It's been known for decades for it's brilliant white colour.  It's self cleaning, and absorbs smog.  Thin films of it are clear  so a number of manufacturers use it in glass applications such as skylights.

The self-cleaning aspect occurs because one processed form of titanium dioxide is what is called superhydrophilic - literally, "water-loving", which means that when water hits a dirty titanium dioxide surface, that surface will draw in a whisper-thin sheet of water across its whole surface, displacing grime which washes neatly away.

iIt removes pollutants which is what has made it an increasingly popular choice for environmentally-minded building projects.

A bit of the ultraviolet light in sunlight frees up electrons from the material, creating "free radicals" that actively break down pollutants including so-called NOx gases (molecules of varying proportions of nitrogen and oxygen) or VOCs (volatile organic compounds).

Laboratory experiments have shown that the breakdown of NOx chemicals can result in the creation of other pollutants such as nitrous acid or ozone. But real-world tests in Japan had not shown significant production of the chemicals.

The data from the tunnel test will help settle the question. And as large building projects make use of titanium dioxide-coated products, such as "The Iceberg" in Aarhus, Denmark, further large-scale experiments can be carried out.