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. © 2013. American Geophysical Union. All Rights Reserved.