Three-dimensional distribution of roadside airborne/tire-wear particles and implications for pedestrian exposure

Non-exhaust traffic particles in roadside air are chemically complex and spatially heterogeneous; however, exposure assessments often rely on limited near-ground measurements. We examined the three-dimensional distributions of resuspension-driven airborne particles, including tire-wear particles (TWP), carbonaceous particles (e.g., carbon black (CB) aggregates), and metals using distance-resolved surface dust (Zone A; 0-30 m), height-resolved airborne sampling within the pedestrian layer (Zone B; 0-2 m), and foliar deposition as a time-integrated sink (Zone C), combined with a rainfall wash-off evaluation. Surface dust and TWP showed strong distance-dependent decline, while CB exhibited weaker distance dependence. Filter-based airborne particles decreased sharply with height (703.8-34.6 µg/m³ from 0 to 2 m), with particulate matter showing mid-height peaks around 0.5-1 m, indicating size-dependent stratification from vehicle-induced turbulence. Leaf deposition decreased with height (deposited area 99% lower at 2 m than at 0 m), and the magnetic Fe-oxide fraction in leaf-deposited material (27.9-17.3%) exceeded that of bulk road dust (6.7-7.1%), suggesting selective metal-rich particulate interception. A 3D concentration field was derived by coupling distance-dependent surface reservoirs with height attenuation using Amato-type loading dependence (exponent 0.81), predicting near-road hotspots and breathing height variability. Heavy rainfall suppressed the off-road reservoir while shifting contaminants toward runoff. These findings suggest that physical removal of deposited roadside dust may help reduce the resuspendable reservoir while avoiding runoff transfer associated with water-based cleaning; however, the short-term airborne effects of vacuum-assisted cleaning were not directly evaluated in this study.