Publication Date:
2015-02-25
Description:
: The ultraviolet absorption cross-sections of the SO 2 isotopologues are essential to understanding the photochemical fractionation of sulfur isotopes in planetary atmospheres. We present measurements of the absorption cross-sections of 32 SO 2 , 33 SO 2 , 34 SO 2 and 36 SO 2 , recorded from 190 to 220 nm at room temperature with a resolution of 0.1 nm (~25 cm -1 ) made using a dual-beam photo-spectrometer. The measured absorption cross-sections show an apparent pressure dependence and a newly developed analytical model shows that this is caused by under-resolved fine structure. The model made possible the calculation of absorption cross-sections at the zero-pressure limit that can be used to calculate photolysis rates for atmospheric scenarios. The 32 SO 2 , 33 SO 2 and 34 SO 2 cross sections improve upon previously published spectra including fine structure and peak widths. This is the first report of absolute absorption cross-sections of the 36 SO 2 isotopologue for the C 1 B 2 -X 1 A 2 band where the amplitude of the vibrational structure is smaller than the other isotopologues throughout the spectrum. Based on the new results, solar UV photodissociation of SO 2 produces 34 ε , 33 Ε and 36 Ε isotopic fractionations of +4.6 ± 11.6 ‰, +8.8 ± 9.0 ‰ and −8.8 ± 19.6 ‰, respectively. From these spectra isotopic effects during photolysis in the Archean atmosphere can be calculated and compared to the Archean sedimentary record. Our results suggest that broad-band solar UV photolysis is capable of producing the mass-independent fractionation observed in the Archean sedimentary record without involving shielding by specific gaseous compounds in the atmosphere including SO 2 itself. The estimated magnitude of 33 Ε , for example, is close to the maximum Δ 33 S observed in the geological record.
Print ISSN:
0148-0227
Topics:
Geosciences
,
Physics
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