Contribution of isotopologue self‐shielding to sulfur mass‐independent fractionation during sulfur dioxide photolysis

Abstract

[1] Signatures of sulfur mass-independent fractionation (S-MIF) are observed for sulfur minerals in Archean rocks, and for modern stratospheric sulfate aerosols (SSA) deposited in polar ice. Ultraviolet light photolysis of SO 2 is thought to be the most likely source for these S-MIF signatures, although several hypotheses have been proposed for the underlying mechanism(s) of S-MIF production. Laboratory SO 2 photolysis experiments are carried out with a flow-through photochemical reactor with a broadband (Xe arc lamp) light source at 0.1 to 5 mbar SO 2 in 0.25 to 1 bar N 2 bath gas, in order to test the effect of SO 2 pressure on the production of S-MIF. Elemental sulfur products yield high d 34 S values up to 140 %, with d 33 S/d 34 S of 0.59 AE 0.04 and Δ 36 S/Δ 33 S ratios of À4.6 AE 1.3 with respect to initial SO 2 . The magnitude of the isotope effect strongly depends on SO 2 partial pressure, with larger fractionations at higher SO 2 pressures, but saturates at an SO 2 column density of 10 18 molecules cm À2 . The observed pressure dependence and d 33 S/d 34 S and Δ 36 S/Δ 33 S ratios are consistent with model calculations based on synthesized SO 2 isotopologue cross sections, suggesting a significant contribution of isotopologue self-shielding to S-MIF for high SO 2 pressure (>0.1 mbar) experiments. Results of dual-cell experiments further support this conclusion. The measured isotopic patterns, in particular the Δ 36 S/Δ 33 S relationships, closely match those measured for modern SSA from explosive volcanic eruptions. These isotope systematics could be used to trace the chemistry of SSA after large Plinian volcanic eruptions.Citation: Ono, S., A. R. Whitehill, and J. R. Lyons (2013), Contribution of isotopologue self-shielding to sulfur massindependent fractionation during sulfur dioxide photolysis,

SO<sub>2</sub> photolysis as a source for sulfur mass-independent isotope signatures in stratospehric aerosols

“…In particular, SO 2 photolysis produces large MIF anomalies, as well as large mass-dependent isotope fractionations (Masterson et al, 2011;Whitehill and Ono, 2012;Ono et al, 2013) that are consistent with the isotopic signatures observed in stratospheric sulfate aerosols in ice cores .…”

“…The differences in the photophysics and photochemistry between the photolysis region (190 to 220 nm) and the photoexcitation region (250 to 350 nm) suggest different mechanisms for MIF formation, as discussed previously (Whitehill and Ono, 2012;Ono et al, 2013;Whitehill et al, 2013). In the 165 to 235 nm wavelength region, SO 2 photolysis occurs through pre-dissociation from the bound C( 1 B 2 ) state.…”

“…et al, 2008) and optical screening effects under high SO 2 column densities (Lyons, 2007(Lyons, , 2008Ono et al, 2013). In the narrow spectral region from 215 to 218.7 nm, where the quantum yield of photodissociation varies, it is possible that quantum yield differences between isotopologues could potentially produce additional isotope effects beyond those predicted from absorption cross sections.…”

Multiple sulfur-isotope signatures in Archean sulfates and their implications for the chemistry and dynamics of the early atmosphere

“…Type 1 bedded barite with high Δ 33 S values (Δ 33 S mean ∼−0.1‰ with a maximum value of +0.24 ± 0.09‰) is fully consistent with the range of modern stratospheric volcanic sulfate aerosols preserved in the ice and snow records ( Fig. 3; δ 34 S-Δ 33 S slope ∼0.07 ± 0.01 and Δ 33 S-Δ 36 S slope ∼ −2.2 ± 0.4) (21-24), as well as laboratory experiments of SO 2 photodissociation in the 190-to 220-nm absorption region (δ 34 S-Δ 33 S slope = 0.086 ± 0.035 and Δ 33 S-Δ 36 S slope = −4.6 ± 1.3) (25,26). This consistency between different sets of data, together with the observation that this type 1 barite is found in felsic volcanic ash, suggests that the same type of photolytic process, likely volcanic SO 2 photolysis, was effective both in the Archean atmosphere and modern stratosphere and potentially in the earliest Earth atmosphere (27).…”

Recombination reactions as a possible mechanism of mass-independent fractionation of sulfur isotopes in the Archean atmosphere of Earth

“…However, the actual chemical process, or processes, responsible for generation of significant enrichments of sulfur-bearing species in heavy isotopes of sulfur, and leading to mass-independent fractionation, still remains unidentified. Some mass-independent fractionation was observed experimentally in SO 2 photolysis (10)(11)(12) and in nonadiabatic dynamics of SO 2 photodissociation (13,14). Another important chemical pathway, specific to a low-oxygen atmosphere (7,8,(15)(16)(17), is a chain of recombination reactions that start from recombination of photolytically produced sulfur atoms (S + S → S 2 ), go through formation of larger sulfur allotropes (S + S 2 → S 3 , S 2 + S 2 → S 4 , and S + S 3 → S 4 ), and end up at the elemental sulfur (S 4 + S 4 → S 8 ) that can be deposited, preserved, and could contribute to S-MIF in the Archean atmosphere.…”