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