Sunlight is practically unpolarized; it has no preferred direction. Such a preferred direction arises when the light path is altered when it is scattered by a molecule. This is why the blue sky is strongly polarized. The degree of linear polarization as seen from the ground is perpendicular to the line from the Sun to the location of that particle. The degree of linear polarization depends strongly on the size and composition of the scattering particle. Thus, the particles leave a unique imprint on the spectrum and polarization of the scattered light. And only by measuring this spectrum as well as the polarization of scattered sunlight at different angles, the properties of the dust particles can be fully determined.

A measurement of spectrum and linear polarization for various scattering angles yields number density size distribution chemical composition shape of the aerosol particles.

Read these papers by Hansen and Travis, and Hasekamp en Landgraf about the theorie behind measuring aerosols with spectropolarimetry.

Each iSPEX unit enables this measurement capability through an add-on and an app for smartphones. A measurement consists of pointing the iSPEX add-on to various patches of blue sky. The app on the smartphone records the spectra and disentangles the polarization information, whilst also determining the phone’s pointing and its geographic location. All measurements will be submitted to a central database, where all data will be interpreted and combined into a map of aerosol parameters. By deploying many iSPEX units over a country like the Netherlands, we create the first measurement network that delivers crucial information on dust and aerosols with national coverage and local detail at the same time. Therefore everybody can contribute to answering urgent questions regarding atmospheric aerosols through crowdsourcing.