The influence of SO₂ as the Criegee intermediate scavenger on the heterogeneous oxidation of oleic acid

As a well-known Criegee intermediate (CI) scavenger, SO₂ can react with CIs in the gas phase to form sulphuric acid, which can be further converted to sulfate aerosols, which make an important contribution to haze formation. Despite the potential importance, the interaction of SO₂ with the CIs produced by heterogeneous oxidation of unsaturated fatty acids remains largely unclear. In this work, the heterogeneous reactions of oleic acid (OA) thin films with O₃ and O₃/SO₂ mixture gases are studied under four different relative humidities (RHs): 5%, 30%, 60% and 80%, using microscopic Fourier transform infrared (micro-FTIR) spectrometer combined with a gas-flow system. A detailed comparison on formation rates of ester and intensity ratios of 1742 cm⁻¹ band (envelope C[dbnd]O of ester) to 1712 cm⁻¹ band (feature C[dbnd]O of carboxyl) between the two reaction systems shows that the addition of SO₂ inhibits the conversion of carboxyl to ester at any RH. As the SO₂ concentration increases, the formation rates and final relative amount of ester decrease. Based on these data, it can be concluded that SO₂ consumes part condensed-phase CIs produced by ozonolysis of OA, resulting in a significant inhibition in ester formation. The present results provide an important atmospheric implication on the reaction between SO₂ and condensed-phase CIs, which may be a missing sulfate production pathway in the atmosphere.