Absorption spectra measurements at ~1 cm^–1 spectral resolution of ³²S, ³³S, ³⁴S, and ³⁶S sulfur dioxide for the 206–220 nm region and applications to modeling of the isotopic self-shielding

Abstract

The sulfur isotope fractionation that occurs during SO₂ photolysis is key to explaining the isotope signatures stored in ancient sedimentary rocks and understanding the atmospheric compositions of the early Earth and early Mars. Here, we report the photoabsorption cross-sections of ³²SO₂, ³³SO₂, ³⁴SO₂, and ³⁶SO₂ measured from 206 to 220 nm at 296 K. The wavelength resolution was set to 1 cm^–1, 25 times higher than that of previous SO₂ isotopologue absorption spectra measurements. The precision of ~10% is in agreement with previously reported SO₂ absorption spectra. In comparison with previously reported high-resolution spectra of natural abundance, SO₂ measurements demonstrate smaller cross-sectional magnitudes at absorption peaks and an offset wavelength by ~0.016 nm. Using the newly recorded isotopologue spectra, we calculated the sulfur isotope fractionation for self-shielding during SO₂ photolysis. The calculated ³⁴S fractionation (³⁴ε) roughly reproduces the observed relationship between ³⁴ε and the SO₂ column density in previous photolysis experiments. Thus, the cross-section is useful for predicting ³⁴S/³²S isotope fractionation in an optically thick SO₂ atmosphere. In contrast, for mass-independent fractionation (MIF-S, i.e., non-zero Δ³³S), the measured spectra predicted a weakly negative Δ³³S/δ³⁴S slope of about –0.1. The small Δ³³S/δ³⁴S slope is consistent with the slopes of SO₂ photolysis experiments under high-pressure atmospheres (i.e., the pressure broadened absorption line width will be comparable to the spectral resolution). Therefore, MIF-S during photolysis experiments was linked to spectroscopic measurements for the first time. We conclude that reasonable precision and high-resolution spectroscopic measurements are key to explaining the origin of MIF-S at column densities below 10¹⁸ cm^–2. However, MIF-S production in chamber experiments or atmospheric conditions may require understanding pressure or temperature effects, such as linewidth broadening on the UV-absorption spectra, and how these effects manifest themselves on isotopologues.