In the Netherlands, common people interested in volunteering for science became "citizen scientists", and helped to map pollution across the country using their smartphones. The volunteers measured aerosols, which are liquid or solid particles suspended in air, many of which include the particulate matter (PM) in vehicle exhaust and industrial emissions, and are therefore regulated under the EU Air Quality Directive.

Atmospheric aerosols are usually monitored from the ground using professional measurement stations, or from space, by instruments on satellites. However, measurements from professional monitoring stations only provide data for a limited number of locations, while satellites, that provide global mapping and characterize aerosols, can often only make observations once a day.

In the study, 3187 citizen scientists collected data through a small optical add-on attached to their smartphones, which makes use of several sensors inside the phone (e.g. GPS, compass). Observations were made by taking photos with the phone's camera and data submitted via an app, iSPEX. The low resolution of the cameras was balanced by the large number of users, allowing many measurements to be averaged, improving accuracy.

The observations were organised in three national measurement days in 2013, during which citizen scientists submitted nearly 10,000 observations. The authors then created maps which showed how aerosol quantities varied across the country. One site was chosen to compare the measurement taken by citizen scientists to the ones collected in the same location at an atmospheric monitoring station, with data showing very good agreement.

The results suggested that participatory monitoring made through smartphones could provide better resolution maps of aerosols than ground-based or satellite observations, in terms of both space and time. These measures cannot of course replace professional measurements, but could complement them, while at the same time rising awareness among the citizens about their cities' air quality conditions.

The study is available at this link