Multiple-sulfur isotope effects during photolysis of carbonyl sulfide

Y Lin,M S Sim,S Ono

doi:10.5194/acp-11-10283-2011

Y Lin, M S Sim + Show 1 more

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https://doi.org/10.5194/acp-11-10283-2011

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Abstract

Abstract. Laboratory experiments were carried out to determine sulfur isotope effects during ultraviolet photolysis of carbonyl sulfide (OCS) to carbon monoxide (CO) and elemental sulfur (S0). The OCS gas at 3.7 to 501 mbar was irradiated with or without a N2 bath gas using a 150 W Xe arc lamp. Sulfur isotope ratios for the product S0 and residual OCS were analyzed by an isotope ratio mass-spectrometer with SF6 as the analyte gas. The isotope fractionation after correction for the reservoir effects is −6.8‰ for the ratio 34S/32S, where product S0 is depleted in heavy isotopes. The magnitude of the overall isotope effect is not sensitive to the addition of N2 but increases to −9.5‰ when radiation of λ > 285 nm is used. The measured isotope effect reflects that of photolysis as well as the subsequent sulfur abstraction (from OCS) reaction. The magnitude of isotope effects for the abstraction reaction is estimated by transition state theory to be between −18.9 and −3.1‰ for 34S which gives the photolysis isotope effect as −10.5 to +5.3‰. The observed triple isotope coefficients are ln(δ34S + 1)/ln(δ34S + 1) = 0.534 ± 0.005 and ln(δ36S + 1)/ln(δ34S + 1) = 1.980 ± 0.021. These values differ from canonical values for mass-dependent fractionation of 0.515 and 1.90, respectively. The result demonstrates that the OCS photolysis does not produce large isotope effects of more than about 10‰ for 34S/32S, and can be the major source of background stratospheric sulfate aerosol (SSA) during volcanic quiescence.

Highlights

Carbonyl sulfide (OCS) accounts for more than 80 % of gasphase sulfur above 8 km as the most resistant reduced sulfur species to oxidation in the troposphere (e.g. Farwell et al, 1995; Turco et al, 1980; Khalil and Rasmussen, 1984; Crutzen, 1976)
Our study focuses on the rare and non-conventional isotopes of sulfur (33S and 36S) because certain gas phase reactions, SO2 photolysis and CS2 photopolymerization, are known to produce non-mass-dependent isotope effects (Farquhar et al, 2001; Colman et al, 1996; Zmolek et al, 1999)
Laboratory photolysis of carbonyl sulfide produced elemental sulfur depleted in heavy isotopes by −6.8 ‰ for the ratio 34S/32S relative to initial OCS

Summary

Introduction

Carbonyl sulfide (OCS) accounts for more than 80 % of gasphase sulfur above 8 km as the most resistant reduced sulfur species to oxidation in the troposphere (e.g. Farwell et al, 1995; Turco et al, 1980; Khalil and Rasmussen, 1984; Crutzen, 1976). The sulfur atom is oxidized by OH/O3/O2 to SO2 and forms sulfate, and is thought to contribute to the stratospheric sulfate aerosol layer (Junge layer) (Junge et al, 1961; Crutzen, 1976; Pitari et al, 2002). The stratospheric sulfate aerosol (SSA) layer at 17–30 km, with lifetime of 3–4 yr, affects the atmospheric radiation balance and catalyzes heterogeneous reactions that recycle the inert halogen species related to the ozone budget The other significant sources of SSA are oxidation of SO2 transported upward from the lower troposphere by diffusion (Crutzen, 1976), uplifted tropospheric H2SO4 (Pitari et al, 2002), and direct stratospheric injection of SO2

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