Photoabsorption cross-section measurements of 32S, 33S, 34S, and 36S sulfur dioxide from 190 to 220 nm

The ultraviolet absorption cross sections of the SO2 isotopologues are essential to understanding the photochemical fractionation of sulfur isotopes in planetary atmospheres. We present measurements of the absorption cross sections of (SO2)-S-32, (SO2)-S-33, (SO2)-S-34, and (SO2)-S-36, recorded from 190 to 220nm at room temperature with a resolution of 0.1nm (25cm(-1)) made using a dual-beam photospectrometer. The measured absorption cross sections show an apparent pressure dependence and a newly developed analytical model shows that this is caused by underresolved 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 (SO2)-S-32, (SO2)-S-33, and (SO2)-S-34 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 (SO2)-S-36 isotopologue for the (CB2)-B-1-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 SO2 produces (34)epsilon, (33), and (36) isotopic fractionations of +4.611.6, +8.89.0, and -8.8 +/- 19.6 parts per thousand, 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 broadband 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 SO2 itself. The estimated magnitude of (33), for example, is close to the maximum S-33 observed in the geological record.