Sulfate formation through copper-catalyzed SO₂ oxidation by NO₂ at aerosol surfaces

Severe urban air pollution in China is driven by a synergistic conversion of SO₂, NOx, and NH₃ into fine particulate matter (PM_2.5). Field studies indicated NO₂ as an important oxidizer to SO₂ in polluted atmospheres with low photochemical reactivity, but this rapid reaction cannot be explained by the aqueous reactive nitrogen chemistry in acidic urban aerosols. Here, using an aerosol optical tweezer and Raman spectroscopy, we show that the multiphase SO₂ oxidation by NO₂ is accelerated for two-order-of-magnitude by a copper catalyst. This reaction occurs on aerosol surfaces, is independent of pH between 3 and 5, and produces sulfate by a rate of up to 10 µg m^-3_air hr^-1 when reactive copper reaches a millimolar concentration in aerosol water – typical of severe haze events in North China Plain. Since copper and NO₂ are companion emitters in air pollution, they can act synergistically in converting SO₂ into sulfate in China’s haze.