The current iSPEX-EU project, which runs until 15 October, is a scaled-up version of the original iSPEX-project that enlisted the general public to contribute to the understanding of air pollution. Participating cities include: Athens, Barcelona, Belgrade, Berlin, Copenhagen, London, Manchester, Milan, and Rome.

iSPEX add-on. Click to enlarge.

The smartphone add-ons are “spectropolarimeters”—optical sensors that, in combination with the phone’s camera, sensors, computing and communications capabilities, measure tiny particles in the atmosphere.

The add-on is essentially a slit spectrograph that uses a transmission grating foil and a plastic lens in addition to the lens inside the smartphone camera. In addition, it contains a combination of stretched (and hence bi-refringent) plastic sheets and Polaroid film to modulate every spectrum by a sine curve.

The relative amplitude of this sine curve directly scales with the degree of linear polarization (DoLP), and its phase is determined by the polarization angle. As such, all the information on both the spectrum and the linear polarization of the light entering the slit is obtained in a single shot. The iSPEX app disentangles the spectral information from the polarization information.

The degree of linear polarization (DoLP) of the cloud-free sky can be measured as a function of wavelength and, by pointing the phone at different directions in the sky, as a function of scattering angle. This DoLP as a function of both wavelength and scattering angle yields unique information on fundamental aerosol properties, including the amount of aerosol, and also the particle size distribution and the chemical composition (through the refractive index).

iSPEX is developed building upon the measurement principles of the Spectropolarimeter for Planetary EXploration (SPEX), a highly sophisticated instrument built for the observation of other planets and their atmospheres. In comparison to SPEX, of which there exist only a few in the world, iSPEX is simple and designed to be available and used by thousands of people, and it is primarily meant for the measurement of aerosols in Earth atmosphere.

The iSPEX app instructs participants to scan the cloud-free sky while the phone’s built-in camera takes pictures through the add-on. Each picture taken through the iSPEX add-on contains information on both the spectrum and the linear polarization of the sunlight. The measurements taken using the phone camera can provide unique information about the properties of the particles in the air, including the amount of particles, their size distribution and the type of particles. This type of measurement is crucial in assessing the impacts of atmospheric aerosols on environment and health.

The iSPEX-EU team, led by Leiden Observatory, is working closely with local partners in all cities, including research organizations and environmental protection agencies. These partners include the National Observatory of Athens (NOA/IAASARS); ICFO—the Institute of Photonic Sciences; the Centre for Research in Environmental Epidemiology (CREAL); the Institute of Physics Belgrade (IPB); MINT Impuls; Freie Universität Berlin (FUB); the Danish Environmental Protection Agency; The Institute of Physics (IOP); the Science & Technology Facilities Council at the Rutherford Appleton Laboratory (STFC-RAL); the University of Manchester; the Italian Aerosol Society (IAS); Cittadini per l’Aria; and the Institute of Atmospheric Sciences and Climate at the National Research Council of Italy (ISAC-CNR).

The iSPEX project is a collaboration between Leiden University (LU), the Dutch Research School for Astronomy (NOVA), SRON Netherlands Institute for Space research, KNMI Royal Netherlands Meteorological Institute, and RIVM National Institute for Public Health and the Environment, all in The Netherlands. The iSPEX add-ons were produced by Bright Led Solutions and the app was programmed by DDQ, both in the Netherlands.

iSPEX-EU is organized as part of the EU-funded project LIGHT2015 and is one of the many activities running during the International Year of Light and Light-based Technologies 2015, a worldwide celebration of light and light-based technologies this year.

Resources

• Frans Snik, Jeroen Rietjens, Arnoud Apituley, Hester Volten, Bas Mijling, Antonio Di Noia, Stephanie Heikamp, Ritse C. Heinsbroek, Otto Hasekamp, Martijn Smit, Jan Vonk, Daphne Stam, Gerard van Harten, Jozua de Boer, Christoph Keller, & 3187 iSPEX citizen scientists (2014) “Mapping atmospheric aerosols with a citizen science network of smartphone spectropolarimeters” Geophysical Research Letters doi: 10.1002/2014GL061462