Take ordinary cement. Mix in an agent called a photocatalyzer(titanium dioxide, if you really want to know), which speed up the natural process that breaks down smog into its component paerts. Now start paving things with thw stuff. That's what they're doing in Segrate, an Italian town near Milan. The smog-eating cement is called TX Active, and the Italian firm Italcementi spent to years developing it. Now there's a busy street in Segarate that's covered with it, and Italcementi claims it has reduced nitric oxides in the area as much as 60%. Bonus: buildings made with TX Active stay cleaner too.