Long-Term Characterization of Non-Exhaust Particulate Concentrations at Two Contrasting Near-Road Sites: Source Contributions and Real-World Emission Factors

Abstract

As exhaust emissions (EE) from motor vehicles decline, non-exhaust emissions (NEE) from brake wear (BW), tire wear, and road dust (RD) are becoming a major source of traffic-related particulate matter (PM). Hence, there is a pressing need to characterize the resulting exposures and health burden. Here we examine the factors affecting NEE concentrations in ambient fine and coarse PM (PM2.5 and PM10–2.5) using speciated PM measurements conducted near a highway and an arterial road in Canada. Receptor model source apportionment analysis showed BW and RD combined contributed 28–49% of traffic-derived PM2.5, with associated near-road concentrations being 4–8 times higher for PM10–2.5 than PM2.5. Copper and iron, primarily contributed by BW, were 2–5 times higher than background levels. Multiple linear regression analysis showed NEE concentrations are affected by meteorology, traffic volume, and fleet composition. Emission factor (EF) calculations showed copper emission per vehicle-kilometer traveled declined at a rate of 7–9% year–1 (p < 0.001), cautioning against its use as a stable BW marker as brake pad formulations evolve. While the EFs for EE were comparable across both sites due to similar heavy-duty vehicle (HDV) shares, BW and RD EFs were 3–4 times higher at the highway site, highlighting driving conditions as key determinants of NEE emissions. Light-duty vehicles (LDVs) dominated BW and RD contributions in PM2.5 due to their much higher traffic volume, yet HDVs became important in PM10–2.5 with their higher EFs for coarse particles. This work highlights the need to account for size-resolved characteristics and real-world driving conditions, in addition to vehicle fleet composition, to accurately quantify NEE contributions.