the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Feb 2024
A Nitrate Ion Chemical Ionization Atmospheric Pressure interface Time-of-Flight Mass Spectrometer (NO3− ToFCIMS): calibration and sensitivity study
Abstract. Volatile organic compounds (VOCs) play a key role in tropospheric chemistry, giving rise to secondary products such as highly oxygenated organic molecules (HOMs) and secondary organic aerosols (SOA). HOMs, a group of low-volatility gas-phase products, are formed through the autoxidation process of peroxy radicals (RO2) originating from the oxidation of VOCs. The measurement of HOMs is made by a NO3¯ ToFCIMS instrument, which also detects other species like small highly oxygenated VOCs (e.g. dicarboxylic acids) and sulfuric acid (H2SO4). The instrument response to HOMs is typically estimated using H2SO4, as HOMs are neither commercially available nor easily synthesized in the laboratory. The resulting calibration factor is then applied to quantify all species detected using this technique. In this study, we explore the sensitivity of the instrument to commercially available small organic compounds, primarily dicarboxylic acids, given the limitations associated with producing known amounts of HOMs for calibration. We compare these single compound calibration factors to the one obtained for H2SO4 under identical operational conditions. The study found that the sensitivity of the NO3¯ ToFCIMS varies depending on the specific type of organic compound, illustrating how a single calibration factor derived from sulfuric acid is clearly inadequate for quantifying all detected species using this technique. The results highlighted substantial variability in the calibration factors for the tested organic compounds, with 4-nitrocatechol exhibiting the highest sensitivity, and pyruvic acid the lowest. The obtained sulfuric acid calibration factor agreed well with the previous values from the literature. In summary, this research emphasized the need to develop reliable and precise calibration methods for progressively oxygenated reaction products measure with NO3¯ CIMS, for example, HOMs.
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RC1: 'Comment on amt-2024-20', Anonymous Referee #1, 22 Mar 2024
Review of the article: “A Nitrate Ion Chemical Ionization Atmospheric Pressure interface Time-of-Flight Mass Spectrometer (NO3− ToFCIMS): calibration and sensitivity study” by Alage et al.
Summary and general comment:
Alage et al. present very interesting and important results obtained from laboratory experiments about the calibration of a fundamental instrument that have been extensively used in several studies. It is well known that VOC play a key role in atmospheric chemistry, forming highly oxygenated organic molecules and secondary organic aerosols. While the measurement of VOC is quite standardized the measurement of the HOM and especially their quantification is still very complicated and so far, there is not a proper method yet. As the authors say, at the moment, the HOMs estimation is made by a nitrate CIMS using H2SO4 as a reference. This is not ideal because these HOM are very different then sulphuric acid. Thankfully, in this study, the authors try to understand these issues by using commercially available small organic compounds, and they compare their signal with the one obtained for H2SO4.
With this study, the authors were able to prove that the sensitivity of the nitrate CIMS varies depending on the organic compound, showing once more that that a single calibration factor from sulfuric acid is not enough for the HOMs quantification.
Because of all these findings, and the needs for a calibration system for HOMs I think that this article is suitable for publication in AMT. Below I have added few minor comments.
Minor comments:
Introduction: This part is very well done with a detail explanation of the problem that the authors are investigating. However, I might suggest moving the second part (equation, table and text connected to it) in the method. So that the introduction goes directly to the point of the problem.
Approach 2: As far as I understood the nitrate CIMS is compared to the FTIR. Since the FTIR has high LOD the authors need to have high concentration in the chamber as they mention after line 220. I’m wondering if this value can be translated also at low concentration. Saying in other words how linear is the nitrate CIMS calibration over different concentrations.
Line 236: Maybe this sentence needs to be rephrased:” A calibration unit was used that was developed based on the work of Kürten et al. (2012).”
Figure 4. A bracket is missing in the Y-axe label. I would show the R2 just as a “normal” value (e.g. 0.992) and not in the exponential form. However, it’s up to the author if they want to change that. X-axe label Pyruvic “acid”, add the word acid. Also, in the caption.
Figure 5 caption: Add the word “acid” after the word “oxalic”. This has been missing in several part of the paper. I would Pyruvic and Oxalic not followed by the word “acid” don’t mean much.
Conclusions: This part is great but I would shorten it and make it more concise so that the message goes streight to the point. Here again is up to the authors to take up on this comment or not.
Citation: https://doi.org/10.5194/amt-2024-20-RC1 -
RC2: 'Comment on amt-2024-20', Anonymous Referee #2, 15 Apr 2024
Comments on “A Nitrate Ion Chemical Ionization Atmospheric Pressure interface Time-of-Flight Mass Spectrometer (NO3− ToFCIMS): calibration and sensitivity study” by Alage et al.
HOMs are important precursors for secondary particles in the atmosphere and subject extensive studies in the past decade. Among various detection techniques, nitrate CIMS appears to be the most widely used instrument owing to its capacity of detecting the least volatile group of compounds in HOMs. An accurate measurement of HOM concentration is required for quantifying the potential contribution of HOMs to SOA or NPF, which however, have not been achieved due to the lack of an effective calibration method. In views of this, this study shows advances in the calibration of nitrate CIMS, which makes it certainly important in this research field. Thus, I would recommend it acceptance in AMT after my comments are addressed.
General comments
- This manuscript have stressed, in a few places, that this study highlights that HOM sensitivity could vary by orders of magnitude. This is of course true, but not new. Trostl et al., (2016) and Hyttinen et al., (2015) both clearly clarified this issue. Similarly, it is well known that the calibration coefficient of sulfuric acid cannot be directly apply to HOM quantification, which merely gives an lower limit of HOM concentration. Therefore, I think this paper contributes to briefly quantify the possible magnitude of such underestimation, rather than show this fact. I hope some relevant statements can be revised accordingly.
Specific comments:
- I think the title is somewhat misleading or overstated. This study tested several commercially available calibrants, which however, are not enough to say it is a systematic or full calibration study of nitrate CIMS.
- In the calibration of any compound, it is essential to show the linear regression between CIMS signals and injected concentrations (just as in Fig.4). I wonder why it is not the case for Fig.5-9.
- In the Approach 1 calibration, do the authors consider the wall loss of these organic compounds, which will directly affect the calibration coefficient. For the significantly higher sulfuric acid calibration coefficient in setup 3 (Table 7), additional loss could rise from particle nucleation of sulfuric acid (that consumes sulfuric acid).
- Line 430. It should be the lower limit rather than the upper limit.
- I found a few places where the expression of terms appear to be inconsistent with the literature or this manuscript itself:
e.g., Line 35 “… in the formation and growth of secondary organic aerosols (SOA)”. SOA is usually used in a single form (secondary organic aerosol); “growth” is preserved for particle growth in size (a later stage of new particle formation).
Line 40-45 ELVOC stands for “extremely low-volatility organic compounds” instead of “extremely low volatile organic compounds”. LVOC and ULVOC are likewise.
Line 60 and 65. NO3- ToFCIMS or nitrate ToFCIMS, please pick one and keep consistent.
Line 81 “can be sued to” should be “can be used to”?
Line 129 “a 1-min intervals”, where “a” should be removed.
- J. Trostl et al., The role of low-volatility organic compounds in initial particle growth in the atmosphere. Nature 533, 527-531 (2016).
- N. Hyttinen et al., Modeling the Charging of Highly Oxidized Cyclohexene Ozonolysis Products Using Nitrate-Based Chemical Ionization. J Phys Chem A 119, 6339-6345 (2015).
Citation: https://doi.org/10.5194/amt-2024-20-RC2
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