Photolysis of long-lived predissociative molecules as a source of mass-independent isotope fractionation:: The example of SO2

Laboratory experiments have demonstrated that a mass-independent fractionation (MIF) signature is present in elemental sulfur produced during SO2 phototysis, but the underlying mechanism remains unknown. I report here the results of chemical kinetics modeling of self-shielding during photodissociation Of SO2 in the (C) over tilde B-1(2) - (X) over tilde (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 rotationalty-resolved lines of (SO2)-S-32 produces MIF signatures in SO and residual SO2. Using approximate synthetic spectra for the sulfur isotopotogues Of SO2, I show that SO2 photolysis yields a sulfur MIF signature that can account for much of the laboratory MIF measured, and is in qualitative agreement with Delta S-33 and Delta S-36 values observed in Archean rocks.