High Resolution Photoabsorption Cross-Sections of Isotopologues of SO2

The timing of the oxygenation of the Earth’s atmosphere is a central issue in understanding the Earth’s paleoclimate. The discovery of mass-independent fractionation (MIF) of sulphur isotopes deposited within Archean and Paleoproterozoic rock samples has given rise to a possible marker, through the transition between MIF within older rock samples ( > 2.4 Gyrs) to mass-dependent fractionation (MDF) within younger samples, for the rise in oxygen concentrations within the Earth’s atmosphere (Farquhar et al., Earth Planet. Sci. Lett., 213:1-13 (2003).). Laboratory experiments (Farquhar et al., J. Geophys. Res., 106:32829-32839, (2001).) suggest isotopic self shielding during gas phase photolysis as the dominant mechanism for MIF. Self shielding is present for SO2 at wavelengths shorter than 220 nm where it undergoes partial predissocation. The UV absorption of SO2is dominated by the C1B2-X1A1 electronic system comprising of strong vibrational bands between 170 - 230 nm. Within an atmosphere consisting of low O2 and O3 concentrations, UV radiation would penetrate deep into the ancient Earth’s atmosphere within the 180 - 220 nm range driving the photolysis of SO2. We have conducted the first ever high resolution measurements of the photoabsorption cross sections of several isotopologues of SO2, namely 32SO2, 33SO2, 34SO2 and 36SO2. The cross sections are being measured at Imperial College at initial resolutions of 1.0 cm-1 which will be increased to resolutions < 0.5 cm-1 for inclusion in photochemical models of the early Earth’s atmosphere. The models will be used to more reliably interpret the sulphur isotope ratios found within ancient rock samples (Lyons, Geophys. Res. Lett., 34:L22811 (2007).). For a detailed discussion of the use of high resolution cross sections within photochemical models and interpretation of the findings see the abstract by Lyons et al.(this meeting). Initial 1.0 cm-1 resolution measurements of several isotopologues of SO2 will be presented in addition to preliminary < 0.5 cm-1 photoabsorption cross section measurements. 1.0 cm-1 resolution photoabsorption cross sections of the 32SO2, 33SO2, and 34SO2 isotopologues recorded at Imperial College.