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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 ³²SO₂, ³³SO₂, and ³₄SO₂and use them to interpret the geological record. The calculated isotopie fractionation factors for SO₂ photolysis give mass independent distributions that are highly sensitive to the atmospheric concentrations of O₂, O₃, CO₂, H₂O, CS₂, NH₃, N₂O, H₂S, OCS, and SO₂ itself. Various UV-shielding scenarios are considered and we conclude that the negative △³³S observed in the Archean sulfate deposits can only be explained by OCS shielding. Of relevant Archean gases, OCS has the unique ability to prevent SO₂ photolysis by sunlight at λ > 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.

Natural climate variation, such as that caused by volcanoes, is the basis for identifying anthropogenic climate change. However, knowledge of the history of volcanic activity is inadequate, particularly concerning the explosivity of specific events. Some material is deposited in ice cores, but the concentration of glacial sulfate does not distinguish between tropospheric and stratospheric eruptions. Stable sulfur isotope abundances contain additional information, and recent studies show a correlation between volcanic plumes that reach the stratosphere and mass-independent anomalies in sulfur isotopes in glacial sulfate. We describe a mechanism, photoexcitation of SO ₂, that links the two, yielding a useful metric of the explosivity of historic volcanic events. A plume model of S(IV) to S(VI) conversion was constructed including photochemistry, entrainment of background air, and sulfate deposition. Isotopologue-specific photoexcitation rates were calculated based on the UV absorption cross-sections of ³²SO ₂, ³³SO ₂, ³⁴SO ₂, and ³⁶SO ₂ from 250 to 320 nm. The model shows that UV photoexcitation is enhanced with altitude, whereas mass-dependent oxidation, such as SO ₂ + OH, is suppressed by in situ plume chemistry, allowing the production and preservation of a mass-independent sulfur isotope anomaly in the sulfate product. The model accounts for the amplitude, phases, and time development of Δ ³³S/δ ³⁴S and Δ ³⁶S/Δ ³³S found in glacial samples. We are able to identify the process controlling mass-independent sulfur isotope anomalies in the modern atmosphere. This mechanism is the basis of identifying the magnitude of historic volcanic events.

A marine ecosystem model that incorporates nitrous oxide (N 2 O) production processes (i.e., ammonium oxidation during nitrification and nitrite reduction during nitrifier denitrification) and N isotopomers was developed to estimate the sea–air N 2 O flux and to quantify N 2 O production processes. This model was applied to water above the depth of 220 m at two contrasting time series sites, a subarctic station (K2) and a subtropical station (S1) in the western North Pacific. The model was validated with observed N concentration and N isotopomer data sets, and successfully simulated the higher N 2 O concentrations, higher δ 15 N values, and higher site preference values for N 2 O at K2 compared with S1. The annual mean N 2 O emissions were estimated to be 32.3 mg N m −2  year −1 at K2 and 2.7 mg N m −2  year −1 at S1. The results of case studies based on this model estimated the ratios of in situ biological N 2 O production to nitrate production during nitrification to be ~0.22 % at K2 and ~0.06 % at S1. It is also suggested that N 2 O was mainly produced via ammonium oxidation at K2, but was produced via both ammonium oxidation and nitrite reduction at S1. A large fraction (~80 %) of the ammonium oxidation at K2 was carried out by archaea in the subsurface water. Isotope tracer incubation experiments using an archaeal activity inhibitor supported this hypothesis.

We report measurements of the ultraviolet absorption cross‐sections of32SO2, 33SO2, 34SO2 and 36SO2 recorded from 250 to 320 nm at 293 K with a resolution of 8 cm−1. This is the first reported measurement of the 36SO2cross‐section. This work improves earlier measurements of the32SO2, 33SO2 and 34SO2cross‐sections and is in good agreement concerning fine structure and peak widths, with localized differences at the peak maxima when isotope effects are taken into account. SO2 samples were produced in an identical process via combustion of isotopically enriched S0, eliminating effects due to variation in oxygen isotopic composition. Peak positions for the rare isotopologues are red shifted relative to the 32SO2 isotopologue. Starting at the origin the shift increases linearly through the band. A linear shift model based on the spectrum of 32SO2was used to estimate the cross‐sections of33,34,36SO2; the average of the wavelength resolved absolute difference between the modeled and experimental spectra is 77.4, 107 and 139 ‰ respectively. While the peak‐to‐valley amplitude of36SO2 tends to be smaller than the other isotopologues throughout the spectrum, integrated band intensities for all isotopologues are conserved to within 4% relative to 32SO2. The cross‐sections were used in a photochemical model to obtain fractionation constants to compare with photochemical chamber experiments. We conclude that planetary atmospheres will exhibit isotopic fractionation from both photoexcitation and photodissociation, and that experiments in the literature have isotopic imprints arising from both the B1B1‐X1A1 and the C1B1‐X1A1 bands. Key Points SO2 UV spectra present large isotopic effects Isotopic effects show a systematic red shifting Models suggest that different light sources influenced chamber experiments results

We report measurements of the ultraviolet absorption cross‐sections of 32 SO 2 , 33 SO 2 , 34 SO 2 and 36 SO 2 recorded from 250 to 320 nm at 293 K with a resolution of 8 cm −1 . This is the first reported measurement of the 36 SO 2 cross‐section. This work improves earlier measurements of the 32 SO 2 , 33 SO 2 and 34 SO 2 cross‐sections and is in good agreement concerning fine structure and peak widths, with localized differences at the peak maxima when isotope effects are taken into account. SO 2 samples were produced in an identical process via combustion of isotopically enriched S 0 , eliminating effects due to variation in oxygen isotopic composition. Peak positions for the rare isotopologues are red shifted relative to the 32 SO 2 isotopologue. Starting at the origin the shift increases linearly through the band. A linear shift model based on the spectrum of 32 SO 2 was used to estimate the cross‐sections of 33,34,36 SO 2 ; the average of the wavelength resolved absolute difference between the modeled and experimental spectra is 77.4, 107 and 139 ‰ respectively. While the peak‐to‐valley amplitude of 36 SO 2 tends to be smaller than the other isotopologues throughout the spectrum, integrated band intensities for all isotopologues are conserved to within 4% relative to 32 SO 2 . The cross‐sections were used in a photochemical model to obtain fractionation constants to compare with photochemical chamber experiments. We conclude that planetary atmospheres will exhibit isotopic fractionation from both photoexcitation and photodissociation, and that experiments in the literature have isotopic imprints arising from both the B 1 B 1 ‐X 1 A 1 and the C 1 B 1 ‐X 1 A 1 bands. SO2 UV spectra present large isotopic effects Isotopic effects show a systematic red shifting Models suggest that different light sources influenced chamber experiments results

Natural climate variation, such as that caused by volcanoes, is the basis for identifying anthropogenic climate change. However, knowledge of the history of volcanic activity is inadequate, particularly concerning the explosivity of specific events. Some material is deposited in ice cores, but the concentration of glacial sulfate does not distinguish between tropospheric and stratospheric eruptions. Stable sulfur isotope abundances contain additional information, and recent studies show a correlation between volcanic plumes that reach the stratosphere and mass-independent anomalies in sulfur isotopes in glacial sulfate. We describe a mechanism, photoexcitation of SO2, that links the two, yielding a useful metric of the explosivity of historic volcanic events. A plume model of S(IV) to S(VI) conversion was constructed including photochemistry, entrainment of background air, and sulfate deposition. Isotopologue-specific photoexcitation rates were calculated based on the UV absorption cross-sections of ... and ... from 250 to 320 nm. The model shows that UV photoexcitation is enhanced with altitude, whereas mass-dependent oxidation, such as SO2 + OH, is suppressed by in situ plume chemistry, allowing the production and preservation of a mass-independent sulfur isotope anomaly in the sulfate product. The model accounts for the amplitude, phases, and time development of ... and ... found in glacial samples. We are able to identify the process controlling mass-independent sulfur isotope anomalies in the modern atmosphere. This mechanism is the basis of identifying the magnitude of historic volcanic events. (ProQuest: ... denotes formulae/symbols omitted.)

Natural climate variation, such as that caused by volcanoes, is the basis for identifying anthropogenic climate change. However, knowledge of the history of volcanic activity is inadequate, particularly concerning the explosivity of specific events. Some material is deposited in ice cores, but the concentration of glacial sulfate does not distinguish between tropospheric and stratospheric eruptions. Stable sulfur isotope abundances contain additional information, and recent studies show a correlation between volcanic plumes that reach the stratosphere and mass-independent anomalies in sulfur isotopes in glacial sulfate. We describe a mechanism, photoexcitation of SO 2 , that links the two, yielding a useful metric of the explosivity of historic volcanic events. A plume model of S(IV) to S(VI) conversion was constructed including photochemistry, entrainment of background air, and sulfate deposition. Isotopologue-specific photoexcitation rates were calculated based on the UV absorption cross-sections of 32 SO 2 , 33 SO 2 , 34 SO 2 , and 36 SO 2 from 250 to 320 nm. The model shows that UV photoexcitation is enhanced with altitude, whereas mass-dependent oxidation, such as SO 2 + OH, is suppressed by in situ plume chemistry, allowing the production and preservation of a mass-independent sulfur isotope anomaly in the sulfate product. The model accounts for the amplitude, phases, and time development of Δ 33 S/δ 34 S and Δ 36 S/Δ 33 S found in glacial samples. We are able to identify the process controlling mass-independent sulfur isotope anomalies in the modern atmosphere. This mechanism is the basis of identifying the magnitude of historic volcanic events.

Natural climate variation, such as that caused by volcanoes, is the basis for identifying anthropogenic climate change. However, knowledge of the history of volcanic activity is inadequate, particularly concerning the explosivity of specific events. Some material is deposited in ice cores, but the concentration of glacial sulfate does not distinguish between tropospheric and stratospheric eruptions. Stable sulfur isotope abundances contain additional information, and recent studies show a correlation between volcanic plumes that reach the stratosphere and mass-independent anomalies in sulfur isotopes in glacial sulfate. We describe a mechanism, photoexcitation of SO 2 , that links the two, yielding a useful metric of the explosivity of historic volcanic events. A plume model of S(IV) to S(VI) conversion was constructed including photochemistry, entrainment of background air, and sulfate deposition. Isotopologue-specific photoexcitation rates were calculated based on the UV absorption cross-sections of 32 SO 2 , 33 SO 2 , 34 SO 2 , and 36 SO 2 from 250 to 320 nm. The model shows that UV photoexcitation is enhanced with altitude, whereas mass-dependent oxidation, such as SO 2 + OH, is suppressed by in situ plume chemistry, allowing the production and preservation of a mass-independent sulfur isotope anomaly in the sulfate product. The model accounts for the amplitude, phases, and time development of Δ 33 S/δ 34 S and Δ 36 S/Δ 33 S found in glacial samples. We are able to identify the process controlling mass-independent sulfur isotope anomalies in the modern atmosphere. This mechanism is the basis of identifying the magnitude of historic volcanic events.

We report measurements of the ultraviolet absorption cross-sections of 32SO2, 33SO2, 34SO2 and 36SO2 recorded from 250 to 320 nm at 293 K with a resolution of 8 cm1. This is the first reported measurement of the 36SO2 cross-section. This work improves earlier measurements of the 32SO2, 33SO2 and 34SO2 cross-sections and is in good agreement concerning fine structure and peak widths, with localized differences at the peak maxima when isotope effects are taken into account. SO2 samples were produced in an identical process via combustion of isotopically enriched S0, eliminating effects due to variation in oxygen isotopic composition. Peak positions for the rare isotopologues are red shifted relative to the 32SO2 isotopologue. Starting at the origin the shift increases linearly through the band. A linear shift model based on the spectrum of 32SO2 was used to estimate the cross-sections of 33,34,36SO2; the average of the wavelength resolved absolute difference between the modeled and experimental spectra is 77.4, 107 and 139 respectively. While the peak-to-valley amplitude of 36SO2 tends to be smaller than the other isotopologues throughout the spectrum, integrated band intensities for all isotopologues are conserved to within 4% relative to 32SO2. The cross-sections were used in a photochemical model to obtain fractionation constants to compare with photochemical chamber experiments. We conclude that planetary atmospheres will exhibit isotopic fractionation from both photoexcitation and photodissociation, and that experiments in the literature have isotopic imprints arising from both the B1B1-X1A1 and the C1B1-X1A1 bands.