[A-CC29] Ice cores and paleoenvironmental modeling

[ACC29-P06] Sulfur isotope fractionation during SO₂ photolysis in low-temperature and low-pressure atmospheres: Implication for sulfur chemistry during stratospheric volcanic events

*Yoshiaki Endo¹, Yasuhito Sekine^1,2, Yuichiro Ueno^1,3 (1.Tokyo Institute of Technology, 2.Kanazawa University, 3.Japan Agency for Marine-Earth Science and Technology)

Keywords:sulfur isitopes, sulfate aerosol, stratosphere, sulfur dioxide, photochemistry, mass-independent fractionation

Signatures of mass-independent sulfur isotope fractionation (MIF-S; Δ³³S = δ³³S – 0.515δ³⁴S ≠ 0) have been observed in stratospheric sulfate aerosols deposited in polar ice. The anomalous isotope signatures are believed to originate from photochemical reactions and to be a tracer of past stratospheric eruptions [1]. However, the origin of the isotope signatures is still debated. In the present study, we first report sulfur isotope fractionations during SO₂ photolysis, including isotopic self-shielding, at both low temperatures, down to 228 K, and low pressures below 10 kPa, where pressure broadening of SO₂ becomes negligible. These experimental conditions should be closer to those of the stratosphere where SO₂ photolysis occurs than previous experiments performed by Whitehill et al. (2015) [2] and Endo et al. (2019) [3]. The results show that magnitude of δ³⁴S, Δ³³S, and Δ³⁶S values become larger at lower temperatures, with the values at 228 K being ~4 times those at 296 K, whereas Δ³³S/δ³⁴S and Δ³⁶S/Δ³³S ratios depend little on temperature (~ +0.12 and ~ –3.1, respectively). We found that the two-component mixing of SO₂ oxidation by OH and SO₂ photolysis in the stratosphere can reproduce δ³⁴S and Δ³³S of the modern SSA when the contribution ratio of SO₂ photolysis to SO₃ production is ~20%. The contribution ratio is consistent with photochemical model results of Whitehill et al. (2015) [2]. Thus, we suggest that the MIF-S signatures in stratospheric sulfate aerosols originate from SO₂ photolysis. Our results are the basis of interpreting the MIF-S signatures and understanding sulfur chemistry during large volcanic events.
[1] Savarino et al. (2003). Geophys. Res. Lett., 30(21), 9−12.
[2] Whitehill et al. (2015). Atmos. Chem. Phys., 15(4), 1843−1864.
[3] Endo et al. (2019). Geophys. Res. Lett., 46(1), 483−491.