When architects design commercial and other nonresidential urban buildings, they have a number of options when choosing the building's exterior material — it's "skin". For any choice other than tinted glass, they also have the option of painting that outside surface. This extra layer of protection against the elements can prolong the life of the building's walls and roof, but it also contributes to environmental degradation, since the paint must be manufactured from chemicals, and either breaks down slowly over time, or becomes part of the rubble if and when the building is demolished.
But what if that paint could make a positive contribution to the urban ecology, by reducing airborne pollution? What if the paint could reduce levels of nitrogen oxides (NOx), which contribute to smog and respiratory problems? That is the purpose of a type of paint developed by a British firm, Millennium Inorganic Chemicals, and originally intended to be called "Ecopaint" — an apt name. It utilizes the same photocatalytic process of the smog-busting cement developed and tested in Italy. Yet it has the advantage of being applicable to far more surface areas.
The paint's base is polysiloxane, a polymer made from silicon. The active ingredients are spherical nanoparticles, with diameters of only 30 nanometers, made of calcium carbonate and titanium dioxide (TiO2). The porosity of its polysiloxane base is sufficient to allow nitrous oxides to diffuse though it, and then adhere to the nanoparticles. When sunlight strikes the painted surface, the nanoparticles absorb the ultraviolet radiation and use that energy to convert the nitrous oxides into nitric acid, which is either washed away by rain, or neutralized by the calcium carbonate (which is alkaline), and thus converted into carbon dioxide, calcium nitrate, and water. Another advantage to the use of nanotechnology, is that the particles are so tiny that the paint is initially colorless and clear, and thus receptive to coloring pigments.
The more smog that is neutralized by the paint, the more that the paint itself is transformed, which raises the issue of longevity. According to the manufacturer, for a typical 0.3-millimetre layer in a heavily polluted urban environment, there will be enough calcium carbonate to last at least five years. Once the calcium carbonate has reached the end of its effective lifetime, the titanium dioxide will continue to break down nitrous oxides, and the resultant nitric acid will begin to discolor the paint, thereby providing a visible indicator that the paint should be replaced, having served its purpose.