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

Abstract

Abstract Severe urban air pollution in China is driven by a synergistic conversion of SO 2 , NOx, and NH 3 into fine particulate matter (PM 2.5 ). Field studies indicated NO 2 as an important oxidizer to SO 2 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 SO 2 oxidation by NO 2 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 NO 2 are companion emitters in air pollution, they can act synergistically in converting SO 2 into sulfate in China’s haze.