Pressure Effects in the Mass-Independent Fractionation Parameters of UV-Photolyte Sulfur from Broadband Irradiation of SO2

Mass-independent sulfur isotope fractionation observed in the Archean rock record, Martian meteorites, and atmospherically derived sulfate have all been attributed to the UV photolysis of sulfur dioxide. Single photon dissociation of ~{C} and ~{D} SO2 below a 219.2 nm threshold occurs primarily by the following reaction: SO2 + hν => SO + O. Subsequent photolysis of SO and lower sulfur oxides generate precipitable elemental sulfur residues. Previous laboratory photochemical studies have documented the influence of the ultraviolet light source in the mass-independent fractionation patterns of photolyte sulfur and residual SOx. [1] We have conducted similar experiments employing a continuous spectrum D2 lamp with varying pressures of SO2 from 20-900 torr and observe a large influence in the mass- independent parameters Δ36S/Δ33S, Δ33S/δ34S, and Δ36S/δ34S. Sulfur from experiments with pSO2 ~ 20, 155, 900 torr yield Δ36S/Δ33S = -2.4±0.1, -5.3±0.1, -12.1±0.7, and demonstrates an order of magnitude reduction in Δ33S/δ34S. Further, photolysis of ~20 torr SO2 with partial pressures of 100 and 300 torr He yield = -2.7±0.1, -3.4±0.1, corresponding to a similar, smaller magnitude effect. These experiments demonstrate that the incident UV light source is not the sole control on the mass-independent minor isotope patterns of photolyte sulfur, and that the observed patterns are a function of the total pressure of the photolyzed system. Further experiments are needed to quantify low pressure behavior, to determine if a pressure effect is observed with alternative broadband light sources, and to ascertain the influence of atmospherically relevant gases. [1] Farquhar et al. (2001) J. Geophys. Res. 106, 32829-32840.