Magnetic biomonitoring of roadside tree leaves: identification of spatial and temporal variations in vehicle-derived particulates

We report here the novel use of rapid and non-destructive magnetic measurem ents to investigate the spatial and temporal pattern of urban dust loadings on leaves of roadside trees. More than 600 leaves were collected from birc h trees and their remanent magnetization (IRM300 (mT)) determined and norma lized for the leaf area. The results show that this normalised 2-D magnetiz ation is dominantly controlled by the tree's distance to the road. The magn etic analyses enabled detailed mapping of the spatial and temporal variatio ns of vehicle-derived particulates. Higher 2D-magnetizations, indicating hi gher magnetic dust loadings, were measured for leaves collected adjacent to uphill road sections than for those next to downhill sections. This sugges ts that vehicle emissions, rather than friction wear or resuspended road du st, are the major source of the roadside magnetic particles. Additional mag netic analyses suggest that the particle size of the magnetic grains domina ntly falls in the range classified for airborne particulate matter as PM2.5 (<2.5 mu m), a particle size hazardous to health due to its capacity to be respired deeply into the lungs. Thus, the leaf magnetizations relate direc tly to release into the atmosphere of harmful vehicle combustion products. For leaves from individual trees, magnetization values fall significantly f rom high values proximal to the roadside to lower values at the distal side , confirming the ability of trees to reduce aerosol concentrations in the a tmosphere. Magnetic analysis of leaves over days and weeks shows that rainf all produces a net decrease in the leaf magnetic loadings. (C) 1999 Elsevie r Science Ltd. All rights reserved.