SO 2 and natural abundance samples were obtained from commercial manufacturers. The spectrum of the natural abundance sample is in agreement with previously published spectra. The spectra of the isotopically pure species were retrieved using the isotopic composition of the samples. The 32 SO 2 , 33 SO 2 , and 34 SO 2 absorption spectra show rich vibrational structure, and the positions and widths of the peaks change with isotopic substitution in a complex fashion. The results imply that large wavelength-dependent and broadband isotopic fractionations are associated with the UV photolysis of SO 2 .

Distributions of sulfur isotopes in geological samples would provide a record of atmospheric composition if the mechanism producing the isotope effects could be described quantitatively. We determined the UV absorption spectra of 32 SO2, 33 SO2, and 34 SO2 and use them to interpret the geological record. The calculated isotopic fractionation factors for SO2 photolysis give mass independent distributions that are highly sensitive to the atmospheric concentrations of O 2, O3, CO2, H2O, CS2, NH3, N2O, H2S, OCS, and SO 2 itself. Various UV-shielding scenarios are considered and we conclude that the negative ⌬ 33 S observed in the Archean sulfate deposits can only be explained by OCS shielding. Of relevant Archean gases, OCS has the unique ability to prevent SO 2 photolysis by sunlight at >202 nm. Scenarios run using a photochemical box model show that ppm levels of OCS will accumulate in a CO-rich, reducing Archean atmosphere. The radiative forcing, due to this level of OCS, is able to resolve the faint young sun paradox. Further, the decline of atmospheric OCS may have caused the late Archean glaciation. mass independent fractionation ͉ carbonyl sulfide (OCS) ͉ sulfur dioxide (SO2) ͉ photochemistry ͉ greenhouse gases M ass independent fractionation (MIF) of sulfur isotopes has been found in geological samples older than 2.3 billion years (Ga) and is believed to be produced by the photochemical reactions of gaseous sulfur compounds in a low-oxygen reducing atmosphere (1-4). The rise of atmospheric oxygen (O 2 and O 3 ) would have oxidized atmospheric sulfur compounds to sulfate, preventing the accumulation of isotopic fractionation in two reservoirs. Moreover, it would have blocked UV sunlight inhibiting the supposed photochemical process(es) producing the isotope anomaly. In contrast, given reducing conditions, two isotopically distinct sulfur species such as elemental sulfur (polysulfur) and sulfate could be formed photochemically and precipitate at the surface, where the MIF signal could be preserved in sediment (3-5). Sulfur MIF in geological samples is regarded as indicating an anoxic atmosphere.This assertion relies on assumptions as to the origin of the MIF, although the mechanisms are still poorly known. Broad agreement in the ⌬ 36 S/⌬ 33 S ratio (ϷϪ1) in pre-2.3 Ga sedimentary rocks (1, 6, 7) and those produced in the laboratory by SO 2 photolysis suggest that photodissociation of gas-phase SO 2 most likely accounts for MIF in the Archean (2). Indeed, SO 2 photolysis is a key rate limiting step, particularly in a low-O 2 reducing atmosphere (4, 8). In addition, two mechanisms have been proposed whereby sulfur MIF could be formed under oxidizing conditions in the modern atmosphere: (i) Photolysis of SO 3 at altitudes greater than Ϸ50 km, where reaction between SO 3 and H 2 O is slow, is claimed to be responsible for the MIF found in modern stratospheric aerosol (8). In contrast, at lower altitudes, SO from SO 2 photolysis reforms SO 2 , so fractionation in SO 2 photolysis would not affect the isotopic compos...

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Ultraviolet absorption cross sections of the main and substituted carbonyl sulfide isotopologues were calculated using wavepacket dynamics. The calculated absorption cross section of (16)O(12)C(32)S is in very good agreement with the accepted experimental spectrum between 190 and 250 nm. Relative to (16)O(12)C(32)S, isotopic substitution shows a significant enhancement of the cross section for (16)O(13)C(32)S, a significant reduction for (18)O(12)C(32)S and (17)O(12)C(32)S and almost no change for the sulfur isotopologues (16)O(12)C(33)S, (16)O(12)C(34)S, and (16)O(12)C(36)S. The analysis of the initial wavepackets shows that these changes can be explained in terms of the change in the norm of the initial wavepacket. Implications for our understanding of the stratospheric sulfur cycle are discussed.