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Leaf samples of Juniperus formosana were collected from an open road environment, in order to establish how particulate matter (PM) generated by vehicles was dispersed in both horizontal and vertical directions. Sampling was conducted at sites with trees of varying height and configuration adjacent to a major road in Lanzhou, Gansu Province, Northwest China. The concentration of remanence-bearing ferrimagnets in the leaf samples was estimated from measurements of Saturation Isothermal Remanent Magnetization (SIRM), while the weight of particles deposited on the leaves and their elemental composition were determined at different heights and in different directions relative to the road. The PM on the surface of needles was predominantly influenced by traffic emissions and by dust resuspension. Rows of roadside trees, as opposed to solitary trees, were more effective at intercepting PM and thus in filtering road traffic pollution. The results indicate that Juniperus formosana needles may be an effective bio-sensor for monitoring variations in the spatial diffusion of road pollutants. Image 1 • Juniperus formosana was an alternative for biomagnetic monitoring. • Significant correlation found between the weight of particles and magnetism. • Tree configuration might affect the accumulation and diffusion of road pollutants. • Rows of trees have higher road pollutants retention abilities than a solitary tree. Rows of roadside trees have a higher road pollutant retention ability than solitary trees, which reveals the important effect of tree configuration on the accumulation and diffusion of road traffic pollutants in an open road environment. [ABSTRACT FROM AUTHOR]