Billboards don't really do anything -- except for Toyota's new Mirai billboards, which are actually engineered to clean the air around them.
From April 3 to May 28, Toyota will put up 37 billboards around Los Angeles and San Francisco, touting the. In order to reinforce how clean a hydrogen-powered electric car can be, the billboards are built to scrub pollution from the air.
A titanium dioxide-coated vinyl reacts with oxygen, converting nitrogen oxide emissions to nitrates and removing them from the air. Other than the coating, the billboard needs nothing other than light, humidity and (obviously) air. Toyota also wrapped a Mirai in the same coated vinyl at the Environmental Media Association Impact Summit in Beverly Hills, California.
The 37 billboards in question add about 25,000 square feet of pollution-removing material to the California landscape. It's estimated that the billboards will remove the equivalent of 5,285 vehicles' worth of nitrogen oxide emissions from the air each month. Nitrogen oxide is a component of both smog and acid rain, so its removal from the air is very much a good thing.
California is the primary market for the Mirai, given its filling infrastructure, however limited it may be at the moment. Its fuel cell powertrain works with compressed hydrogen, converting it to electricity that powers the vehicle, with the only byproduct being (technically) potable water, although I'm not sure how many owners will be willing to take a sip of what comes out of the tailpipe.