Excitation band dependence of sulfur isotope mass-independent fractionation during photochemistry of sulfur dioxide using broadband light sources

Ultraviolet photolysis of sulfur dioxide (SO2) is hypothesized to be the source of the sulfur isotope mass-independent fractionation (S-MIF) observed in Archean sulfate and sulfide minerals and modern stratospheric sulfate aerosols. A series of photochemical experiments were performed to examine the excitation band dependence of S-MIF during the photochemistry of SO2 under broadband light sources (a xenon arc lamp and a deuterium arc lamp). Optical filters (200 +/- 35 nm bandpass and 250 nm longpass filters) were used to separately access two different excitation bands of SO2 in the 190-220 nm and the 250-330 nm absorption regions, respectively. UV irradiation of SO2 in the 190-220 nm and 250-330 nm regions both produced elemental sulfur (S-0) and sulfur trioxide (SO3) as end products but yielded very different sulfur isotope signatures. The elemental sulfur products from direct photolysis in the 190-220 nm region were characterized by high delta S-34 values (154.7-212.0 parts per thousand), modest Delta S-33 anomalies of 21 +/- 3 parts per thousand, and relatively constant (33)lambda (= ln(delta S-33 + 1)/ln(delta S-34 + 1)) values of 0.64 +/- 0.3, all with respect to the initial SO2. Photoexcitation in the 250-330 nm region produced elemental sulfur with delta S-34 values of 7.7-29.1 parts per thousand and Delta S-33 values of 15.0 +/- 1.6 parts per thousand. In both excitation regions, the SO3 products were mass dependently fractionated relative to the SO2 reservoir. The two different absorption regions produced contrasting Delta S-36/Delta S-33 signatures in the elemental sulfur products, with Delta S-36/Delta S-33 = -1.9 +/- 0.3 and 0.64 +/- 0.3 for the 190-220 nm and 250-330 nm bands, respectively. Our results provide several critical constraints on the origin of the S-MIF signatures observed in modern stratospheric aerosols and in the Archean geological record. A lack of S-MIF in the sulfate product and positive Delta S-36/Delta S-33 ratios for the elemental sulfur from SO2 photo-oxidation demonstrate that photoexcitation in the 250-330 nm region is not a likely source for the S-MIF observed in modern stratospheric aerosols. Large Delta S-34 fractionation, (33)lambda values, and Delta S-36/Delta S-33 ratios observed for the 190-220 nm band are qualitatively consistent with predictions from synthetic isotopologue-specific cross sections. These isotope patterns, however, are not compatible with the Archean rock record. We explore the possibility that SMIF from both the 190 to 220 nm and the 250 to 330 nm absorption bands could have contributed to the Archean S-MIF signatures. (C) 2012 Elsevier Ltd. All rights reserved.