Distance decay of tire wear particles and potentially toxic elements near Canada's busiest highway: Assessing lichen transplants as biomonitors

Non-exhaust emissions from road traffic, including tire wear particles (TWPs), potentially toxic elements (PTEs), and magnetic metallic particles (MMPs), represent an emerging component of urban air pollution. Here we evaluated the suitability of lichen transplants (Evernia prunastri) as a biomonitor of TWPs, PTEs and MMPs along a 150 m transect from Highway 401, Toronto, Ontario (Canada). Lichens were exposed for 2 months at six different distances from the highway and analysed for TWPs, PTEs, and MMPs. Lichen transplants showed an exponential decrease in the accumulation of TWPs with distance from the highway (estimated at >17,500 to ∼1500 TWP g−1; R2 = 0.98). The concentration of Sb and MMPs, the latter deduced by magnetic susceptibility values, declined sharply (by 70%) within 35 m of the road, while most other PTEs decreased by 50% at 150 m. The strong associations between TWPs and non-exhaust tracers (Sb, Cr, and MMPs; r = 0.69, 0.68, and 0.83, respectively) indicate a shared traffic-related source arising from combined tire and brake wear and highlight the potential of magnetic susceptibility as a rapid proxy for assessing the dispersion of TWPs.