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 (S[superscript 0]). 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 S[superscript 0] 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 [superscript 34]S/[superscript 32]S, where product S[superscript 0] 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 [superscript 34]S which gives the photolysis isotope effect as −10.5 to +5.3‰. The observed triple isotope coefficients are ln(δ[superscript 34]S + 1)/ln(δ[superscript 34]S + 1) = 0.534 ± 0.005 and ln(δ[superscript 36]S + 1)/ln(δ[superscript 34]S + 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 [superscript 34]S/[superscript 32]S, and can be the major source of background stratospheric sulfate aerosol (SSA) during volcanic quiescence.