Barcelona is going to build a road bridge which may be the cleanest bridge yet. Of course it’ll have pedestrian walks and bike paths, however, what makes the bridge really noteworthy is that it will clean the air.
Concrete is notoriously energy-intensive to create so any carbon offset is beneficial. The Barcelona bridge will make use of photocatalytic concrete.
But the real prize of this thing is its basic building material, photocatalytic concrete. The principal of photocatylitics is that ultraviolet light naturally breaks down dirt, both natural and synthetic. It’s that old adage about sunlight being the best disinfectant. Photocatalytic concrete is used with titanium dioxide, which helps accelerate the natural UV-breakdown process, turning the pollution into carbon dioxide, and oxygen and substances that actually belong in the atmosphere.
The actual process has to do with semiconductors and electrons and other things that you may or may not care to read about. (At any rate, the Concrete Society of the United Kingdom does a better job of explaining it.)
An air-cleaning bridge makes for a neat news story and a sci-fi-ish novelty that environmentalists can blog about. But the important point is that Barcelona has taken a piece of infrastructure that exists solely to accommodate car culture, and re-invented it to partially offset the effects of car pollution.