I found this work for Friends of the Earth on the always wonderful I Believe in Advertising site. If you go there to poke around, dwell on the Ambient & Outdoor section for a real reward.
If you can't read the copy on this even if you blow it up, it goes like this:
"The poster is blank with a clear sticky coating over the headline and visual. Over time, dust and dirty particles on the street get stuck on the poster, gradually revealing the message. In doing so, we let polluted air speak for itself." From JWT Hong Kong.
If I could come up with an idea like this JUST ONCE . . .