High-Resolution UV Absorption Cross-Section Measurements of 32S, 33S, 34S, and 36S Sulfur Dioxide for the B~1B1−X~1A1 Absorption Band

Abstract

We report newly measured high-resolution and high-precision ultraviolet absorption cross-sections of ³²SO₂, ³³SO₂, ³⁴SO₂, and ³⁶SO₂ for the ${\tilde{B}}^1{B}_1-{\tilde{X}}^1{A}_1$ band over the wavelength range of 240 to 320 nm at a resolution of 0.4 cm^–1. The resolution was improved 20 times compared to that in a previous study. A least absolute deviation linear regression method was applied to calculate cross-sections and derived spectral errors from a set of measurements recorded at a wide range of pressures to ensure the optimal signal-to-noise ratio at all wavelengths. Based on this analysis, error bars on the measured cross-sections ranged between 3 and 10%. The overall features of measured cross-sections, such as peak positions of the isotopologues, are consistent with previous studies. We provide improved spectral data for studying sulfur mass-independent fraction (S-MIF) signatures during SO₂ photoexcitation. Our spectral measurements predict that SO₂ photoexcitation produces S-MIF enrichment factors ³³E = – 0.9 ± 0.2‰ and ³⁶E = – 3.8 ± 0.4‰ (where ³³E and ³⁶E are 1000 × [ln(³³J/³²J) – 0.515 ln(³⁴J/³²J)] ‰ and 1000 × [ln(³⁶J/³²J) – 1.90 ln(³⁴J/³²J] ‰, and ^3xJ is the ^3xSO₂ photoexcitation rate constant). Based on this new result, we found that calculated SO₂ photoexcitation isotope effects are smaller than previously thought and generally do not match photoexcitation experimental observations supporting the hypothesis of an intersystem crossing origin of MIF on those experiments.