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 (S⁰) 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 δ³⁴S values (154.7-212.0‰), modest Δ³³S anomalies of 21 ± 3‰, and relatively constant ³³λ (=ln(δ³³S + 1)/ln(δ³⁴S + 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 δ³⁴S values of 7.7-29.1‰ and Δ³³S values of 15.0 ± 1.6‰. In both excitation regions, the SO₃ products were mass dependently fractionated relative to the SO₂ reservoir. The two different absorption regions produced contrasting Δ³⁶S/Δ³³S signatures in the elemental sulfur products, with Δ³⁶S/Δ³³S = -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 Δ³⁶S/Δ³³S 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 δ³⁴S fractionation, ³³λ values, and Δ³⁶S/Δ³³S 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 S-MIF from both the 190 to 220 nm and the 250 to 330 nm absorption bands could have contributed to the Archean S-MIF signatures.

© 2012 Elsevier B.V