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

Abstract

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 cm3 volume) for preserving the OCS amount and δ34S(OCS) values at −80 ∘C for up to 90 days and 14 days; 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 δ34S(OCS) values as 0.4 ‰ for overall procedures during measurements for atmospheric samples. Additionally, this report presents diurnal variation of δ34S(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 δ34S(OCS) values were, respectively, 447–520 pmol mol−1 and from 10.4 ‰ to 10.7 ‰ with a lack of diurnal variation. The observed δ34S(OCS) values in ambient air differed greatly from previously reported values of δ34S(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 δ34S(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 δ34S(OCS) values is expected due to a link between anthropogenic activities and OCS cycles. 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.

Constraining the atmospheric OCS budget from sulfur isotopes

“…Based on the similarity of atmospheric δ 34 S(OCS) at two sites (Israel and the Canary Islands), with values of 12.8 to 13.4‰, Angert et al (19) reported sulfur isotopic homogeneity of atmospheric OCS. However, differences between δ 34 S(OCS) of (10.5 ± 0.4)‰ at Yokohama and δ 34 S(OCS) of (13.2 ± 0.6)‰ obtained in Israel and the Canary Islands indicate that the atmospheric δ 34 S(OCS) values might not always be constant (21). In addition, several observations have shown variations in [OCS], such as terrestrial seasonality (2) and high level of [OCS] spatially distributed in the boreal summer above the Indo-Pacific region (22) or downwind of air masses from continental China (23).…”

“…Sampling and the corresponding measurements applied a method developed in our earlier works (20,21). Briefly, the system consisted of two parts: a large volume air sampling system and an online OCS purification system.…”

“…Sulfur isotopic constraints of the atmospheric OCS budget revealed that anthropogenic OCS sources, and not only oceanic OCS sources, are likely to be major constituents of the missing source of atmospheric OCS. atmospheric δ 34 S(OCS) values of 4.9‰ were reported from a commercial compressed air sample obtained from one location in Japan (Kawasaki), but this sample from compressed air might have been affected by anthropogenic OCS sources at the sampling site or contamination during compression or preservation processes (21). Furthermore, this method requires hundreds of liters of air per analysis to obtain several nanomoles of OCS.…”

Tropospheric carbonyl sulfide mass-balance based on direct measurements of sulfur isotopes

“…10 Measurements of sulfur isotope ratios (δ 34 S) in COS may be used to track COS sources and thus reduce the uncertainties in their ux estimations. 12,22,23,24 However, the isotopic mass balance approach works best if the COS end-members are directly measured and have a signi cantly different isotopic signature. Previous δ 34 S measurements of atmospheric COS are scarce and there have been no direct measurements of two important components: the δ 34 S of oceanic COS emissions and, the isotopic fractionation of COS during plant uptake.…”

Use of Isotope Effects To Understand the Present and Past of the Atmosphere and Climate and Track the Origin of Life

“…As COS was proposed as a major greenhouse gas in the Archean, its role in the early Earth should be further investigated. Recent development of δ 34 S (and potentially Δ 33 S) analytical methods for ambient COS may provide additional isotopic constrains for such investigation in the future.…”