Direct Organosulfate Production from Terpenoid-SO₂ Interactions in the Aqueous Phase

Organosulfates (OSs) are key components of atmospheric secondary organic aerosols (SOAs), yet much of their ambient mass remains unexplained. This study demonstrates that dissolved SO₂ reacts directly with major biogenic terpenoids (α-pinene, β-pinene, d-limonene, β-caryophyllene, and α-terpineol) in atmospheric condensed phases to form OSs without the need for traditional oxidants. Electrospray ionization mass spectrometry confirms product molecular formulas consistent with field measurements. Kinetic experiments reveal that α-pinene reacts with SO₂ at a second-order rate constant of 4.0 ± 0.9 M^-1 s^-1 in a 50 vol % acetonitrile/water mixture, with the rate increasing nonlinearly as water content rises, reaching 12 ± 1 M^-1 s^-1 at 70 vol % water. pH-dependent experiments suggest that this reaction of α-pinene can occur at rates of 60-80 M^-1 s^-1 at pH 3-5, typical of cloud and aerosol water. This pathway could substantially contribute to OS formation and improve multiphase model predictions. Given that dissolved SO₂ concentrations (∼10^-3 M) far exceed those of OH, O₃, or NO₃ in aqueous phase, this pathway may rival or even surpass established oxidant-based sinks for terpenoids, contributing substantially to OS formation. Incorporating terpenoid + SO₂ aqueous chemistry into multiphase models could thus enhance predictions of aerosol composition, particle acidity, and climate-relevant properties.