by Daniel Bruno, Miguel Sevilla-Callejo, Enrique Navarro and Francisco Sanz
Did you know that we could use plants as sensors to monitor the air quality and pollution in our cities? The Ibercivis Foundation together with the Pyrenean Institute of Ecology has developed the “Vigilantes del Aire” project to measure air quality in Spain. Using citizen science, samples from 205 municipalities in 26 Spanish provinces were successfully obtained and processed.
Human activities are responsible for the emission of many toxic substances into the atmosphere, including a wide variety of heavy metals. The pollution measuring stations that are spread throughout our cities are capable of measuring particulate matter (PM) and the main pollutants continuously and accurately. However, the high cost of these infrastructures means that there are only a few per city, which limits the spatial representativeness of the data. According to scientific literature, the magnetic properties of strawberry leaves are related to the concentration of particulate matter (PM) in the air below 10 microns, especially PM10, PM2.5 and PM0.1, which allows their use as an indicator of air quality and the presence of pollutants, especially metals.
In recent years, citizen science has established itself as a collaborative and open tool, with great potential to increase the spatial, temporal and social scope of many scientific projects. New technologies facilitate societal collaboration from the local to the global scale, which is particularly important for environmental projects, such as monitoring air and water quality in the context of global change. One of the great benefits of citizen science is the ability to source quality environmental data from virtually any location on the planet. In this way, a large amount of environmental data can be obtained, and remote locations can be monitored that would otherwise have been unfeasible due to cost, logistics and socio-economic factors.
Citizen science allows social factors to be considered in its experimental design, so it can also be an inclusion tool for disadvantaged groups or those who experience gender, economic or educational biases. With this in mind, the Ibercivis Foundation [L1] together with the Pyrenean Institute of Ecology [L2] developed the Air Watchers projects [L3] - Vigilantes del Aire - to measure the air quality in Spain.
Through the Ibercivis Foundation, 5,790 organic strawberry (Fragaria vesca L.) plants were distributed to citizen scientists in different urban and rural areas between late September and early November 2020. The geographical distribution was designed to cover population centres of different sizes representative of both urban and rural areas at a national level, reaching a wide spatial coverage that includes large cities and towns (Madrid, Barcelona, Valencia, Zaragoza, Erandio district in Bilbao), to medium and small cities (Vitoria, Granada, Castellón, Algeciras, Burgos, Girona, Pontevedra, Torrelavega). With regard to smaller towns, the aim was to cover a population gradient from large (Villanueva de la Serena, Tuy, Atarfe, A Estrada) through to small towns (Belorado, Báguena, Luna, Erla, El Poyo del Cid) and even villages (Luco de Jiloca, Las Pedrosas).
Figure 1: Strawberry leaves SIRM results, Madrid and Barcelona metropolitan areas.
With the help of the regional ambassadors of the project, each of the citizen scientists was responsible for installing the plant outside his or her home as well as for the care of his or her plant and sending the leaves after more than two months of exposure to the environment (between the end of September 2020 and the end of January 2021, depending on the date of installation of the plant in the house) by filling in a form characterising its location (site, type of exterior, height, number of days exposed, degree of environmental protection, etc.) for subsequent analysis and mapping. The samples of strawberry leaves were received in pre-franked envelopes from the citizen scientists of the different geographical areas, grown under controlled conditions in greenhouses at the facilities of the Pyrenean Institute of Ecology (IPECSIC).
The SIRM (Saturation Isothermal Remanent Magnetization) technique was used to analyse the strawberry leaves. This technique measures the remanent or residual magnetisation remaining after an intense magnetic field has been applied to a material; a variable related to PM10, PM2.5 and PM0.1, as well as to several metals (especially Zn, Cd, Pb and Cr) present in the leaf after exposure to the urban environment .
Participation and results
A total of 2,755 samples were received from 205 municipalities in 26 Spanish provinces, which means that Airwatch 2020 has operated in more than half of the Spanish territory. This represents a plant return rate of 47.8%, a figure that can be considered successful in this type of citizen science project. Moreover, participation was higher than that obtained in the pilot project carried out in the city of Zaragoza in 2017 where 30% of the samples were recovered (300 of the 1,000 pots distributed).
Figure 2: Clockwise from upper left: a) Highly sensitive 2G 755 superconducting magnetometer (10-12 A m2) with alternating field demagnetiser system, used for SIRM analysis, b) Distributing strawberry pots in Zaragoza, c) Distributing strawberry pots in a retirement home, d) Distributing strawberry pots La Puebla de Fantova (Aragón-Pyrenees).
As expected, pollution levels follow a decreasing gradient from cities with higher population, traffic and polluting industrial activities in their periphery to sparsely populated towns and rural areas with little traffic, with natural, semi-natural or extensive land uses. Monitoring with biosensors (burrs) combined with environmental magnetism analysis in a citizen science framework seems to be a very useful and affordable tool for air quality monitoring, both economically and logistically, while increasing the knowledge and training of the participating citizens, establishing interesting synergies between citizenship, environmental monitoring and science. There is a complete analysis  available for download.
We would like to thank not only the Spanish Foundation for Science and Technology-Ministry of Science and Innovation, the founder of this project, but also all citizen scientists who selflessly and passionately contributed to increase our knowledge of air quality in Spain. Special mention should be made of regional ambassadors and local coordinators in each site, as well as their respective teams, without whom we would not have had the opportunity to reach so many corners of Spain.
 Hofman, J., Maher, B. A., Muxworthy, A. R., Wuyts, K., Castanheiro, A., & Samson, R. (2017). Biomagnetic monitoring of atmospheric pollution: a review of magnetic signatures from biological sensors. Environmental Science & Technology, 51(12), 6648-6664.
 Bruno, D., Sevilla-Callejo, M., Navarro, E., Vigilantes del aire, Informe Técnico (2020). https://vigilantesdelaire.ibercivis.es/wp-content/uploads/sites/7/2021/07/Informe_cientifico_rdos_Vigilantes_del_aire_2021.pdf
Ibercivis Foundation, Spain