Multiple-sulfur isotope effects during photolysis of carbonyl sulfide

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.