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

Abstract

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±35nm bandpass and 250nm longpass filters) were used to separately access two different excitation bands of SO2 in the 190–220nm and the 250–330nm absorption regions, respectively.UV irradiation of SO2 in the 190–220nm and 250–330nm 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 190–220nm 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 250–330nm 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 190–220nm and 250–330nm 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 250–330nm 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 190–220nm 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 220nm and the 250 to 330nm absorption bands could have contributed to the Archean S-MIF signatures.