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.