Most people picture vehicle pollution as exhaust smoke, but tire and brake wear now rival — and in electric vehicles exceed — tailpipe particulates. Every rolling wheel abrades a fine mix of rubber, metal and mineral dust that lifts into the air behind the car.
Stokes terminal settling velocity:
v_s = (2/9) · (ρ_p − ρ_air) · g · r² / μ_air
ρ_p ≈ 1200 kg/m³ (tire/brake-wear composite)
ρ_air ≈ 1.2 kg/m³
g = 9.81 m/s²
μ_air ≈ 1.81×10⁻⁵ Pa·s
PM10 (r=5.0 µm): v_s ≈ 3.6 mm/s
PM2.5 (r=1.25 µm): v_s ≈ 0.22 mm/s → 16× slower
Because these terminal velocities are tiny — real particles can hang in the air for minutes — the sim runs the vertical settling ~40× faster than real time so you can watch it happen, while the crosswind (in true m/s) carries lighter, slower-falling PM2.5 dust much farther from the road than the heavier PM10 fraction before it ever reaches the ground. Raise the fine-fraction slider and watch more dust drift clear over the verge instead of settling near the curb. The top-down map fades and shrinks each dust dot as it falls, so remaining altitude is visible even without a 3D view; the side settling panel plots the same physics directly as height vs. time for one particle of each size.
Rain wash-off (first-order kinetics):
dM_soil/dt = −k·M_soil (k ≈ 0.15 s⁻¹, accelerated)
While it's dry, particles that land in the grassy verge stay there as accumulating roadside soil contamination. Once rain starts, freshly landing dust is carried straight into surface runoff instead, and the dust already resting in the soil washes off exponentially into the storm drain — this is the dominant real-world pathway by which road dust reaches streams and rivers.
- Traffic density — more cars per minute means more emission events, raising the airborne particle count.
- Crosswind speed — pushes particles across the verge; strong wind can carry the slowest (PM2.5) particles beyond the monitored zone entirely, so they never register as local deposition.
- Fine fraction — sets the split between fast-settling PM10 (orange) and slow-drifting PM2.5 (light blue) grains, shown live as a stacked bar in the settling panel.
- Rain — switches the verge from a dry accumulator to an active source feeding the storm drain.