Contribution of isotopologue self-shielding to sulfur mass-independent fractionation during sulfur dioxide photolysis

Signatures of sulfur mass-independent fractionation (S-MIF) are observed for sulfur minerals in Archean rocks, and for modern stratospheric sulfate aerosols (SSA) deposited in polar ice. Ultraviolet light photolysis of SO ₂ is thought to be the most likely source for these S-MIF signatures, although several hypotheses have been proposed for the underlying mechanism(s) of S-MIF production. Laboratory SO₂ photolysis experiments are carried out with a flow-through photochemical reactor with a broadband (Xe arc lamp) light source at 0.1 to 5 mbar SO₂ in 0.25 to 1 bar N ₂ bath gas, in order to test the effect of SO₂ pressure on the production of S-MIF. Elemental sulfur products yield high δ³⁴S values up to 140 ‰, with δ³³S/ δ³⁴S of 0.59 ± 0.04 and Δ³⁶S/ Δ³³S ratios of -4.6 ± 1.3 with respect to initial SO₂. The magnitude of the isotope effect strongly depends on SO ₂ partial pressure, with larger fractionations at higher SO ₂ pressures, but saturates at an SO₂ column density of 10¹⁸ molecules cm^-2. The observed pressure dependence and δ³³S/δ³⁴S and Δ³⁶S/ Δ³³S ratios are consistent with model calculations based on synthesized SO₂ isotopologue cross sections, suggesting a significant contribution of isotopologue self-shielding to S-MIF for high SO₂ pressure (>0.1 mbar) experiments. Results of dual-cell experiments further support this conclusion. The measured isotopic patterns, in particular the Δ³⁶S/Δ³³S relationships, closely match those measured for modern SSA from explosive volcanic eruptions. These isotope systematics could be used to trace the chemistry of SSA after large Plinian volcanic eruptions.