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 (PM2.5). 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-3air 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.