Heat Islands Cause Bare Roofs to Release Air Pollutants

by Dr. Anna Zakrisson on Tuesday, September 8, 2020

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Urban Heat Islands can cause asphalt and roof coverings to release more secondary organic aerosols (SOA) than road traffic. Cars still release the most pollutants overall, but the SOA produced from rooftops and asphalt is significant and has, until now, largely gone unreported.

This study is yet another argument for implementing green infrastructure such as vegetated roofs to cool our cities, restore the natural water cycle, and improve biodiversity.

What causes urban heat island effects?

Urban heat island effects describe the phenomenon in which cities are heating up at accelerated rates due to vegetation losses. Stone, asphalt, and glass are fast to heat up and emit this heat to the surroundings. Further, traffic, air conditioning units, industrial activity, all contribute to heating the city.

The geometry of the cities further traps heat between the houses, in so-called street canyons. All of these processes contribute to making cities several degrees hotter than greener, and more open surroundings.
This heat exacerbates the formation of several dangerous compounds, such as SOAs.

Read more about urban heat islands in this article:
How Green Roofs Mitigate Urban Heat Island Effects



Urban heat islands and air pollution

A recent study from Yale University shows that during hot conditions, asphalt and many roof coverings can release staggering amounts of secondary organic aerosols (SOA). These aerosols can be damaging to health and well-being.

Secondary organic aerosols are produced through multiple generational oxidations of the parent molecule, also referred to as a primary aerosol. Primary aerosols are directly emitted from the source. The researchers report that the SOA emissions doubled as the temperature increased from 40°C (104°F) to 60°C (140°F), and then continued to increase by approximately 70% with every 20°C (68°F) increase.
Further, a 300% increase in asphalt emissions were observed when the material was exposed to sunlight.

This is bad news for a heating world.

The future city

It is estimated that by 2050, 68% of the world’s population will live in cities. To create livable urban spaces, we urgently need to plan for an unpleasantly hot future. With the right adaptation measures, we can cushion some of these effects and make lives better for the majority of the world’s population.

In the US, asphalt and roof surfaces are estimated to make up 65% of the urban areas. Though this might at first seem daunting, it also presents an opportunity… most of these rooftops are unused and this space is neglected. Let’s make use of it!

Nature-based solutions such as green roofs, walls, and permeable green pavers are simply a necessity for a sustainable future city. With these measures, heat islands can be reduced through evaporative cooling, flooding can be prevented (using detention-type green roofs), pollution captured, combined sewer overflows reduced (or prevented entirely), and biodiversity improved. So, what are we waiting for? The technology is there. Further, the next-generation green roofs also offer stormwater detention, which in turn creates a strong return on investment for the green roof. Read more about that HERE!

CONTACT us who work with the Purple-Roof concept if you have any questions!

Read more in this article on detention roofs:
Blue Roofs, Blue-Green Roofs, and Purple Roofs all DETAIN water