Unexpectedly rapid sulfate formation on the surface of vehicular brake wear particles

The missing mechanisms of atmospheric sulfate formation remain a challenging issue for urban haze mitigation worldwide. Over the past decades, with the significant reduction of exhaust emissions and the electrification of vehicle fleets, non-exhaust emissions from vehicle braking have become a major source of aerosol particles in urban environments. Here, we demonstrate that brake wear particles (BWPs), an emerging urban aerosol source, possess exceptional catalytic efficiency for SO₂ oxidation and sulfate production under dark ambient conditions. Their SO₂ uptake coefficient (up to 1.5 × 10^-5) is orders of magnitude higher than those of mineral dust or soot. This remarkable reactivity originates from a self-sustained synergy between Fe₂O₃ and carbonaceous components: oxygen vacancies in Fe₂O₃ continuously activate atmospheric O₂ and H₂O to generate reactive oxygen species and Fe-OH for SO₂ oxidation, while organics and elemental carbon promote H₂O dissociation through proton abstraction and enhance SO₂ adsorption at carbon defects, respectively. Together, these processes sustain cyclic catalysis and mitigate site deactivation. Our findings establish BWPs as a previously overlooked class of reactive aerosols, with broad implications for multiphase chemistry, atmospheric modeling, and air quality management.