Green Roofs Mitigate Air Pollution

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How living roofs can help mitigate air pollution

We simply don’t talk as much about air pollution as it’s not one single cause, it’s hard to grasp (though it makes us gasp!). It’s invisible, and it creeps upon us.

However, we insist that we talk about air pollution and present a case to how well-designed green roofs, and green infrastructure in general, can help us improve this very serious issue. There are green roof solutions available that can mitigate this silent killer that currently sends about 4.2 million people annually to an early grave (WHO).

This article will take a look at the effectiveness of green roofs and living walls in mitigating air pollution and improving air quality. We will also take a glimpse at the roof nitrogen budget and runoff since this is a topic very much related to air pollution.

Is air pollution only an issue for far away, massive Asian cities like Beijing?

You will probably be surprised to hear that in the beautiful and clean European city of Amsterdam, air pollution, in the form of photochemical smog and airborne particles, exceeded the WHO guidelines at 77% of all locations where air pollution is regularly measured [2].

US-American air quality is also declining rapidly. The Environmental Protection Agency reported that there was a 15% increase in days with unhealthy air in 2017-2018 compared with 2015-2016 [3].
Currently, around 100,000 US-Americans die each year due to this preventable issue [4].

Air pollution is an issue that affects us all. But we can fix it!

Green roofs can reduce the formation of dangerous smog, absorb dangerous particles, reduce the heat island effect and energy needs of buildings, and improve local water quality.

What is air pollution?

Outdoor air pollution is a mixture of naturally occurring and man-made substances. Naturally occurring air pollutants would be, for example, gasses from volcanoes, ash, and soot but also things like radon and combustion gasses from fires. For places like NYC and Amsterdam man-made pollutants, stemming from traffic and industry are causing most of the problems.

The majority of the man-made urban air pollution originates from combustion engines which add both gasses and aerosols to the atmosphere, but there are many other processes responsible for air pollution ranging from mining to agriculture.

Aerosols are formed when liquid or solid particles end up in the air. It might be good to note that particles that are smaller than 10 micrometers can directly enter the bloodstream through the lungs. Once in the body, they can cause severe damage. This is something we very much would like to prevent from happening!

The cost of lost working days attributed to air pollution damages has been estimated to a whopping 348 million EUR annually for a small country like the Netherlands [5]. Yes, of course, pollution abatement measures cost, but business-as-usual will be a lot more costly!

Green infrastructure is a nature-based solution to reduce mortality and the socio-economic costs of air pollution.

NO_x and ozone – the main urban air pollutant culprits

So, what are the main pollutants that we are fighting?

One of our common pollutants is ozone! Ozone (O₃) is a rather spectacular molecule that is protecting our planet from dangerous ultraviolet (UV) radiation. Well, ozone is great as long as the molecules stay far up in the stratosphere. Ground ozone, on the other hand, is a completely different cup of tea…

Down at ground level, ozone is formed through an unholy concoction of NO_x (NO₂+NO+N₂O and other nitrogen oxides), volatile organic carbon compounds (VOC), and sunlight. This air is often being trapped at ground level between houses on a hot summer day creating optimal conditions for the formation of ozone and smog!

Ozone is an extremely reactive molecule and this reactivity can cause it to react with lung tissues resulting in asthma and many other pulmonary diseases. Especially children are at risk.

What is ozone and how can green roofs help?

Urban heat island effects accelerate the formation of ozone and this issue will worsen with time due to the warming climate. Thus, we need to reduce the temperatures in our cities.

Green roofs and walls are excellent ways of achieving this temperature reduction.

If you are interested in learning more about the data available for heat island mitigation using green infrastructure, take a look at our article: How Green Roofs Mitigate Heat Island Effects.

However, there is more to the NO_x-story that warrants investigation.

The dangers of NOx – photochemical smog

The reaction between NO_x and volatile carbon-containing (organic) compounds, especially during hot and sunny days becomes smog. Further, when NO_x reacts with sulfur dioxides, acid rain forms.

Exposure to NO_x causes a range of serious health problems where decreased fertility, mental confusion, and birth defects are only a few, which is why it is so critical to pick up the available tools and do something about it.

The capacity of green roofs in removing air pollutants

It has been shown that vegetation can remediate air and absorb NO_x [6]. However, badly placed trees have been shown to sometimes worsen the situation due to trapped air beneath the canopies. Green roofs and walls do not block air circulation in this way but at the same time are able to absorb NO_x.

Interestingly, green walls have even been shown to break up harmful circulation patterns, especially those found in skyscraper canyons and aid the removal of the trapped pollutants, hence, green walls and green roofs are a swiss army knife tool in the fight against the deadly air pollution.

To put in context what a green roof can achieve for the local air quality: A modeling study showed that if 20% of the roof surfaces in Washington D.C. of buildings with a roof area of more than 900m² were covered with green roofs, about 6 metric tonnes of ozone and 6 tonnes of particulate small matter would be removed from the air annually. That is equivalent to the pollutants that would be absorbed by 25,000-33,000 trees [7].

Many other studies have also concluded the effectiveness of green roofs and walls in mitigating pollution such as ozone, particulate matter, and NO_x. We should also not forget that by reducing urban heat islands, green infrastructure also reduces smog formation.

When modeling green roofs pollutant capture, many parameters must be considered such as building height, temperature and weather, wind patterns, and pollutant levels.

Nitrogen air pollutants

Other issues caused by these nitrogen-containing molecular species such as nitrogen oxides (NO_x), but also ammonia (NH₄) and nitrate (NO₃) as wet deposition, are that they may cause eutrophication. Eutrophication is when an ecosystem has received too many nutrients, especially nitrogen and sometimes phosphorus.

This nutrient excess in water bodies can result in an overgrowth of fast-growing bacteria and algal species that often deplete available oxygen resulting in dead, stinking waters that can no longer be used for recreation or as water deposits.

This, of course, is an ecological disaster and a costly one at that. Who wants to go swimming in a thick, stinking algal soup? Remediation of the water bodies is costly and difficult, and the tourism industry can take a major downturn.

On study in China showed that around 2300mg nitrogen was deposited per square meter and year with high monthly variations showing the highest deposition rates during the sunny and hot summer months [8]. But, also in Europe, very high levels of atmospheric nitrogen deposition have been observed with annual concentrations frequently reaching 2000mgN m^-2 year^-1 in European hotspots such as the Benelux countries [9].

To put the scope of the European eutrophication situation into context: According to the German Ministry of Environment, 68% of German vulnerable ecosystems are threatened by eutrophication.

Are green roofs sources of nitrogen?

We have concluded that green roofs are able to absorb nitrogen-containing air pollutants and so improving the urban environment significantly. This is fantastic, but as you should not merely look at different system compartments in separation but put them in context to see the bigger picture. This is why I’d like to take a step back and not only look at the air pollutants but also take a quick look at the total nitrogen budget of the roof system.

Some studies have reported that green roofs can become nitrogen sources due to fertilization practices. Other studies saw no difference compared with a control gray roof [10].

Even though the roofs absorb NO_x and other pollutants from the air, we still must take a critical look at this to ensure that the roofs do not leak a surplus of nitrogen i.e. become nitrogen sources due to excessive fertilization.

However, it should be noted that many of the studies showing an increase in runoff total nitrogen were carried out on newly planted roofs which have been shown to leak significantly more nutrients than older roofs as the freshly used compost in the soil blend contains high levels of nitrogen. After the initial high runoff nitrogen concentrations, levels quickly decline.

Plant coverage appears to be a critical factor where nutrient leeching appeared to be significantly lower for roofs with healthy coverage [10].

Bare soil is exposed to the elements and hence, nutrients are not held in the system by the biota. Based on this, I would like to put forward the hypothesis that using instant fully vegetated installations are favored over plugs, as instant vegetation is more likely to reduce nutrient runoff due to the instant coverage

Nutrient concentrations vs nutrient loads

It should also be noted that even though the absolute concentrations of nitrogen in the runoff may be higher compared with gray/black roofs, the total volume of runoff is lower.

A green roof retains about 50% of the annual rain volume. That means that 50% of the rain never enters the sewage system. During these rain events, no nutrients ever hit the ground. For a black roof, this volume most certainly hits the ground and with-it nitrogen and other pollutants.

That means that though concentrations may be high, total loads can still be very low.

Vegetated roofs and the future

Our future urban environment needs green infrastructure. Urban Heat Islands are threatening our future. We have the tools to make a great future for ourselves. We just need to pick them up and continue hammering away, improving as we go along. Let’s start by installing green roofs!

We need climate-adapted green roofs that perform optimally in your local climate. Green roofs 2.0 that react to your local needs. For this, we have developed the Purple-Roof Green Roof Modeler. The next-generation green roofs are already here!

Even though I hate the word “holistic” as it sounds like someone selling fragrance candles and henna over a daytime shopping channel, but we must start thinking “holistic” when it comes to green roofs. No compartment is separate from the others. Also, these compartments are most likely not additive.

Read more about what Detention roofs can do here:
Purple-Roof Detention Explained
How Detention Roofs Generate a Live Roof ROI!

Nothing is more important than our future. The future city is green, clean, and water resilient.

- Dr. Anna Zakrisson for Purple-Roof

Bibliography

[1] WHO, “WHO | Air pollution,” World Health Organization. 2016.

[2] I. Dekker, “NOx concentrations and exposure in Amsterdam and over Europe,” no. x, pp. 1–39, 2014.

[3] C. NUNEZ, “Air pollution, facts and information,” National Geographic. p. 4, 2019.

[4] S. Borenstein and N. Forster, “US air quality is slipping after years of improvement,” Associated Press. 2019.

[5] M. Amann, M. Holland, R. Maas, T. Vandyck, and B. Saveyn, “Costs, Benefits and Economic Impacts of the EU Clean Air Strategy and their Implications on Innovation and Competitiveness,” pp. 1–59, 2017.

[6] X. Wei et al., “Phylloremediation of air pollutants: Exploiting the potential of plant leaves and leaf-associated microbes,” Front. Plant Sci., vol. 8, no. 2, pp. 1–23, 2017.

[7] US EPA, “Reducing Urban Heat Islands: Compendium of Strategies. Cool Pavements,” Epa, pp. 1–23, 2008.

[8] J. Yu et al., “Wet and dry atmospheric depositions of inorganic nitrogen during plant growing season in the coastal zone of Yellow River Delta,” Sci. World J., vol. 2014, 2014.

[9] W. Aas et al., “in emissions and implications for habitat protection,” 2018.

[10] B. G. Johannessen, E. Fassman-beck, and Y. Cheng, “Investigating substrate amendments to prevent nutrient leaching from extensive sedum green roofs,” pp. 1–15.