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.