Oxidation of sulfur dioxide by nitrogen dioxide accelerated at the interface of deliquesced aerosol particles

Although the multiphase chemistry of SO₂ in aerosol particles is of great importance to air quality under polluted haze conditions, a fundamental understanding of the pertinent mechanisms and kinetics is lacking. In particular, there is considerable debate on the importance of NO₂ in the oxidation of SO₂ in aerosol particles. Here experiments with atmospherically relevant deliquesced particles at buffered pH values of 4-5 show that the effective rate constant for the reaction of NO₂ with SO₃^2- ((1.4 ± 0.5) × 10¹⁰ M^-1 s^-1) is more than three orders of magnitude larger than the value in dilute solutions. An interfacial reaction at the surface of aerosol particles probably drives the very fast kinetics. Our results indicate that oxidation of SO₂ by NO₂ at aerosol surfaces may be an important source of sulfate aerosols under polluted haze conditions.