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A quantitative understanding the origin of sulfur isotope mass‐independent fractionation (MIF) is essential to a full interpretation of the Archean sulfur geochemical record. Laboratory experiments have demonstrated that a MIF signature is present in elemental sulfur produced during SO 2 photolysis, but the underlying mechanism remains unknown. Here, I report the results of atmospheric chemistry modeling of isotope‐selective photodissociation of SO 2 in the 1 B 2 − 1 A 1 bands from 190 to 220 nm. This band system is dominated by a bending mode progression that produces shifts in the absorption spectrum upon sulfur isotope substitution. Self‐shielding in the rotationally‐resolved lines of 32 SO 2 produces MIF signatures in SO and residual SO 2 . A self‐shielding origin for sulfur MIF implies that the variations observed in Δ 33 S in Archean rocks reflect variation in atmospheric SO 2 concentration, and demonstrates that MIF in terrestrial rocks can be derived from photochemistry independent of molecular symmetry.
J. R. Lyons (Thu,) studied this question.