Hong Kong-based Frederick Givens, an architect, and Benny Chow, a sustainable building specialist, have developed a skyscraper design that would chemically clean polluted air:

Envisioned for Qingdao, China, a city with particularly bad air quality, the tower would have a skin featuring a nano-coating of titanium dioxide. When smog hits the skin, sunlight would trigger a chemical reaction that would neutralize grease and toxins, with oxygen and water as byproducts.

The tower would work both day and night, by simultaneously collecting solar energy and chemically cleaning the air during the daytime:

Photovoltaic panels would capture enough energy from the sun to allow the production of indigo light (near the UV portion of the spectrum) during evening hours. The purple light that would let the chemical reactions continue would be visible for miles: “The indigo glow will become symbolic of the cleansing, counteracting the yellow haze that dominates the daytime hours.”

(Via Freakonomics)