iSPEX is a hardware add-on for the iPhone (4, 4S, 5) camera plus a dedicated app that analyzes the light that enters it in terms of its spectrum and its polarization. By measuring these properties for the blue sky at various angles from the Sun, indirect information is obtained on the aerosols in the air. The app submits the data to a central database, where many measurements are collected and combined into a map of air pollution properties.

The iSPEX app also delivers a real-time plot of the spectrum and a real-time "compass" of the polarization.

The iSPEX app only works in combination with the iSPEX hardware add-on for the iPhone camera. At this moment this add-on is only available in the Netherlands. The iSPEX project is still in an experimental stage. A Dutch national measurement day with thousands of citizen participants will prove how well the iSPEX measurements work, and what (additional) information on air pollution they yield. Upon a successful outcome of this massive experiment, the aim is to go international with this project.
More information: http://ispex.nl/en/