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

Abstract

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 SO2 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 N2 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 SO2 partial pressure, with larger fractionations at higher SO2 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.