The Netherlands today experienced a novel approach to citizen science with over 5000 participants who did a quick scan of local atmospheric dust levels (particulates) with just a smartphone, an app and a small set-up piece . The results of the scan were already in this evening (Link).
So how did the measurement work? The set-up piece was placed in front of the camera. A slit sits in front followed by a quarter-wave plate, a multi-order wave plate, a polarizer and a diffraction grating. The participants pointed the device at the blue sky and took photos at different angles. Back in HQ dust levels were calculated from the visible spectrum and the degree of polarization of this spectrum that the photos produced. The same principle by the way is used in SPEX for the measurement of dust and clouds in the atmosphere of the planets in our solar system.

The end result is a map of the Netherlands dotted with 5000 data points with each point a dirty brown for polluted air or a healthy blue for clear air (we are generalizing) and everything in between. The map does not immediately makes sense. You would expect to see oceans of blue outside the major cities but it does not work that way. Someone took a measurement almost in front of my house and it is an ugly brown! Must be the heavy road reconstruction work taking place in the next street? I hope so.

Movie! (in Dutch)