Volunteers track Parisian pollution with the help of tree bark

Along the streets of Paris, more than 40,000 tall, leafy plane trees provide picturesque backdrops, offer shade to city dwellers, and improve air quality, among numerous other benefits. Since 2016, some of these trees have also served as low-cost, passive biological sensors for Ecorc'Air, a crowdsourced science initiative that monitors levels of fine-particle pollutants associated with road traffic.