Really interesting talk at Boston CHI by Prof. Dietmar Offenhuber a few days ago, where one of the projects he detailed was "Dust Mark". So, in Stuttgart, Germany, some areas of the city suffer from air quality issues, as is true in many cities. One way to measure this over time is with reverse graffiti, which struck me as a really interesting concept and approach to urban marking. Instead of making a long-term mark or addition to measure something (say, with paint or a physical object like a meter of some sort), you can power-wash away accumulated air particulate from concrete surfaces. This can get around anti-graffiti laws, since you aren't adding anything (paint, chalk) to a surface, instead you're removing and actually cleaning the surface!
