Even little children know that bigger is better. ( I think there is a cell phone commercial that continuously reminds us of that.!) And when you are talking about filtering out air pollution, you really do need BIG.
Scientists have known that nanoparticles of Titanium Dioxide can fiter out pollutants from air–pollutants like nitrogen oxides that come from car exhausts, for example. Titanium Dioxide is the stuff that makes white paint white, and if you make the particles small enough they react with pollutants and remove the noxious gases from the air. So why not use paint as a filter! And as long as you are painting, why not paint a whole building !!
Scientists at the University of Sheffield in England have just done that! They turned the side of a building into a giant nanotechnology pollution eating poster, 10 meters x 20 meters ( that is about 30 ft x 60 ft). When light hits the poster, it excites the nanoparticles and makes then more reactive to the pollutants, removing them from the air. (That is a recurring theme in nanotechnology—nanoparticles can be more chemically reactive and can interact with light differently than larger particles of the same material.)
Just think how much pollution could be removed if lots of buildings were covered with “nano-painted posters”!