An Estimate of Sulfur Isotope Fractionation Due to SO2 Self-Shielding in the Upper Atmosphere of Venus

Sulfur dioxide is a trace constituent of the upper atmosphere of Venus but plays a dominant role in the photochemistry above the cloud tops. Because SO2 undergoes indirect dissociation to a relatively long-lived excited state, it has a line-type absorption spectrum in the dissociation region (~190–220 nm). This leads to strong isotopic fractionation under optically thick conditions, a process referred to as self-shielding. Here, I use SO2 cross-sections, shielding functions, and a simple steady-state photochemical model to estimate sulfur isotope ratios in SO2. The results indicate that large isotope depletion relative to SO2 in the deep atmosphere is expected in SO2 below 70 km altitude, with δ34S ~ −100 to −200 permil. This is readily detectable by the VTLS tunable laser spectrometer planned for the NASA DAVINCI mission.