Sulfur Chemistry at the Air-Water Interface

The oxidation of gas-phase SO₂ to sulfate aerosols has been a driver of urban air pollution since the Great Smog of London in 1952. Traditionally, this reaction has been perceived as a quintessential atmospheric aqueous reaction, occurring within condensed water such as cloud and fog droplets. This established view has been challenged by recent studies showing that, in urban air pollution, sulfate aerosols form predominantly through a heterogeneous SO₂ conversion at aerosol surfaces. This review summarizes recent advances in understanding this heterogeneous process, focusing on (<i>a</i>) why S(IV) oxidation is faster at the air-water interface, (<i>b</i>) how to experimentally determine the reaction location with the scaling relationships of apparent reaction kinetics, and (<i>c</i>) how to predict, or retrieve, the localized surface reaction kinetics with multiscale models. We conclude by discussing open questions and remaining challenges, with the central theme of how the interfacial heterogeneous process may redefine our understanding of atmospheric sulfur chemistry.