Story at a glance:
- Air pollution causes millions of deaths each year, but urban green infrastructure helps fight it.
- Common airborne pollutants are often divided into primary and secondary pollutants. Secondary pollutants Are often exacerbated by high temperatures.
- There is often a solid ROI for investing in green infrastructure outside and inside buildings.
Air pollution is now in the top five out of 87 risk factors in the WHO Global Assessment. Air pollution is estimated to cause 4 to 9 million deaths annually, on par with deaths attributed to unhealthy diets and smoking. In 2019 a staggering 90% of the world’s population lived in areas where they breathed air with concentrations of PM2.5 that exceeded the WHO air quality guidelines. These statistics are staggering, but not insurmountable. Urban green infrastructure is a valuable tool in battling air pollution and could help save and improve the quality of many lives.
Air Pollution Soup
The air we breathe contains pollutants from many sources, both natural and anthropogenic (man-made). However, in urban areas, anthropogenic sources dominate and can primarily be attributed to the combustion of fossil fuels from transportation, energy generation, and industrial processes. Nonetheless, air pollution particles from other sources can also contribute to the urban air pollution soup, namely dust from construction sites, tire particles, and long-range transport of particles from far-away sources like forest fires, burning of agricultural land, or dust storms.
Another important air pollution source is the resuspension of particles within a city. This can be particularly bothersome when boundary layers form, trapping and resuspending polluted air at street levels where people walk and live. For example, road dust settles, gets stirred up by vehicle movement, and then hangs in the air. Children and elderly living within particularly polluted areas are especially vulnerable due to their limited range of movement and prolonged exposure to these dangerous, carcinogenic compounds.
Common airborne pollutants are often divided into primary and secondary pollutants. Particulate matter (PM) is an example of so-called primary pollutants, i.e. they are directly emitted from the source in measurements of PM2.5 (with a diameter of 2.5µm), PM10 (with a diameter of 10 µm), and ultrafine particles (UFP) (0.1µm-100nm in diameter). PM2.5 can penetrate deep into the lung tissue and impair alveolar function.
Secondary pollutants like organic carbon, nitrates, ozone, and sulfates are formed from precursors originating from combustion sources, agriculture, and other processes. The formation of these secondary pollutants is often exacerbated by high temperatures, which is bad news when we consider the urban heat island effects combined with ever-increasing summer temperatures in most western cities. The good news is that green infrastructure can affect the formation of these secondary compounds by reducing the heat island effect through evaporative cooling.
It is concerning that not only has a relationship been found between PM2.5 exposure and acute respiratory infections, COPD, lung cancer, stroke, and heart diseases, but also with neonatal mortality like low birth weights and short gestation. Recent research also suggests an increase in the prevalence of Alzheimer’s and other neurological diseases after prolonged exposure to elevated air pollution concentrations.
A recent academic article was published explaining the biological background of how air pollution causes cancer by awakening dormant mutations in our lungs. Contrary to smoking, we have no control over the air we breathe and are completely dependent on decision-makers and policymakers for our health. Hence it is critical that people become more aware of this issue to ensure good political decisions for the protection of the urban inhabitants.
It should also be noted that the mortality and disease burden mentioned above is only based on PM2.5 and ozone, as the health effects of the other pollutants are poorly understood and reported. We are likely dealing with a much higher mortality and disease burden from air pollution than what is currently reported.
Apart from the suffering and death from air pollution, there are also economic costs. First are those directly related to the health care system. The second cost pathway is lost labor productivity. The World Bank (2013) estimated that the global economic impact from lost labor productivity due to exposure to PM2.5 was $143 billion. The global welfare losses due to PM2.5 were estimated to be $3.55 trillion. In 2016 the World Bank estimated air pollution-related welfare losses were equivalent to 5% of the gross domestic product in high-income countries.
It is essential to put the installation cost of green infrastructure in this context to show that there is often a good solid return on investment for an investment in green infrastructure outside and inside the buildings.
Old is New for Pollution Reduction
Using urban green spaces to improve air quality and cool areas and buildings is far from novel technology. Go back to the hanging gardens of Babylon or the city gardens of the Mediterranean more than 2000 years ago for examples of the human need for greenery. The industrial revolution introduced issues of urban sprawl, overpopulation and crowding, increased lack of open public spaces, and severe air pollution. These urban challenges led to the emergence of public parks. The famous NYC Central Park is a modern, western example of how a green space was created within an urban area to provide a place for recreation and fresh air. You can read more about public parks in Handbook on Green Infrastructure: Planning, Design, and Implementation.
From 1868 to 69 a comprehensive plan for an interconnected network of green spaces was laid out for Buffalo, New York. In 1894 Boston followed suit. This development was a response to the increasingly unhealthy urban environment and poor air quality. Today green infrastructure is seen as an interconnected network of open green spaces, including green roofs, green facades, and living walls. We are dealing with old technology adapted and developed in new ways for our current times.
How do Green Areas Remove Air Pollution?
We cannot continue with business as usual and fall into a sense of complacency in thinking that the air pollution problem has been solved merely by adding green infrastructure. The most critical step is to reduce the pollution at the source.
Air pollution can be removed via several processes, often broadly divided into wet and dry deposition. Wet deposition tends to involve less pollutant deposition than dry deposition.
Wet deposition involves things like mist and fog and is more effective on vegetation than on built surfaces. This is due to the structure of the plant surface area. However, wet deposition of rain and snow has been reported to be equally good on vegetation and built surfaces.
Dry deposition involves gas and particle transfer to various surfaces and is a continuous process contrary to wet deposition, which is episodic. Therefore, dry deposition is the primary process for the removal of air pollutants by green infrastructure. Dry deposition mainly occurs through three processes:
Movement from a high to a low concentration on a surface.
Removal of sedimentation from the atmosphere.
Direct interception and impaction on a surface.
The deposition rate depends on the concentration, mass, size, turbulence, and the nature of the surface; is it sticky (does it have ridges, wax, hairs), and what is the size and form? The rate of uptake of gasses is also determined by the stomatal resistance of the plants. This refers to how easily gasses can be exchanged through the “breathing holes” on the leaves.
It is also vital to consider if the pollutant can be re-suspended into the air (short-term removal) or if it has become sequestered (integrated into a structure for long-term removal). It is also possible that the pollutant is transferred from surfaces to the soil after a rain event or when a leaf is composted and broken down in the soil.
Due to all these factors, there is significant variation in how plants can alleviate air pollution. Here are several points to consider when determining the effect of green infrastructure on air pollution mitigation:
- Which pollutant is targeted?
- How does the local climate influence deposition?
- Which plant species are we planning to add? And what is the mix of trees, vines, shrubs, and groundcover?
- How is the vegetation positioned in relation to the pollution source?
This list is not exhaustive, but it is a start.
It’s Not Always About Direct Pollution Capture
Wind flow is a significant factor when it comes to the effect of green infrastructure on air pollution removal in cities. Direct deposition is critical and high when it comes to urban vegetation, but there is also a secondary factor governed by the vegetation generated changes in thermodynamics.
Polluted air can be trapped at street level where it does the most damage. Many people then assume that a green roof on top of a building will do nothing for the street level pollution simply because of the considerable distance between the roof and the street. But on hot summer days when street air pollution is high, the green roof cools the air. This, because of the density changes, flows downward and creates circulation within the street canyon, which disperses the pollutants and leads to better localized air quality.
When it comes to directly protecting pedestrians from traffic-related PM, vegetated greenscreen trellis systems or shrubs placed between the traffic and the pedestrians have been shown to be the most effective method. The greenscreen acts as both a direct capture of the pollutants and a dispersion agent, disrupting the airflow and diverting the poor quality air away from the sidewalk.
How Much Air Pollution Can be Captured by GI?
How much pollution is removed by green infrastructures? A computer modeling study in 2006 examined the pollutant removal capacity of the urban tree canopy in 55 US cities. The study found that the trees alone were responsible for removing 711,000 tonnes of air pollution annually. The pollution types included in this study were PM, ozone, nitrogen dioxide, sulfur dioxide, and carbon monoxide.
An interesting aspect is how outdoor green infrastructure can help improve indoor air quality. One study showed that terraced houses with trees in front of them had 50% lower indoor air pollution compared with similar houses without trees. Another modeling study showed even stronger effects with up to 60% indoor air pollution reduction attributed to the presence of trees in front of the building.
Green facades were estimated to reduce PM10 by 50% and NO2 by 60% in a study by Abhijith et al. 2017. When it comes to living walls, active living walls with the possibility for air flowing through the system appear to be more effective in capturing PM than passive living walls. An active green wall utilizes a mechanical device to force large volumes of air through the substrate, root zone and foliage.
When Green Spaces Can Contribute to Air Pollution
Vegetation, particularly trees, can, in some instances, also contribute to air pollution, e.g., through pollen, fungal spores, and so-called BVOCs (biologically derived volatile organic compounds). In the presence of nitrogen oxides (NOx), some BVOCs can act as a precursor to dangerous ground ozone, a principal component of smog.
The severity of the BVOC release depends on the species and the stress level of the plant. The best practice is to select tree species with a low propensity for BVOC formation and to make sure that the plants are healthy and live as stress-free as possible. Selecting native tree species is presumably favorable as they are adapted to the local climate and thus tend to be less stressed.
Nonetheless, a special case exists for green roofs. Green roofs are not natural environments with temperature and water fluctuations that do not resemble the local climate. For green roofs, the plants need to be adapted to the roof environment, not the at-grade environment.
Another issue that can result in higher air pollution levels at street level is too closely planted trees. Densely planted trees can result in closed canopies that trap air and dangerous pollutants at street level. This can be avoided by using other types of green infrastructures such as green facades from greenscreen, green roofs, or hedges in street canyon locations.
In summary, green infrastructure is a multi-pronged approach that allows for better urban living conditions. It has the capacity to cool our cities thus protecting us from the urban heat island effects and allowing for energy savings. GI can introduce more biodiversity, reestablish the natural water cycle and help reduce volumes of wastewater reaching the sewage treatment plants. Green infrastructure can also extend and improve our lives by capturing dangerous air pollutants. But we have to make sure we implement GI using a data-based and science-driven approach to achieve these benefits.