This article was originally published in August 2019 in WSP’s magazine The Possible
In January, Porsche announced that part of the factory producing its all-electric Taycan model would be built from aluminium panels coated with titanium dioxide (TiO2). When exposed to sunlight, the coating reacts with moisture in the air to “absorb” NOx by breaking it down into water and nitrates — a process known as photo-catalysis. Porsche claims its 126m2 of coated panels will be “as effective as ten trees” in removing NOx from the air.
The technology first came to prominence in 2007 after Italian cement manufacturer Italcementi used TiO2 additives to create a self-cleaning concrete. Dirt on the surface of the concrete is broken down by the same photo-catalytic process, and it was then discovered that it was also removing NOx.
The jury is still out, however, on how far TiO2 technology can help with air pollution. Concerns have been raised that without specialist drainage the resultant nitrates eventually wash into water courses where they could encourage algal blooms. In addition, the manufacture of TiO2 itself is associated with the release of toxic chemicals. It is also expensive, especially when used in concrete, where only the surface is active in removing dirt or NOx.
Meanwhile, another German company, Green City Solutions, has developed the pollution-removing CityTree. This comprises a semi-portable framework, typically covering some 4m2, densely packed with several varieties of moss, each known for its ability to trap particulates and absorb NOx. A built-in system delivers a controlled supply of water and nutrients to the mosses, which would otherwise struggle to thrive in polluted air. As air passes through the CityTree, particulates are trapped by the large surface area of moss. They are eaten by moss-friendly bacteria which are then themselves absorbed by the moss. Solar-powered electric fans can adjust the quantities of air passing through the moss wall — increasing air flow at rush hour, for example.
CityTrees cost around US$25,000 each, and while they are unlikely to make much of an impact on a city’s overall air pollution problem, they could be strategically placed in highly polluted areas that may be unsuitable for actual trees. Several have already been “planted” in cities including Berlin, Beijing, Dresden, Paris and London.
This article appeared in The Possible issue 05, as part of a longer feature on improving air quality in cities