Measure aerosols with iSPEX on your smartphone

Many people carry advanced instrumentation with them, all day, every day. Modern smartphones contain excellent cameras, are connected to the internet, have considerable computing power, and know exactly where they are and how they are oriented: they are nearly fit for performing scientific measurements! With the iSPEX add-on device,¹ we supply the last piece to turn a smartphone into a spectropolarimeter that can be used by anyone to perform measurements of aerosols in the atmosphere overhead. (The name iSPEX derives from that of the SPEX instrument,² the Spectropolarimeter for Planetary EXploration, which is currently being developed for accurate global observations of aerosol properties from orbit around Earth or other planets.)

Detailed measurements of aerosols are crucial for a number of reasons. First, many of them affect your health. Just inhaling the air in an average industrialized city takes half a year off your life expectancy. To assess health risks due to aerosols, the size and composition of the particles need to be known: it is the smallest particles, which penetrate deep into your lungs, and the ones that cannot be broken down by the body that do most damage. Aerosol particles also deserve more attention in climate research. They constitute the largest unknown factor within our current understanding of climate change.³ Finally, one particular type of aerosol even has the power to ground airplanes. Remember that Icelandic volcano eruption back in 2010?

With iSPEX, we introduce a new measurement technique for aerosols, but, more important, also a modern measurement philosophy: citizen science. This approach is fully enabled by the robust yet cheap design of the iSPEX add-on for smartphones. The measurement process itself is very easy to perform. The user simply mounts the iSPEX add-on before the camera (see Figure 1), starts up the iSPEX app, and points the device at various patches of blue sky. iSPEX measures both the spectrum and the linear polarization of the sunlight that is scattered by the combination of molecules and aerosols in the sky. The greater the quantity of aerosols present, the less blue and polarized the sky is. The large amount of information gathered (spectrum and polarization as a function of scattering angle from the sun) allows various aerosol properties to be inferred, including the amount of aerosol, and also the particle size distribution and the chemical composition (through the refractive index).⁴The app will give the user direct feedback in the form of a qualitative color code of the air quality, and will submit all relevant data to our central database. There, data from all iSPEX participants will be collected and combined with data from professional measuring stations. After analysis with an advanced radiative transfer model,⁵ a unique, crowd-sourced map of aerosol properties will be created.

Figure 1. The iSPEX add-on.

The add-on is essentially a slit spectrograph, employing a transmission grating foil and a plastic lens in addition to the lens inside the smartphone camera. The measurement of linear polarization is effectuated by the ‘spectral modulation’ technique, developed within our group.⁶ A combination of stretched (and hence birefringent) plastic sheets and Polaroid film modulates every spectrum by a sine curve. The relative amplitude of this sine curve directly scales with the degree of linear polarization, and its phase is determined by the polarization angle (see Figure 2). This way, all the information on both the spectrum and the linear polarization of the light entering the slit is obtained in a single shot, without requiring any moving parts. The app disentangles the spectral information from the polarization information. The same polarimetric technique lies at the core of the SPEX instrument.^{2, 7}

Figure 2. Data from the iSPEX prototype. Top: Unpolarized light (cloud). Middle: Fully polarized light. Bottom: Partially polarized blue sky. The amplitude of the intensity modulation is proportional to the degree of linear polarization. The polarization angle determines the phase of the modulation.

In October, the iSPEX team won a Dutch science outreach award. With the $130,000 (€ 100,000) prize money, we are currently manufacturing 10,000 iSPEX add-ons (primarily designed to fit recent models of iPhone) and developing the iSPEX app. On a sunny day in 2013, we will organize the first national iSPEX measurement day in the Netherlands. Many people have already registered to participate in this event, which will be one of the largest citizen science experiments ever. With this initial experiment, we intend to validate the accuracy of the iSPEX crowd-sourced measurements. After a successful outcome, we will ask the participants to take part in further campaigns of measurements, thus creating a very flexible and adaptive measurement network for aerosols. At that point, we will consider implementing iSPEX in other countries.

In the end, we hope to contribute to the solution of any of the major societal issues related to aerosols. But perhaps more important, with iSPEX anybody can experience the fun of being a scientist!

The author gratefully acknowledges the contributions of his colleagues on the iSPEX team at Leiden University, NOVA (Dutch Research School for Astronomy), SRON (Netherlands Institute for Space Research), RIVM (National Institute for Public Health and the Environment), and KNMI (Royal Netherlands Meteorological Institute).