Large volume sample system for measuring sulfur isotopic compositions of carbonyl sulfide

Abstract. Knowledge related to sulfur isotopic composition of carbonyl sulphide (OCS or COS), the most abundant atmospheric sulfur species, remains scarce. Earlier method developed for sulfur isotopic analysis for OCS using S+ fragmentation is inapplicable for ambient air samples because of the large samples required (approx. 500 L of 500 pmol mol−1 OCS). To overcome this difficulty, herein we present a new sampling system for collecting approx. 10 nmol of OCS from ambient air coupled with a purification system. Salient system features are (i) accommodation of samples up to 500 L (= approx. 10 nmol) of air at 5 L min−1, (ii) portability of 7 inch tubes (approx. 1 cm3) for preserving samples, and (iii) purification OCS from other compounds such as CO2. We tested the OCS collection efficiency of the systems and sulfur isotopic fractionation during sampling. Results show precision (1σ) of δ33S(OCS), δ34S(OCS), and Δ33S(OCS) values, respectively, as 0.4 ‰, 0.2 ‰, and 0.4 ‰. Additionally, this report presents diurnal variation of δ34S(OCS) values collected from ambient air at Suzukakedai campus of Tokyo Institute of Technology located in Yokohama, Japan. The observed OCS concentrations and δ34S(OCS) values were, respectively, 447–520 pmol mol−1 and from 10.4 ‰ to 10.7 ‰. No significant difference was found between values obtained during the day and night. The observed δ34S(OCS) values in ambient air differed greatly from previously reported values ((4.9 ± 0.3) ‰) for compressed air collected at Kawasaki, Japan, presumably because of sampling conditions and collection processes for that sample. Consequently, previous values of δ34S(OCS) = (4.9 ± 0.3) ‰ were not representative samples for a global signal. When considering (10.5 ± 0.4) ‰ is postulated as the global signal of δ34S(OCS), this revised δ34S(OCS) value is consistent with previous estimation based on terrestrial and oceanic sulfur sources. The δ34S(OCS) value explains the reported δ34S(OCS) values for background stratospheric sulfate. The system presented herein is useful for application of δ34S(OCS) for investigation of OCS sources and sinks in the troposphere to elucidate its cycle and its contribution to background stratospheric sulfate.