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 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 SO 2 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 SO 2 in 0.25 to 1 bar N 2 bath gas, in order to test the effect of SO 2 pressure on the production of S‐MIF. Elemental sulfur products yield high δ 34 S values up to 140 ‰, with δ 33 S/δ 34 S of 0.59 ± 0.04 and Δ 36 S/Δ 33 S ratios of −4.6 ± 1.3 with respect to initial SO 2 . 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 SO 2 column density of 10 18 molecules cm −2 . The observed pressure dependence and δ 33 S/δ 34 S and Δ 36 S/Δ 33 S ratios are consistent with model calculations based on synthesized SO 2 isotopologue cross sections, suggesting a significant contribution of isotopologue self‐shielding to S‐MIF for high SO 2 pressure (>0.1 mbar) experiments. Results of dual‐cell experiments further support this conclusion. The measured isotopic patterns, in particular the Δ 36 S/Δ 33 S 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.