the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 19 Feb 2024
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 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.
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RC1: 'Comment on amt-2024-16', Anonymous Referee #1, 01 Mar 2024
This study developed a dynamic dilution method for generating COS reference gas mixtures at ambient level, which would avoid issues related to poor stability of COS stored in high-pressure cylinders. Good performance in terms of repeatability, repeatability, linearity, and accuracy was obtained, demonstrating this method can be used for accurate long-term COS observations and applied to the preparation of other reference gases containing unstable species. Overall, the manuscript is well-written. Therefore, I suggest accepting this manuscript for publication after addressing my comments below.
- It is better to mention the WMO goal for COS measurement (such as inter-laboratory compatibility or other requirements) somewhere if available.
- Page 11: Please check the two formulas carefully. The comma at top right of the parentheses in Formula (1) should be removed, as well as the superscript “2” at far right of Formula (2).
- Page 11, line 26: According to Figure 2, the uncertainty “increased” rather than “decreased” with a decreasing flow rate.
- Page 23, line 3: Generally, uncertainty of reference gas is combined by a series of components which include but not limited to measurement error. So is it appropriate to define the uncertainty as standard error here? Or if there are some literatures to support this?
- According to Figure 4 and Table 3, the measurement of COS seemed to be more stable than that of HCFC-22, although the latter is identified as the one with greater stability. Are there some potential reasons related to this discrepancy?
Citation: https://doi.org/10.5194/amt-2024-16-RC1 -
RC2: 'Comment on amt-2024-16', Anonymous Referee #2, 07 Mar 2024
This paper describes a method to produce a reproducible, traceable gas stream containing carbonyl sulfide at the parts-per-trillion level for calibration purposes. While dynamic dilution systems have been described previously, this paper introduces some additional controls to stabilize pressure, which they show are important for large dilution ratios and a small (10 cc/min) flow of parent gas that interacts via adsorption/desorption with surfaces, such as carbonyl sulfide. The scientific methods are sound and the conclusions are reasonable. Some of the terminology used could be improved (see comments below).
Specific comments:
Pg 3. Line 11: Need a space before “Wehr”
Pg 3, line 20: Are you sure you have the correct citation? COS is known to be unstable in some cylinders, but I don’t recall that being demonstrated in the Hall et al. (2014) paper, and certainly not to the point we would say that stability is the most important factor limiting our understanding its global distribution and budget. The seasonal cycle of COS is several 10s of ppt in the northern hemisphere, and while smaller in magnitude, is clearly detectable in the southern hemisphere. Spatial gradients in COS are observable, despite issues related to stability in cylinders.
Pg 4, line 5: Consider changing the sentence to “In addition, gravimetric dilution into cylinders may introduce uncertainties related to surface effects that can vary by cylinder type.”
Pg 6, line 19: How much is a small amount of COS in the dilution gas? Can you be more specific? < 1 ppt?, 5 ppt?
Pg 8, line 13: The term “flow pressure” is used several times, while “pressure” is also used. For “flow pressure” do you mean the pressure applied to the MFC? I can see how you would want to keep the pressures on the mass flow controllers constant, but the term “flow pressure” may not be a well-known term.
Pg 10, line 23: Were internal surfaces of the static mixer also coated with Sulfinert?
Pg 14, line 9: You say that ambient air reference standards prepared in aluminum cylinders are unstable. Please clarify if these were dry air, or humidified air. In the previous section you discussed a reference standard that was humidified to 500 ppm H2O to enhance COS stability.
Pg 14, line 24: It seems strange why you chose to perform value-assignments to compressed air in a manganese (steel) cylinder as opposed to humidified ambient air in aluminum cylinders discussed on page 13. I am not questioning the results, only that the discussion includes dry air in aluminum cylinder, humidified air in aluminum cylinders, and “compressed” air in manganese (steel) cylinders. This leads to some confusion. I recommend being more specific with respect to dry air and humidified air.
Pg 15. line 5: You mention that you determined the linearity of the GC/MS using a totally separate dilutions system. Why? You created what appears to be a very good dilution system as the subject of this paper. And then later in the document you show that the dynamic dilution appears to be very linear on the GC/MS (figure 5). I don’t really understand the need for the commercial system, unless you are trying to compare a commercial system to the one you built.
Pg 15, figure 3: Please provide more details about the aluminum cylinder (dry air, no passivation treatment?, etc?). Although Aculife-treated aluminum cylinders from Airgas can drift, they tend to perform much better than this. This picture gives the impression that aluminum cylinders are almost useless for COS. Also, I would argue that would be difficult to conclude that this is exponential decay based on only the 3 data points shown.
Pg 16, line 6: This is confusing. How can the commercial dynamic dilution system be both stable and unstable?
Pg 20, line 21: I suggest referring to the traceability path here because the GCMS measurements are used in two ways:
consider: “There was a slight difference between the two average values; HCFC-22 measurements traceable to the gravimetrically-prepared mixture were approximately 2 pmol/mol lower than those traceable to the Molbloc dilution”.
Pg 24, line 19: change “On the other hands” to “On the other hand”
Pg 28, line 8: WMO compatibility goal for CO
Table 2: I think I understand this table, but the labels could lead to some confusion. This appears to be a comparison of GCMS results based on two traceability paths: dilution of a 1 ppm parent mixture to 500 ppt using the Molbloc system and a gravimetric dilution of the same 1 ppm parent mixture to 500 ppt in a cylinder (from figure 6)? In the text you say that there was a 2 ppt difference between the gravimetric standard and the dilution system. But as written, both rows in the table 2 refer to dynamic dilution. What exactly is meant by “sample determination system”? It might be easier for the reader to understand if you refer to traceability instead since evaluation requires GC/MS analysis. You have two traceability paths to make value-assignments based on GC/MS analysis a) dynamic dilution of a 1 ppm gravimetric mixture, and b) static gravimetric dilution of the same 1 ppm gravimetric mixture.
Table 3 caption: I also find this caption confusion. Would it be correct to say:
“Table 3: Mole fraction assignments of ambient air reference gas mixture based on GC/MS analysis, calibrated using a Molbloc dilution system and a 1.01 ppm COS (1.00 ppm HCFC-22) reference standard.”
Supplement
Pg. 2, line 10: I am still having a hard time understanding the value assignments. Since both are based on the Molbloc dilution, shouldn’t they be the same? Would is suffice to say in this section, that mixing ratios based on the Molbloc (flow) were expected to have been stable, but the GC/MS measurements showed changes over time (figure S1).
Pg 7, line 11: You mention a COS-specific bias. The COS problem looks like it is related to stability (e.g. surfaces not fully equilibrated), rather than bias. It’s not like you get a stable COS result that differs from what is expected (=bias). You saw a time-dependent change for COS. I fully agree that the problem was probably related to adsorption/desorption of COS, and that your efforts improve the system by adding exhaust flow paths are significant.
Citation: https://doi.org/10.5194/amt-2024-16-RC2 -
RC3: 'Comment on amt-2024-16', Anonymous Referee #3, 20 Mar 2024
Comments on the manuscript “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” by Hideki Nara, Takuya Saito, Taku Umezawa, and Yasunori Tohjima, submitted to Atmospheric Measurement Techniques.
The authors present a very valuable technical solution to a long-standing problem of calibrating analytical instruments to measure carbonyl sulfide (COS) mole fractions at the parts per trillion (ppt) level in the atmosphere. Most analytical instruments to measure trace gas mole fractions are calibrated by means of a suite of reference gases in compressed air cylinders that span a range of target gas mole fractions including the range measured in the atmosphere. Such reference gases are typically designed to last several years or even decades of continuous analysis and represent an integral component of the achievable analytical continuity. Atmospheric COS mole fractions range between 400-600 ppt. At such low COS mole fractions, compressed gas mixtures including COS show that COS mole fractions are not stable with time, thereby breaking a fundamental requirement to a suitable calibration system. The authors present a technique that circumvents the problem of low mole fractions within compressed air cylinders by using a cylinder with COS mole fractions orders of magnitude higher than atmospheric levels, followed by consecutive dilution to atmospheric levels prior to analysis. This is a technically very challenging analysis. I congratulate the authors and would like to state that this work deserves a lot of credit for the achieved instrumental performance. Even though this general idea has been applied to other gases before, it is new to COS to my best knowledge and has great scientific merit. In my view, this will be a very valuable addition to the scientific literature that is well within the scope of AMT.
However, I’d suggest that the manuscript needs some major revisions. In my view, some clarification on major analytical aspects would be very helpful or even required. Also, the manuscript lacks a comparison with other atmospheric laboratories measuring COS or available scale gases to provide a linkage to the global observations as outlined in the introduction. This may be out of scope for this publication, but it should be discussed.
General comments
- As a person without personal experience with GC/MS but great interest in all things COS, I suggest adding a paragraph in the main text on how the GC/MS system is setup, how it is calibrated, and how it performs. It isn’t clear to me how the GC/MS system is calibrated for COS and HCFC. At some place the manuscript talks about a system blank. This is hard to understand without a clear description. The figures show delta values between Molebloc and GC/MS, but how GC/MS raw values are calibrated and then used for comparison isn’t clear to me. At several places, the main text talks about normalising data. If that is related to this, please explain in detail what is done, how it’s against what gas. Please explain in detail how you calibrate your GC/MS system, what are the COS/HCFC-22 ranges in your calibration gases and how stable is your system with time. Somewhere in the supplements you talk about the uncertainty of the analytical system on its own, please include that in the main text as well and expand on this.
- Maybe related to the previous point, please explain in more detail what the “reference gas mixture” in Figure 1 is used for.
- Commercial providers of gas mixtures do often not meet the strict requirements of the atmospheric community. Therefore, the atmospheric community developed an approach of Central Calibration Laboratories (CCLs). If I understand the manuscript and the described technique correctly, the scale realisation for COS and HCFC-22 enabled by this instrument is a propagation of the gravimetric scale provided by the gas provider. Mixing ratio calculations in the diluted gas depend on the values in the parental mixture. Given that there are other laboratories making such measurements since a long time and that provide reference gases to the atmospheric community, I wonder if the authors have thought about a comparison of their measurements with those within the atmospheric community? It is my understanding of the manuscript that the authors tested for accuracy by comparing their results with gravimetric dilutions of the very same parental tank used by the gas provider. I wonder if this comparison is showing whether or not the presented system and the gravimetric system of the gas provider achieve different results using the same gases. To some degree, this is a great local verification as the same gases are used, but it isn’t a global verification where laboratories have to use materials from different providers. The authors state that “absolute determination of the target gases in the parent gas mixture is required…” (P22-L23+24) and that “COS-specific bias is not clearly understood” (P24-L5). I would expect that those effects are a limitation on what can be stated on the achievable accuracy of this method. However, this is in strong contrast to the title statement this as “high accuracy… method”. Do the authors think this system is fit for purpose as it is, or is absolute calibration still required to fully assess the accuracy performance of this method? This is not clear to me, but I’d consider this a major point in need of clarification.
- Also, can the authors comment on the potential accuracy of the gravimetric standards, and the purity of the applied gases? The authors provide purity levels of the gases used, i.e., 99.99995% etc. Theoretically, this allows for 0.00005% of contaminating gases. Even if a very small fraction of the contaminating gases was COS, this method would have a severe bias. Can you elaborate on the composition of the contaminating gases, is there a certificate?
- Pressure regulators and cylinders are known to have huge effect on cylinders. Please provide exact details on those components, i.e., pressure regulator manufacturer, model, materials used in wetted surfaces, materials avoided in wetted surfaces. Same for cylinders, Molebloc and all other components. Really important for people attempting to implement your work.
- Generally, the wording could be shortened or simplified throughout the manuscript without reducing the level of information. As an example, the authors talk about the GC/MS system and the “sample determination system”. Is this not referring to the same system? If it was, can you consider referring to it as the “GC/MS system” consistently?
Specific comments
P2-L9: you mention that the system is validated for 10 years. Can you show quality control data over 10 years of how the system is performing? What variability do you see in COS on that time period? I know this is the abstract, but showing this in the main text would be great.
P2-L12+L13+20 and elsewhere in the text: you use statistical concepts of repeatability, reproducibility, standard error. Please explain how you calculate each (in the main text, not the abstract).
P2-L16: GC/MS in the main text and GCMS in the supplements. It would be easier if this was consistent.
P3-L3 delete the work “on”.
P3-L17: “that allow precise determination of atmospheric COS levels”. I understand that the goal of this manuscript is more about accuracy than precision. Maybe “that allow precise and accurate determination…”?
P3-L19+20: I am not sure if the statement of both sentences is warranted, and if the technique presented in this manuscript would provide a breakthrough to better understand COS globally. I’d suggest starting P3-L20 with “One fundamental…”.
P4-L14: “eliminates need for storage”. Your system still requires long term storage of the 1 ppm COS gas and this seems to be assumed here. Is there any proof or demonstration that a 1 ppm COS gas is stable in a cylinder over long time periods, i.e., the 10 years mentioned in the abstract? Can you elaborate on what is known and your thinking here?
P4-L22: “which we expect”… this sounds as if you are not fully convinced that the method is fit for purpose? Otherwise, could you say “which has the demonstrated capability to support….”?
P5-L9: “COS-free diluent gas”, can you provide an estimate of the COS blank? Have you tried to reduce the COS blank? (Nickel very effectively removes COS, a piece of 1/16” Ni tubing in the supply line might effectively remove any remaining COS?
P5-L20: change “consisted” to “consists”
P5-L25 and following lines: Provide all details possible on gases, purity levels, contaminants and their certified levels, cylinder manufacturers, cylinder valve type, wetted materials of cylinder, valve body, valve spindle, valve seat, burst disc, same for pressure regulators, Molebloc, static mixer, and all other components. Could be as table or text.
P6-L8: It seems that you and the gas provider use pure COS and pure HCFC-22 in pure N2. Is there any evidence that COS in an N2 matrix behaves the same as COS in air matrix as is the case for samples?
P6-L10-11: Why do you use single and dual stage regulators? Could the use of single stage pressure regulator cause the pressure variability and therefore COS variability? We found severe differences in levels of COS contamination between different pressure regulators, despite all being high-quality. Because this is so critical for the success of the method, I strongly suggest to provide further information.
P6-L19: “small amounts of COS…” because this is fundamental for the method, this needs a quantitative estimate. As asked above, what has been done to remove the blank, i.e., with Ni tubing?
P7-L10: provide details on wetted materials of Molebloc
P8-L4: spell out abbreviations in headers
P8-L8: “as the controller…” which one?
P8-L23: change “prevent” to “minimise”
P8-L25 (and everywhere else): spell out abbreviations in headers
P11-L15: Explain the use of all symbols, i.e., “δ”
P11-L19: Is the assumption of δ[x]parent = 0 valid? It seems critical for this method, please discuss the accuracy and consequence of this assumption.
P12-L5: RSD is mentioned in the figure for the first time and continuously throughout the main text (I believe the abbreviation is explained in the supplements but needs to be stated in main text). Please explain how you use the terminology and how it is calculated.
P13-L7: Please provide specs and precise details on mirror polished aluminium cylinders, manufacturer, valve, wetted materials, etc.
P14-L7: Please provide cylinder details, valve details etc…
P14-L15-17: Is this due to the amount of COS, i.e., passifying the cylinder, or is it due to the rate of COS change, that it can be neglected at ppm COS levels?
P15-L2: It would be interesting to plot the pressure within the cylinder over this time.
P15-L9: you use “dilutor” and “diluter”. Are these referring to different components or typo?
P15-L14: “until the measured values stabilised”, can you elaborate on this? How long does this take, how large is the effect with time, how many measurements does it need, is this constant?
P15-L16: you could change “The measurements of COS in the compressed air showed high repeatability, of which standard deviations (1σ) were within 0.5 pmol mol−1.” to “COS measurements in the compressed air showed high repeatability with standard deviations (1σ) of 0.5 pmol mol−1.”
P15-L17: I am not sure I understand this sentence: “The assigned COS values fell outside the 2-sigma range but were within 2%.” Can this be clarified, assigned by what method, outside 2 sigma from what?
P15-L20: I am not sure I understand the underlying experiment leading to this statement, also Table 1. What exactly has been measured, why is nominal COS at 5 ppm and assigned value around 270 ppt? I understand the parent gas has 1 ppm COS, where does the 5 ppm originate from, what is 270 ppt when the system is used to dilute to 500 ppt?
P16-L1: Change the sentence “These results imply that the impact of drift during storage is practically negligible when using a gas mixture containing COS at a micromole-per-mole level” to “These results imply that the impact of drift during storage is practically negligible when using a gas mixture containing COS at a micromole-per-mole level in this specific setup.” or similar.
P16-L4: “Periodic absolute determination…” I wonder how you do this to meet the compatibility levels required for atmospheric research. It would be great to provide a perspective or guidance, including on the frequency required to achieve this over a range of COS mole fractions.
P16-L7+8: Please explain how you calculate repeatability and reproducibility and what you learn from both parameters in your study.
P16-L11: Does this table suggest that you used your dilution system to produce dilutions with ~5 ppm COS and you measured air from different cylinders with different air fillings made in 2006, 2011 and 2015 and that you kept for COS measurements in 2023 and in those COS measurements you found COS values ranging from 270 to 275 ppt? Were the gases in cylinders stable over that period of time? Is the standard deviation in column 4 referring to the averages in column 3? What is n? What does the comment mean that COS was determined in 2023 if not what above? I am not sure I understand what the nominal mole fraction refers to, and even what this table exactly shows. Please clarify.
P17-L13: Spell out RSD
P17-L15: Here and everywhere else: are the 2nd digits significant and needed, or is 0.3 %, 0.6 %, 0.2 % and 0.4 % a sufficient approximation? I wonder because you refer to a typical GC/MS repeatability of 0.5 % in L18.
P17-L18-22: I don’t understand the discussion of the values, what we should learn from them and what the drawn conclusions are. Please clarify this paragraph.
P18-L4: Here and everywhere else: What does “normalize” mean in your system/data processing? Please clarify.
P20-L10: Can you provide a time over which the COS decrease of 1/13th occurred? If you use this as an argument to not base your accuracy test on COS but solely on HCFC-22, then this should be clearly stated after this sentence (see comment on P20-L26+27).
P20-L17: “, which measures the mass flow rates of the parent and diluent gases” has been mentioned before and can be deleted.
P20-L23: What points to the gravimetric dilution as the source for the offset? If a cause for the offset is decided upon, it should be explained. What is the uncertainty of the gravimetric system? What is typical and what uncertainties have been reported in the literature for other gravimetric systems?
P20-L26+27: Do I understand correctly that the argument here is that the dynamic dilution system is considered accurate for COS, because it works for HCFC-22? If that’s the case, please spell this out very clearly (see comment P20-L10). Is that a valid assumption? It sounds like there is a systematic bias as an offset in 2 ppt, but this is acceptable within the uncertainty of the gravimetric system (which needs to be demonstrated/referenced).
P21-L1: Are all gases really made from the very same parent gas mixture, or are all mixtures made using the same gravimetric system? Please ensure that the figure represents this accurately. If COS is not used in the accuracy test, maybe remove COS values from this figure to avoid confusion?
P21-L5: Does this table really need column 1 and 2, or could GC/MS, Molebloc and GC/MS – Molebloc be sufficient? How many digits are significant?
P22-L12: Explain all parameters and symbols in equation, what is Ř?
P22-L17-19: This sentence sounds like a conclusion before the values are presented and discussed.
P22-L21: What does “sufficiently accurate” mean? You determine 554.51 ppt, but what is the target value that this gas has and that you assess the accuracy with? How is that target value determined, gravimetrically?
P23-L1: Is the purpose is of table 3 to show results of 5 measurements and the average of those? If that’s the case, the table structure seems overly complicated to me.
P23-L3: Describe how you define/calculate the standard error.
P23-L15: “a greater bias was observed for COS than for HCFC-22.”
P23-L15-17: Isn’t the systematic COS increase referred to here (Fig S1-d, S2-a, S3-a) showing that significant absorption problems are apparent in this system that are not balancing and achieving equilibrium state, and therefore preclude from achieving high accuracy? And isn’t this just as likely a cause for the accuracy offset of 2 ppt, that has been assigned to the gravimetric dilution method in section “4.3 Validation of dilution accuracy”? Please discuss this in the text.
P24-L21+22: You state that “the precision of the dynamic dilution method is inferior to that of gravimetry”, yet the latter is suspected to have caused accuracy problems in “4.3 Validation of dilution accuracy”. Is this contradictory? What is the typical precision/accuracy or gravimetric methods, can you provide examples from literature?
P24-L25: What is automated and what is not in the presented system? Are you manually setting mass flow rates?
P24-L25: “This will expand the application of the dilution system to other analytical systems for different target gases and reduce labour and time required to conduct experiments.”
P26-L5: Excellent repeatability and reproducibility do not warrant accuracy.
PS2-L1-2: “…to eliminate blank COS accumulated in the GC/MS based sample determination system and was excluded from further analysis (described in…”
PS2-L11-13: Is this a circular argument?
PS2-L17: Clarify what “Arg min” stands for
PS3-L9+10: Where do the values 564 ppt and 293 ppt come from? Uncertainties important?
PS3-L14: Here and at other places, I am not familiar with the concept/terminology of “flow pressure”, but either flow or pressure. What does it mean here?
PS3-L17: These values are great, can the y-axes in Figure S1-e be changed to show relative flow changes?
PS4-L14: You could use “error bars” instead of “vertical bars”, as there are no horizontal error bars in this figure?
PS5-L18: “…no gradual increase with pressure was observed…”
PS-6-L19: Is this supposed to be “EX-S3” instead of “EX-S2”?
PS7-L4+5: “These results showed that the occurrence of the COS-specific bias was not proportional to the flow rate of the parent gas, but the magnitude of this systematic pattern was.”
Citation: https://doi.org/10.5194/amt-2024-16-RC3
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