Real-time measurements of gas-phase organic acids using SF<sub>6</sub><sup>−</sup> chemical ionization mass spectrometry

Nah, Theodora; Ji, Yi; Tanner, David J.; Guo, Hongyu; Sullivan, Amy P.; Ng, Nga Lee; Weber, Rodney J.; Huey, L. Gregory

doi:https://doi.org/10.5194/amt-11-5087-2018

Articles | Volume 11, issue 9

https://doi.org/10.5194/amt-11-5087-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/amt-11-5087-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 11, issue 9

Research article

|

07 Sep 2018

Research article |

| 07 Sep 2018

Real-time measurements of gas-phase organic acids using SF₆⁻ chemical ionization mass spectrometry

Theodora Nah, Yi Ji, David J. Tanner, Hongyu Guo, Amy P. Sullivan, Nga Lee Ng, Rodney J. Weber, and L. Gregory Huey

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Final revised paper (published on 07 Sep 2018)
Supplement to the final revised paper
Preprint (discussion started on 19 Feb 2018)
Supplement to the preprint

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Review of Nah et al., Organic Acid CIMS', Anonymous Referee #1, 15 Mar 2018
- AC1: 'Reply to referees', Theodora Nah, 24 May 2018
RC2: 'Review of Nah et al', Anonymous Referee #2, 16 Mar 2018
- AC2: 'Reply to referees', Theodora Nah, 24 May 2018

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Theodora Nah on behalf of the Authors (24 May 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (04 Jun 2018) by Bin Yuan

RR by Anonymous Referee #1 (19 Jun 2018)

Suggestions for revision or reasons for rejection

The revised manuscript "Real-time measurements of gas-phase organic acids using SF6- chemical ionization mass spectrometry" by Nah et al has many improvements. In particular, there are many technical corrections to the instrument description and explanations of the numbers displayed in figures. However, the paper still devotes too much attention to interpreting speculative peak assignments. This new measurement technique appears to be useful, unique and promising, but these advantages are not sufficiently discussed in the manuscript. Without further comparison with other ion chemistries, it is difficult to know if SF6- should be used. The paper will have a much greater impact if the advantages of SF6- are highlighted.

Although several caveats have been introduced noting that the peak assignments are speculative, the same interpretations follow. The speculation is ok, but there should be no further quantification or interpretation based upon that. Without further justification, the organic acids should not be identified as a single compound and their abundance should not be interpreted. Instead, please show them as masses, or as a sum of compounds. The figures may be very misleading by showing individual compounds as mixing ratios. If molecule names are used, then some simple tests ought to be performed to inform the identification of the masses. Can an HR-TOF be used with SF6- in a similar environment to see which compounds dominate? That would be far more powerful than the comparison with WSOCg. Comparison to aerosol composition (section 3.2.3) is not a compelling reason to choose the peak assignments. As is stated in the manuscript, the low volatility compounds measured in the particle phase are expected to have low gas phase concentrations. This could be used to argue that oxalic, malonic... that are present in the particle phase do not explain the signals measured in the CIMS. The IC aerosol measurements are never discussed quantitatively: Do the gas phase concentrations estimated here make sense relative to the particle concentrations?

It would be much more useful to further justify the technique, which may have some advantages over iodide ion chemistry. For example, the sensitivity to propionic acid is much better than iodide. Is the same true for other acids that iodide does not detect well, like acrylic acid? Iodide often suffers from a large lactic acid interference that compromises oxalic acid quantification. What is the sensitivity to lactic acid? Since the motivation was to quantify organic acids, further discussion of the advantages (possibly unique) of SF6 for quantifying organic acids would be particularly valuable.

I don't understand the normalization to F2SO2. F2SO2 has a water dependence that the organic acids do not, so the normalization is not appropriate, even if the water dependence is later adjusted (which I didn't understand). Why not normalize to the reagent ion signal, as is common in most other CIMS papers? This would make the work more easily compared to other CIMS techniques, and avoid correcting and then uncorrecting the signals by using F2SO2 normalization.

With the removal of speculation in sections 3.24 and 3.3, there is room to emphasize some of the measurement technique accomplishments. The reagent ion signal is never mentioned in the text, though it is stated in the authors comments. Achieving a signal of 9 MHz is extraordinary given the low activity of the ionizer. Few CIMS do any better than this, so lines 352-354 are not correct in general. The paper should detail how the large reagent ion signal was achieved, as this seems to be a major new advance. Likewise, the high sensitivity to acetic acid and possible insensitivity to lactic acid are important. The former is discussed, but it would be more powerful to have a table that compares the sensitivities here for all the acids to other CIMS techniques. The 1/e2 time decays are given, but the time response for most of the system is very fast. It looks like 1/e is <13s. Where does the time response come from? It is slower than many other CIMS measurements, even though there is a very large flow through the flow tube, and the inlet appears to be only 25 cm and with a large flow. Do the high dilution or reduced pressure affect time response?

throughout the paper, more quantitation is necessary. for example:
line 242: what were the impurities in glyoxylic?
line 244: what are the vapor pressures? would heating generate sufficient vapor pressure?
line 353: how much higher can the sensitivity and Po210 activity go?
line 393: replace "noticable increase" with "xx% increase"
line 406: how was water vapor dependence accounted for?
section 3.2.1: ratios to CO may be helpful, in case transported air explains increase
line 535: I don't understand comparison of organic ion signal to F2SO2 sensitivity. the ratio here isn't unitless.
line 559: what does zeroth order check mean?
line 591: How long are these spikes? How close is the nearest power plant or urban area?
line 663: what is a reasonable detection limit? give a number

smaller points:
Please define all acronyms at first use. I could not find definitions for m/z, slpm

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ED: Reconsider after major revisions (26 Jun 2018) by Bin Yuan

AR by Theodora Nah on behalf of the Authors (04 Aug 2018) Author's response Manuscript

ED: Publish subject to minor revisions (review by editor) (12 Aug 2018) by Bin Yuan

AR by Theodora Nah on behalf of the Authors (15 Aug 2018) Author's response Manuscript

ED: Publish as is (24 Aug 2018) by Bin Yuan

AR by Theodora Nah on behalf of the Authors (24 Aug 2018) Author's response Manuscript

Short summary

The sources and atmospheric chemistry of gas-phase organic acids are currently poorly understood, due in part to the limited range of measurement techniques available. We evaluated the use of SF₆⁻ as a sensitive and selective chemical ionization reagent ion for real-time measurements of gas-phase organic acids at a rural site in Yorkville, Georgia. We found that ambient concentrations of organic acids ranged from a few ppt to several ppb, and are dependent on ambient temperature.

Real-time measurements of gas-phase organic acids using SF6− chemical ionization mass spectrometry

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Real-time measurements of gas-phase organic acids using SF₆⁻ chemical ionization mass spectrometry