Abstract Airborne particulate matter (PM) is known to have adverse health effects and is a growing concern in urban areas. Spatial and temporal variability is difficult to assess with the few air quality stations typically available in cities. As an alternative, tree bark acts as a passive captor on which PM is deposited. Magnetic susceptibility measurements can be used as a proxy indicator to estimate pollution from vehicle sources. We present the citizen science project called Ecorc’Air, in which volunteers collect plane tree bark samples, mainly in Paris, which are then sent to laboratories and used for various measurements. The project has developed since 2016, leading to the production of annual maps that identify variations in magnetic particle concentrations. The correlation between magnetic susceptibility and metal content has been verified using multi elemental analysis. Thanks to these numerous samples, we were able to identify areas with consistently high susceptibility values over time. We also quantified the decrease in susceptibility values with the distance between the tree and the road, as well as the shielding effect of parked cars on pedestrians. We showed trends in susceptibility variations over time along a specific well‐sampled road, suggesting that this type of sampling and measurement could be used to quantify variations in metallic pollution at a local scale. Finally, through interviews, we found that there is a growing interest and participation of city dwellers, especially those involved in local associations, to act in favor of environmental research, as well as of municipalities to offer support.