Particulate matter deposition on roadside trees in urban area: Evaluation of Elemental carbon and magnetic minerals.

In France, 97% of cities exceed the WHO guide values for PM2.5 (2022 data). The transport sector is responsible for about 10% of PM2.5 emissions. The development of green infrastructure along roads, among other ecosystem services, could enable air pollution removal by deposition and needs to be better quantified and monitored.

We studied the chemical composition of particle deposits and the temporal evolution of the quantity of deposits on tree leaves over a 9-month period in Toulouse (~500,000 inhabitants), France. Leaf samples were taken from 9 deciduous trees and 2 evergreen trees at three roadside sites. We analyzed the deposition of elemental carbon (EC) and magnetic minerals on the leaf surface of different tree species. The EC deposit was extracted from the leaves using double-deionized water for on-surface deposition and chloroform for in-wax deposition. EC deposit was then estimated using thermo-optical techniques. Total magnetic mineral deposition was analyzed directly on total leaves by acquiring saturation isothermal remanent magnetization (SIRM) using a JR6 magnetometer.

Our results show that the average on-surface EC and total magnetic minerals deposit were respectively 4 and 12 times higher for evergreen than for deciduous trees. On-surface EC for evergreen trees was 2 times lower than in-wax EC. In contrast, on-surface EC for deciduous trees was found 11 times higher than in-wax EC. The analyses enabled the interpretation of changes in particle deposition over time as a function of meteorological conditions. Our results highlighted the potential of leaves to be used as biosensors to monitor ambient air pollution.