Large-volume air sample system for measuring 34 S/ 32 S isotope ratio of carbonyl sulfide

Abstract

Knowledge related to sulfur isotope ratios of carbonyl sulfide (OCS or COS), the most abundant atmospheric sulfur species, remains scarce. An earlier method developed for sulfur isotopic analysis for OCS using S + fragmentation by an isotope ratio mass spectrometer 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 approximately 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 adsorption tubes (1/4 in. (0.64 cm) outer diameter, 17.5 cm length, approximately 1.4 cm 3 volume) for preserving the OCS amount and δ 34 S(OCS) values at -80 °C for up to 90 days and 14 days; and (iii) purification OCS from other compounds such as CO 2 . We tested the OCS collection efficiency of the systems and sulfur isotopic fractionation during sampling. Results show precision (1σ) of δ 34 S(OCS) values as 0.4% for overall procedures during measurements for atmospheric samples. Additionally, this report presents diurnal variation of δ 34 S(OCS) values collected from ambient air at the Suzukakedai campus of the Tokyo Institute of Technology located in Yokohama, Japan. The observed OCS concentrations and δ 34 S(OCS) values were, respectively, 447-520 pmol mol -1 and from 10.4% to 10.7% with a lack of diurnal variation. The observed δ 34 S(OCS) values in ambient air differed greatly from previously reported values of δ 34 S(OCS)=(4:9±0:3)% for compressed air collected at Kawasaki, Japan, presumably because of degradation of OCS in cylinders and collection processes for that sample. The difference of atmospheric δ 34 S(OCS) values between 10.5%in Japan (this study) and ∼ 13% recently reported in Israel or the Canary Islands indicates that spatial and temporal variation of δ 34 S(OCS) values is expected due to a link between anthropogenic activities and OCS cycles. The system presented herein is useful for application of δ 34 S(OCS) for investigation of OCS sources and sinks in the troposphere to elucidate its cycle.