Determination of the Sulfur Isotope Ratio in Carbonyl Sulfide Using Gas Chromatography/Isotope Ratio Mass Spectrometry on Fragment Ions 32S+, 33S+, and 34S+

Little is known about the sulfur isotopic composition of carbonyl sulfide (OCS), the most abundant atmospheric sulfur species. We present a promising new analytical method for measuring the stable sulfur isotopic compositions (delta S-33, delta S-34, and Delta S-33) of OCS using nanomole level samples. The direct isotopic analytical technique consists of two parts: a concentration line and online gas chromatography-isotope ratio mass spectrometry (GC-IRMS) using fragmentation ions S-32(+), S-33(+), and S-34(+). The current levels of measurement precision for OCS samples greater than 8 nmol are 0.42 parts per thousand, 0.62 parts per thousand, and 0.23 parts per thousand for delta S-33, delta S-34, and delta S-33, respectively. These delta and Delta values show a slight dependence on the amount of injected OCS for volumes smaller than 8 nmol. The isotope values obtained from the GC-IRMS method were calibrated against those measured by a conventional SF6 method. We report the first measurement of the sulfur isotopic composition of OCS in air collected at Kawasaki, Kanagawa, Japan. The delta S-34 value obtained for OCS (4.9 +/- 0.3 parts per thousand) was lower than the previous estimate of 11 parts per thousand. When the delta S-34 value for OCS from the atmospheric sample is postulated as the global signal, this finding, coupled with isotopic fractionation for OCS sink reactions in the stratosphere, explains the reported delta S-34 for background stratospheric sulfate. This suggests that OCS is a potentially important source for background (nonepisodic or nonvolcanic) stratospheric sulfate aerosols.