A GC-IRMS method for measuring sulfur isotope ratios of carbonyl sulfide from small air samples

A new system was developed for measuring sulfur isotopes δ 33 S and δ 34 S from atmospheric carbonyl sulfide (COS or OCS) on small air samples of several liters, using a pre-concentration and gas chromatography – isotope ratio mass spectrometry (GC-IRMS) method. Measurements of COS isotopes provide a tool for quantifying the COS budget, which will help towards better understanding climate feedback mechanisms. For a 4 liter sample at ambient COS mixing ratio, ~500 parts per trillion (ppt), we obtain a reproducibility error of 2.1 ‰ for δ 33 S and 0.4 ‰ for δ 34 S. After applying corrections, the uncertainty for an individual ambient air sample measurement is 3.3 ‰ for δ 33 S and 0.9 ‰ for δ 34 S. The ability to measure small samples allows application to a global-scale sampling program with limited logistical effort. To illustrate the application of this newly developed system, we present a timeseries of ambient air measurements, during the fall and winter of 2020 and 2021 in Utrecht, the Netherlands. The observed background values were δ 33 S = 1.0 ± 3.4 ‰ and δ 34 S = 15.5 ± 0.8 ‰ (VCDT). The maximum observed COS mixing ratios was 620 ppt, suggesting that the Netherlands receives little COS-containing anthropogenic emissions. We observed a change in COS mixing ratio and sometimes also δ 34 S with different air mass origin, as modelled with the Hybrid Single-Particle Lagrangian Integrated Trajectory model (HYSPLIT) backward trajectory analyses. An increasing trend of 40 ppt was observed in the COS mixing ratio between fall and winter, which is consistent with the expected seasonal cycle in the Netherlands. Additionally, we present the results from samples taken inside a highway tunnel in Utrecht to characterize vehicle COS emissions and isotopic composition. The vehicle emissions were small, with a COS/CO 2 ratio of 0.4 ppt/ppm; the isotopic signatures are depleted relatively to background atmospheric COS.