Unexpectedly rapid SO₂ heterogeneous reaction on the surface of vehicular brake wear particles

Sulfate is a major aerosol component whose rapid growth in polluted air remains insufficiently explained. 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 2.87 × 10^-5) is orders of magnitude higher than those of mineral dust or soot. This remarkable reactivity originates from a self-sustained synergy between α-Fe₂O₃ and carbonaceous components: oxygen vacancies in α-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.