Citizen science network produces accurate maps of atmospheric dust

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Measurements by thousands of citizen scientists in the Netherlands using their smartphones and the iSPEX add-on are delivering accurate data on dust particles in the atmosphere that add valuable information to professional measurements. The iSPEX team, led by Frans Snik of Leiden University, analyzed all measurements from three days in 2013 and combined them into unique maps of dust particles above the Netherlands. The results match and sometimes even exceed those of ground-based measurement networks and satellite instruments.

The iSPEX maps achieve a spatial resolution as small as 2 kilometers whereas satellite data are much courser. They also fill in blind spots of established ground-based atmospheric measurement networks. The scientific article that presents these first results of the iSPEX project is being published today in Geophysical Research Letters.

The iSPEX team developed a new atmospheric measurement method in the form of a low-cost add-on for smartphone cameras [1]. The iSPEX app instructs participants to scan the blue sky while the phone’s built-in camera takes pictures through the add-on. The photos record both the spectrum and the linear polarization [2,3] of the sunlight that is scattered by suspended dust particles, and thus contain information about the properties of these particles. While such properties are difficult to measure, much better knowledge on atmospheric particles is needed to understand their effects on health, climate and air traffic.

Thousands of participants performed iSPEX measurements throughout the Netherlands on three cloud-free days in 2013. This large-scale citizen science experiment allowed the iSPEX team to verify the reliability of this new measurement method.

After a rigorous quality assessment of each submitted data point, measurements recorded in specific areas within a limited amount of time are averaged to obtain sufficient accuracy. Subsequently the data are converted to Aerosol Optical Thickness (AOT), which is a standardized quantity related to the total amount of atmospheric particles. The iSPEX AOT data match comparable data from satellites and the AERONET ground station at Cabauw, the Netherlands. In areas with sufficiently high measurement densities, the iSPEX maps can even discern smaller details than satellite data.

iSPEX map compiled from all iSPEX measurements performed in the Netherlands on July 8, 2013, between 14:00 and 21:00. Each blue dot represents one of the total of 6007 measurements that were submitted that day. At each location on the map, the 50 nearest iSPEX measurements were averaged and converted to Aerosol Optical Thickness, a measure for the total amount of atmospheric particles. This map can be compared to the AOT data from the MODIS Aqua satellite, which flew over the Netherlands at 16:12 local time. The relatively high AOT values were caused by smoke clouds from forest fires in North America, that were blown over the Netherlands at an altitude of 2-4 km. In the course of the day, the northerly winds brought clearer air to the northern provinces.

iSPEX maps compiled from all iSPEX measurements performed in the Netherlands on September 5, 2013, between 7:00-14:00 and 14:00-21:00. Each orange dot represents one of the total of 2444 measurements that were submitted that day. The skies were relatively clear that day. At regional scales, patches of aerosol at ground level are discerned, which are blown in northeast direction. Some of these patches have a probable origin in Germany and France.

Team leader Snik: “This proves that our new measurement method works. But the great strength of iSPEX is the measurement philosophy: the establishment of a citizen science network of thousands of enthusiastic volunteers who actively carry out outdoor measurements. In this way, we can collect valuable information about atmospheric particles on locations and/or at times that are not covered by professional equipment. These results are even more accurate than we had hoped, and give rise to further research and development. We are currently investigating to what extent we can extract more information about atmospheric particles from the iSPEX data, like their sizes and compositions. And of course, we want to organize many more measurement days.”

With the help of a grant that supports public activities in Europe during the International Year of Light 2015, the iSPEX team is now preparing for the international expansion of the project. This expansion provides opportunities for national and international parties to join the project. Snik: “Our final goal is to establish a global network of citizen scientists who all contribute measurements to study the sources and societal effects of polluting atmospheric particles.”

The iSPEX citizen science project is a collaboration between Dutch research institutes [4] and thousands of volunteers ( The scientists behind the iSPEX project are all involved in studying the atmospheres of the Earth, other solar-system planets and planets around other stars (exoplanets).


  1. The iSPEX add-on currently fits on the iPhones 4, 4S, 5 and 5S.
  2. Polarization is a fundamental property of light and describes the preferential direction of the electric field of the light wave. Direct sunlight is unpolarized. Sunlight that has been scattered by molecules or dust particles becomes polarized. This polarization is highest for the blue sky at 90 degrees scattering angle from the sun. The degree of polarization decreases with increasing particle densities, and the exact polarization properties depend strongly on particle properties like the size and composition.
  3. The polarization measurement principle of iSPEX is derived from developments in astronomy and space research. The optical implementation of iSPEX was invented for the prototype of the SPEX satellite instrument to record highly accurate aerosol data on a global scale. The robust polarization measurement solution for a satellite instrument also proved simple enough for use with smartphones, and consists of a series of static optical elements (quarter-wave retarder, multiple-order retarder and polarizer) that can also be made of plastic.
  4. The iSPEX project is a collaboration between Leiden University, 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. LU/NOVA hosts the project management team and is responsible for the development of the iSPEX add-on and app. SRON developed the off-line data reduction pipeline. KNMI provided weather forecast, and analyzed comparative ground-based and satellite data. RIVM leads the application of SPEX and iSPEX instrumentation for ground-based air quality measurements.

The project partners Longfonds, CNG Net, KIJK and Avantes helped recruiting participants. The iSPEX add-ons were produced by Bright Led Solutions, and the app was programmed by DDQ. The iSPEX project was supported by the €100’000 award from the Academische Jaarprijs 2012.

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Welcome at

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An increasing number of people carries a mobile phone with internet, camera and large computing power. iSPEX, an add-on with complementary app, instantly turns a smartphone into a scientific instrument to measure dust in our atmosphere. The user attaches the add-on on the phone and points it towards blue sky and takes a picture, it is as simple as that. The data is sent automatically to a central database that combines all the measurements with other specialized stations to make a national dust chart. Every iSPEX user will gain awareness to several urgent problems related to dust in our atmosphere, and by means of crowdsourcing everyone can do one’s bit to contribute to solve these problems. The data collected through iSPEX will lead to a better assessment of the health risks due to fine dust particles and whether or not volcanic ash clouds are dangerous for air traffic. It can also contribute to the understanding of the relationship between atmospheric aerosols and climate change.

Thousands of Dutch citizens take part in the first national iSPEX-measure-day

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The first Dutch national iSPEX-measure-day yielded more than 5000 measurements. With this enough data has been gathered to call the iSPEX-measure-day a valid scientific experiment. iSPEX-teamleader Frans Snik is happy with the massive response of the participants, despite the long wait for good weather and dawning summer holiday.

During the day there were two measure moments: one in the morning (at 8:00-10:00 o’clock) and one in the afternoon (at 16.00-18.00 o’clock). During the measure moment in the morning there were so many visits to the live map at, that the servers got overloaded. However, all measurements were collected without any problems. At the end of the day at 18:00 o’clock, the counter was at 4957 measurements.

Map of the iSPEX-measurements at the end of the measure-day on the 8th of July 2013

The next coming months, the data will be analyzed in full detail. After the analysis the measurements by the public will be compared to the aerosol-measurements by scientific equipment. Today, this equipment was set-up at the CESAR-research station at Cabauw, near Lopik. The primary goal of the iSPEX-experiment is to find out how accurate the massive the iSPEX-measurements are, and what kind of additional information about aerosols the measurements can provide. Snik: “Eventually the most important aspect of this project is that thousands of volunteers were able to perform scientific measurements themselves. We are convinced that this kind of citizen science, in which citizens help scientists with there research, has a great future ahead.”

To give all the holidaymakers (and all the students) a chance to also participate, a second national iSPEX-measure-day will be held in September, if the weather is good enough. That is why the participants are asked to keep their iSPEX-unit. Measurements in the meantime are welcome, so as soon as there is a day in the summer with a clear blue sky, people can perform measurements and send these through the App to the central database.

This unique science-project has been made possible because last year, iSPEX (a project by the University of Leiden, NOVA, SRON, KNMI and RIVM) won the Academische Jaarprijs, and thanks to the support of our partners Longfonds, CNG Net, Sanoma en Avantes. The members of the iSPEX-team all work in the development of the use of instrumentation and interpretation-models for researching the Earth, other planets in our solar system and planets orbiting other stars.

Below you will find an animation of the course of the amount of measurements that were done on the first iSPEX- measure-day.