A high-accuracy dynamic dilution method for generating reference gas mixtures of carbonyl sulfide at sub-nanomole-per-mole levels for long-term atmospheric observation

Abstract. Atmospheric carbonyl sulfide (COS) has received increasing attention as a potential tracer for investigating the global carbon cycle. Owing to the irreversible photosynthetic absorption of COS, changes in the atmospheric COS mole fraction can be related to terrestrial gross primary production. However, the instability of COS in high-pressure cylinders has hampered the accurate determination of atmospheric COS. Here, we report a dynamic dilution method for generating reference gas mixtures containing COS at ambient levels (ca. 500 pmol mol−1). Our method combined a dynamic dilution system employing a high-accuracy mass flow measurement system and a dry reference gas mixture prepared gravimetrically as a parent gas mixture containing a micromole-per-mole level of COS filled in a high-pressure aluminium cylinder. The storage stability of COS at this level was experimentally validated for three gravimetrically prepared dry reference gases over a period of more than 1 decade. We evaluated the dilution performance of the developed method using a gravimetric parent gas mixture containing approximately 1 µmol mol−1 of COS and chlorodifluoromethane (HCFC-22). Excellent repeatability (0.2 % for COS and 0.4 % for HCFC-22 in terms of relative standard deviation; RSD), reproducibility (COS: 0.1 %; HCFC-22: 0.3 %), and dilution linearity (R2>0.99 for both COS and HCFC-22) were obtained and were corroborated by the nearly constant ratio of the normalized gas chromatography–mass spectrometry (GC/MS) response of COS to HCFC-22. The dilution accuracy was examined by comparing the determined HCFC-22 mole fractions in a dynamically diluted parent gas mixture from a mass flow rate measurement system and GC/MS calibrated using a gravimetrically diluted parent gas mixture. The mole fractions of HCFC-22 from these two methods agreed within an acceptable difference of approximately 2 pmol mol−1, validating the dilution accuracy of the developed method. By re-evaluating the experimental data, we determined the mole fractions of COS and HCFC-22 in an ambient-air-based reference gas mixture, with relative standard deviations of 0.1 % for COS and 0.3 % for HCFC-22. These results demonstrated that the developed method can accurately generate reference gas mixtures containing COS at ambient levels, which we expect will support long-term observations of atmospheric COS.