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).

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).

 

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).