Preprints
https://doi.org/10.5194/amt-2024-16
https://doi.org/10.5194/amt-2024-16
19 Feb 2024
 | 19 Feb 2024
Status: this preprint is currently under review for the journal AMT.

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

Hideki Nara, Takuya Saito, Taku Umezawa, and Yasunori Tohjima

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 gravimetrically prepared parent gas mixture containing a micromole-per-mole level of COS filled in a high-pressure aluminium cylinder, the COS stability of which we experimentally validated for at least 10 years. 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). In our evaluation experiments, excellent repeatability (0.23 % for COS and 0.43 % for HCFC-22 in terms of relative standard deviation), reproducibility (COS: 0.04 %, HCFC-22: 0.28 %), and dilution linearity (R2 > 0.99, for both COS and HCFC-22) were obtained. 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 gas chromatography–mass spectrometry (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 errors of 0.02 % for COS and 0.12 % 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.

Hideki Nara, Takuya Saito, Taku Umezawa, and Yasunori Tohjima

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on amt-2024-16', Anonymous Referee #1, 01 Mar 2024
  • RC2: 'Comment on amt-2024-16', Anonymous Referee #2, 07 Mar 2024
  • RC3: 'Comment on amt-2024-16', Anonymous Referee #3, 20 Mar 2024
Hideki Nara, Takuya Saito, Taku Umezawa, and Yasunori Tohjima
Hideki Nara, Takuya Saito, Taku Umezawa, and Yasunori Tohjima

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Short summary
We have developed a high-accuracy dynamic dilution system for generating reference gas mixtures containing carbonyl sulfide (COS). Although COS at ambient levels generally has poor storage stability, our approach involves dilution of a gas mixture containing micromole-per-mole levels of COS, the stability of which was validated for at least 10 years. Developed system has excellent dilution performance and will facilitate accurate instrumental calibration for atmospheric COS observation.