SO₂ photolysis as a source for sulfur mass-independent isotope signatures in stratospehric aerosols

Abstract. Signatures of sulfur isotope mass-independent fractionation (S-MIF) have been observed in stratospheric sulfate aerosols deposited in polar ice. The S-MIF signatures are thought to be associated with stratospheric photochemistry following stratospheric volcanic eruptions, but the exact mechanism responsible for the production and preservation of these signatures is debated. In order to identify the origin and the mechanism of preservation for these signatures, a series of laboratory photochemical experiments were carried out to investigate the effect of temperature and added O₂ on the S-MIF produced by two absorption band systems of SO₂: photolysis in the 190 to 220 nm region and photoexcitation in the 250 to 350 nm region. The SO₂ photolysis (SO₂ + hν → SO + O) experiments showed S-MIF signals with large ³⁴S/³⁴S fractionations, which increases with decreasing temperature. The overall S-MIF pattern observed for photolysis experiments, including high ³⁴S/³⁴S fractionations, positive mass-independent anomalies in ³³S, and negative anomalies in ³⁶S, is consistent with a major contribution from optical isotopologue screening effects and data for stratospheric sulfate aerosols. In contrast, SO₂ photoexcitation produced products with positive S-MIF anomalies in both ³³S and ³⁶S, which is different from stratospheric sulfate aerosols. SO₂ photolysis in the presence of O₂ produced SO₃ with S-MIF signals, suggesting the transfer of the S-MIF anomalies from SO to SO₃ by the SO + O₂ + M → SO₃ + M reaction. This is supported with energy calculations of stationary points on the SO₃ potential energy surfaces, which indicate that this reaction occurs slowly on a single adiabatic surface, but that it can occur more rapidly through intersystem crossing. Based on our experimental results, we estimate a termolecular rate constant on the order of 10⁻³⁷ cm⁶ molecule⁻² s⁻¹. This rate can explain the preservation of mass independent isotope signatures in stratospheric sulfate aerosols and provides a minor, but important, oxidation pathway for stratospheric SO₂. The production and preservation of S-MIF signals requires a high SO₂ column density to allow for optical isotopologue screening effects to occur and to generate a large enough signature that it can be preserved. In addition, the SO₂ plume must reach an altitude of around 20 to 25 km, where SO₂ photolysis becomes a dominant process. These experiments are the first step towards understanding the origin of the sulfur isotope anomalies in stratospheric sulfate aerosols.