Sulfur mass-independent fractionation patterns in the broadband UV photolysis of sulfur dioxide: Pressure and third body effects

The production of mass-independent fractionation (MIF) in the products of the middle/far UV photolysis of SO2 isotopologues has been linked to large S-MIF signatures observed in the pre-2.45 Ga rock record, and provides a valuable proxy for the evolution of atmospheric O2. The origin of such large MIF signatures in photolytic products has recently been ascribed to the optical self-shielding of SO2 isotopologues in photon-limited regions of the lower atmosphere. To better characterize the origin of such a signal we have performed pressure-variant broadband UV photolysis experiments of SO2 and SO2–He mixtures. Low pressure (~ 20 Torr) photolysis of SO2 produces extractable sulfur with large δ34S ≈ 120–180‰, Δ33S ~ 20‰, and Δ36S/Δ33S ≈ − 2.5, while near atmospheric pressures produce sulfur with Δ33S ~ 2‰, and Δ36S/Δ33S ≈ − 12. Similar experiments conducted with variable pressures of helium bath gas produce a similar diminution in Δ33S, and in both cases a Δ36S/Δ33S ratio that is a linear function of pressure over several orders of magnitude. We postulate a photochemical and kinetic origin for the large S-MIF observed in these static cell experiments, produced from the excited state predissociative dynamics of SO2, but cannot expressly rule out self-shielding as a contributor to fractionations observed here.