Multiphase Reactions between Secondary Organic Aerosol and Sulfur Dioxide: Kinetics and Contributions to Sulfate Formation and Aerosol Aging

As two of the most important components of atmospheric fine particulate matter (PM_2.5), secondary organic aerosol (SOA) and sulfate are tightly coupled in terms of their formation and evolution, yet our knowledge of the kinetics and mechanisms of such coupling remains incomplete. Here we show that the multiphase reaction between α-pinene-derived SOA and sulfur dioxide (SO₂) results in efficient production of inorganic sulfate (85–90%) and organosulfates (10–15%) and chemical evolution of SOA. The reactive uptake coefficient of SO₂ (γ_SO2) on α-pinene SOA was determined to be ∼10^–4–10^–3, depending markedly on the organic peroxide content (implicating an important role of particle-phase peroxides in SO₂ oxidation) and also being strongly affected by the aerosol liquid water content. We estimate an aqueous-phase reaction rate constant (k^II) of 154–1545 M s^–1 for S­(IV) with α-pinene-derived peroxides under weakly acidic conditions (pH ∼ 4–5). These k^II values range between those for commercially available cumene hydroperoxide and 2-tert-butyl hydroperoxide and those for the smallest peroxides such as hydrogen peroxide and methylhydroperoxides. A quantitative analysis based on our estimated k^II values indicates that the multiphase reaction between SOA and SO₂ is an important pathway for sulfate formation and SOA aging that needs to be considered in the modeling of aerosol budgets and impacts on air quality and climate.