‘GI’ but remember we’re talking about trees, not Joes) refers to all vegetation in urban areas, including parks, private gardens, green roofs and walls, grass verges and street trees. GI is ‘infrastructure’ in the sense that it brings a multitude of environmental benefits to our towns and cities.1 Some of these benefits are at risk of being underestimated while we lack a means of measuring them, that is, of measuring the natural capital associated with the ecosystem services that GI provides.

Urban practitioners are familiar with the notion that GI provides space and connectivity for nature, providing and linking habitats for plants and animals and thereby increasing biodiversity. It is also widely understood that GI can increase urban resilience to extreme weather, such as heavy rainfall events and very hot summers, expected to increase in frequency as a result of climate change.2 As examples, sustainable urban drainage systems (SUDS) store rainwater and attenuate its release, reducing demands on mains drainage; and trees mitigate the urban heat island effect through the creation of cooler microclimates via the provision of shade and transpiration (the uptake of groundwater and the release of water vapour).

Steps are being taken to quantify these ecosystem services. The Construction Industry Research and Information Association has developed a freely available Benefits of SUDS Tool (B£ST). Meanwhile, a recent study by the Forestry Commission highlighted the monetary value of tree transpiration, estimating an annual saving in air conditioning costs across London ranging from £2.1–84 million.3 There are also economic benefits: attractive placemaking increases footfall and potential customer numbers increase, benefitting local businesses and stimulating the ‘business of serendipity’, thus fuelling greater productivity.

Many of the benefits of GI have a direct bearing on human health, translating into health costs saved and working days gained. For urban inhabitants, GI can provide space for recreation and physical activity, and confers benefits for mental health too, including: psychological relaxation, stress alleviation and increased social cohesion.4 Public Health England recently commented, ‘If green infrastructure was a pill, every GP in the country would be prescribing it’.5

GI also offers significant physical health benefits via improved air quality, but not as we perhaps expect. Pollutants are deposited to leaf surfaces, but the fraction of pollution removed by this mechanism in the urban environment is typically just a few percent, owing to the small scale of realistic planting schemes6 and the relatively slow rate of transfer of pollution particles and molecules to (leaf) surfaces. The value of GI for urban air quality lies in its ability, not to remove pollution, but rather to control its distribution by strategically enhancing (or reducing) its dispersion close to its source7. For instance, in an open-road environment and under the right wind conditions – blowing from vehicles towards pedestrians – a vegetation barrier can halve the concentrations of pollutants in its immediate wake.6 As explained below, GI can be of benefit, dis-benefit or of little consequence for air quality. However, used strategically, i.e. with the right vegetation in the right place, GI offers considerable benefits in terms of the public health impact of urban air pollution by altering the public’s exposure to it.

The University of Birmingham is developing a software platform with urban practitioners to enable them to predict quantitatively the impacts of a range of interventions on exposure, on a site-by-site basis. Meanwhile, as outlined below, certain interventions will reliably reduce exposure.

Urban air pollution
The World Health Organization identifies air pollution as the greatest environmental risk to human health:8 90 per cent of the world’s urban population live in cities exceeding its air quality guidelines, and outdoor air pollution claims roughly 3 million lives each year. In the UK alone, outdoor air pollution contributes to approximately 50,000 deaths each year,9 and road transport has been identified as the main source of directly emitted emissions in urban areas.10 Roadside air pollution often exceeds national air quality objectives and has been the subject of litigation against the UK government.11 In these reports, and in what follows, the key pollutants are microscopic particulate matter (PM) and nitrogen dioxide (NO2). However, there are some pollutants – notably ozone and secondary PM – where the relationship with emissions is not linear,12 but we do not consider those here.

Figure 1. The role of trees and other green infrastructure in urban air quality.13 (© Trees and Design Action Group Trust).


Urban form (the size, shape and configuration of our built environment) affects the location and strength of road transport sources of pollution and, importantly, its subsequent dispersion (dispersion refers to the way that air dilutes pollutants and carries them away from their sources). Critically, the impact of road transport pollution on human health depends on the concentration of pollutants at point of exposure – in other words, not only the amount of pollution emitted at source but also how much it has dispersed en route to its human ‘receptors’. The total public health impact also depends on the number of people exposed, the length of time for which they are exposed, and their vulnerability; the very young and the elderly are particularly vulnerable, as are people with certain pre-existing medical conditions. 

Through its impact on the emission and dispersion of pollution, and hence the extent of exposure, urban design has a significant bearing on the public health impacts of pollution emissions. Good urban design provides a tool with which to reduce these impacts and improve health outcomes (and health equality) via the application of three key concepts, listed here in order of priority:13

  1. Reduce emissions, particularly from road transport. This is by far the most effective way to reduce urban air pollution and improve public health outcomes.
  2. Extend the distance between sources of pollution and human receptors (this is called the source–receptor pathway). Pollutant concentration is highest close to the emissions source but decreases with distance, initially very quickly, as a result of mixing with cleaner ambient air. Increasing the dispersion of pollutants between source and receptor reduces the concentration at the point of exposure.
  3. Protect the most vulnerable people. Anyone can suffer negative health impacts from air pollution, but children under 14, adults over 65 and those with pre-existing health conditions, such as chronic obstructive pulmonary disease (COPD) or asthma, are most vulnerable.14
Figure 2. Green infrastructure can be used to create heterogeneous surfaces that stimulate the mixing, and hence dilution, of relatively polluted air at street level with relatively clean air above it.13 (© Trees and Design Action Group Trust).
GI – already an ingredient in good urban design – can help to reduce, extend and protect (see Figure 1). Green open space often takes the place of what would otherwise include further sources of pollution, implicitly reducing emissions. Parks and tree-lined roads can also create green corridors that encourage active transport, such as walking and cycling, in preference to driving, further reducing emissions. GI, whether it is green open space or trees, hedges and green walls, helps to create an urban form with a more variable topography and texture. This creates more turbulent air flows, stimulating mixing between relatively polluted air at street level and the relatively clean air above it15,16 (see Figure 2) and tending to extend the source–receptor pathway. Parks, meanwhile, tend to draw people, including vulnerable people, away from polluted areas into cleaner ones, and hence have a role to play in protecting people. The strategic use of trees, hedges and green walls as vegetation barriers in urban canyons (i.e., streets bounded by buildings on both sides) to extend the source–receptor pathway, and thereby protect people at the kerbside, is the subject of the next section. 
Grenn infrastructure for roadside air quality
In the urban environment the value of green infrastructure for roadside air quality – what we are dubbing ‘GI4RAQ’ – lies in the strategic use of vegetation as physical barriers to extend the source–receptor pathway. At the scale of realistic planting schemes, deposition on leaf surfaces typically removes just a few percent of particulate matter (PM) and a similarly small fraction of nitrogen dioxide (NO2); what NO2 is deposited is offset by accompanying soil emissions of nitrogen monoxide (NO) – subsequently converted into NO2.
Likewise, the emission of volatile organic compounds (VOCs) associated with ozone formation from this scale of vegetation has only a minor impact on air quality.17 Vegetation is only responsible for a small fraction of total urban VOCs, and as ozone formation takes a certain length of time, their minor impact is predominantly felt at a distance downwind, rather than at the point of VOC emission.6 Some VOCs, such as isoprene, have greater impact than others– and emissions from vegetation are expected to increase somewhat as the climate warms. It may be prudent to plant fewer trees of species known to be particularly strong isoprene emitters,18 but simply planting a mixture of species will mitigate any concerns, and species selection19 must take many other factors into account, not least those governing successful long-term growth. The key to GI4RAQ in urban canyons is controlling the distribution of pollutants, by either enhancing or reducing their dispersion (dependent on the site in question) to reduce their concentrations at point of exposure.
Figure 3. The effect of a dense avenue of trees in an urban canyon depends on whether the air at street level is cleaner or more polluted than the air above it. By reducing mixing between the two, a dense canopy on a quiet street protects relatively clean air at street level from the import of polluted air from above (top panel). On a busy street, however, a dense canopy risks trapping pollution at street level (bottom panel).13 (© Trees and Design Action Group Trust).


The first consideration in identifying what GI will be beneficial is how the air quality at street level compares with the average air quality above the surrounding buildings. We often first think about options to reduce exposure on highly trafficked roads, where pollutant concentrations are highest. There is potential, however, to reduce the overall public health impact of road transport pollution by protecting roads with little or no traffic from the import of pollution from above. A dense avenue of trees, forming a near-continuous canopy, can provide very effective protection from downward dispersion. Meanwhile, the increased residence time of air beneath the canopy makes the deposition of pollutants to leaf surfaces more effective. The combination of protection from more polluted air above and enhanced deposition below can create a clean, green corridor (see Figure 3, top panel). Note, however, that trees spaced more widely have little effect on vertical dispersion but do deliver the many further environmental, health and socio-economic benefits outlined earlier.
Meanwhile, vegetation barriers to the horizontal transport of pollution may be of considerable benefit in reducing public exposure at the kerbside on highly trafficked roads. The addition of a hedge (or green wall) between vehicles and pedestrians in an urban canyon (or on a more open suburban road) may not achieve the 50 per cent reduction in pollutant concentrations achievable under idealised conditions (see above).6 However, in all but the deepest and/or narrowest canyons, it will reliably extend the source–receptor pathway and thereby reduce concentrations at point of exposure; see Figure 4. (In canyons with a height/width ratio >2, the air flow is complex and the addition of barriers is not recommended without fluid dynamic modelling of the specific situation.)
If sufficiently dense and suitably maintained, green walls can be used in place of hedges as effective vegetation barriers between vehicles and pedestrians. They also offer some potential to reduce road transport pollution in highly trafficked urban canyons when mounted to building facades,20 but further research is needed to quantify their benefits. A computer-modelling study found that they not only provide surfaces for pollutant deposition but interact with air flow (via surface roughness) to alter the average residence time of air in the canyon. The significant modelled reductions in PM10 (and NO2 to a lesser extent) justify further research.21
Figure 4. Green infrastructure barrier to extend the distance between emissions source and receptor, and protect vulnerable people on the roadside.13 (© Trees and Design Action Group Trust).


He who plants a tree plants a hope22
The world is becoming increasingly urbanised and the United Nations estimates that by 2050, 68 per cent of the global population will live in urban areas. It is imperative that our urban areas are resilient to extreme weather and future climatic change, and are healthy, liveable places for their inhabitants. The importance of GI in creating resilient urban environments is becoming acknowledged nationally and internationally. Cities are leading this green revolution: Birmingham is part of the international biophilic cities network,23 aspiring to place nature at the heart of all planning decisions; Greater Manchester aims to be the UK’s first zero-carbon city by 2038 and is planting a tree for every resident within a generation.24 In June 2019, London will become a National Park City making the city greener, wilder and healthier for its residents.
Strategic green infrastructure has a role in reducing exposure to urban air pollution. First and foremost, we must reduce road transport emissions at source. Reducing exposure to what is emitted, however, provides a means of further reducing the impact on public health. As part of good urban design, GI can also be used to create cleaner spaces where people prefer to spend time – and choose to walk or cycle instead of hopping in the car. Meanwhile, GI provides a multitude of further benefits, including: increased biodiversity; urban resilience to extreme weather in the form of increased thermal comfort and sustainable urban drainage; mental and physical health benefits; and attractive placemaking for communities and business. There is no need to over-sell the benefits, but there is a need to state them clearly and often. Our most valuable urban infrastructure is green.

Emma Ferranti and James Levine are senior researchers at Birmingham Instistute of Forest Research (BIFoR) at the University of Birmingham. Rob MacKenzie is Professor of Atmospheric Science and Director of BIFoR. Ferranti holds​ NERC Knowledge Exchange (NE/N005325/1) and EPSRC Living With Environmental Change (EP/R007365/1) Fellowships; Levine and MacKenzie have NERC Innovation funding for​ GI4RAQ (NE/S00582X/1, NE/S00940X/1, NE/S013814/1). All authors contribute to GI work strand in WM-Air (NE/​S003487/1).


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17. Hewitt, C.N., Ashworth, K. and MacKenzie, A.R., 2019. Using green infrastructure to improve urban air quality (GI4AQ). Ambio, pp.1-12.

18. Donovan, R.G., Stewart, H.E., Owen, S.M., MacKenzie, A.R. and Hewitt, C.N. (2005) Development and application of an urban tree air quality score for photochemical pollution episodes using the Birmingham, United Kingdom, area as a case study. Environmental Science & Technology, 39 (17), pp.6730–6738. https://pubs.acs.org/doi/pdf/10.1021/es050581y

19. Hirons, A.D. and Sjöman, H. (2018) Tree Species Selection for Green Infrastructure: A Guide for Specifiers. http://www.tdag.org.uk/species-selection-for-green-infrastructure.html

20. Weerakkody, U., Dover, J.W., Mitchell, P., and Reiling, K. (2019) Topographical structures in planting design of living walls affect their ability to immobilise traffic-based particulate matter. Science of The Total Environment, 660, pp.644–649. https://doi.org/10.1016/j.scitotenv.2018.12.292

21. Pugh, T. A., MacKenzie, A. R., Whyatt, J. D., and Hewitt, C. N. (2012) Effectiveness of green infrastructure for improvement of air quality in urban street canyons. Environmental Science & Technology, 46 (14), pp.7692–7699. https://pubs.acs.org/doi/10.1021/es300826w

22. Lucy Larcom (1824–93) Plant a Tree. https://www.poetrynook.com/poem/plant-tree

23. Biophilic Cities. http://biophiliccities.org/

24. Manchester City of Trees. http://www.cityoftrees.org.uk/


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