the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Aug 2025
Laboratory and Field Characterization of an Atmospheric Pressure Transverse Chemical Ionization Ion-Molecule reaction Region
Abstract. We introduce a custom-built, field-deployable, atmospheric pressure Ion-Molecule reaction Region (IMR) for use with Chemical Ionization Mass Spectrometry (CIMS), the so-called "t-IMR". The design is described in quantitative detail and shows significant improvements in potential measurement interference compared to other IMR configurations, particularly those operating at low pressure. The relatively large laminar flow and inner chamber diameter reduces the probability of sampled air and ion clusters interacting with the Teflon surfaces of the IMR before being detected by Time-of-Flight (ToF) mass spectrometry. This also leads to a substantial reduction in wall effects and artificial background signals for even low volatility organic products, exhibited by alpha-pinene ozonolysis. An electric field is induced perpendicular to flow in the t-IMR to accelerate ions and consequent charged sample clusters to the MS interface. The strength of this field is modulated and optimized to simultaneously maximize total ion flux and instrument sensitivity. A sheath flow apparatus is introduced to provide small N2 flows counter to ion and sample cluster flow into the MS to reduce the likelihood of particulate buildup and clogs to the pinhole separating the IMR from the MS, ensuring uninterrupted sampling for extended periods of time. Finally, we demonstrate the capability of the t-IMR to be deployed to the field to measure down to sub-ppt level ambient concentrations of important trace gases including reactive bromine at a ground-based site in the marine boundary layer. We find that the t-IMR design considerably reduces artificial signals from surface contact and wall effects, and improves detection of very low concentration species in the ambient atmosphere. Future studies are recommended to evaluate the extent to which humidity effects instrument sensitivity to key compounds for the atmospheric pressure t-IMR setup.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-3709', Anonymous Referee #1, 29 Aug 2025
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RC2: 'Comment on egusphere-2025-3709', Anonymous Referee #2, 10 Sep 2025
This manuscript introduces and characterizes a novel atmospheric-pressure transverse Ion-Molecule Reaction Region (t-IMR) for iodide-CIMS. The inlet employs laminar sample flow, a transverse electric field, a VUV-based iodide ion source, and a sheath counter-flow to reduce wall effects and turbulent mixing. Laboratory tests demonstrate significantly reduced wall-memory artifacts and improved sensitivity compared to low-pressure or earlier transverse IMR designs. Field deployment during the BLEACH campaign in Bermuda shows that the instrument can reliably measure sub-ppt levels of HOBr, HNO3, and other trace gases.
Although atmospheric-pressure IMRs, transverse geometries, and VUV ion sources are not new, but their integration into a field-deployable configuration and systematic performance evaluation is valuable. The laboratory experiments are solid, and the field deployment provides convincing proof-of-concept, despite some limitations (e.g., HNO3 contamination, challenges in background determinations). The manuscript is generally clear and well supported by figures. I recommend minor revision before acceptance for publication.
Major Comments
- Since the t-IMR represents an improvement over traditional IMR designs, it would be valuable to provide additional quantitative data. For example, the authors could briefly summarize the limits of detection, sensitivity, and wall effects for representative species under conventional configurations. This would allow readers to more directly appreciate the extent of the improvements achieved with the new IMR.
- The authors acknowledge that one limitation of the t-IMR is the inability to control humidity. This raises the question of how the reported concentration data were calculated during field observations. Was it assumed that humidity has no effect on sensitivity? Given that specific concentration values are presented and that these species are known to be influenced by humidity in traditional IMRs, a detailed description of how humidity effects on sensitivity were accounted for is essential. Furthermore, was any on-site calibration performed during the observation period? If so, these data could be used to further analyze and discuss the role of humidity. Since the treatment of humidity directly determines whether the instrument is truly field-deployable, and given that the authors have already applied the t-IMR in field studies, it would be highly valuable to provide recommendations regarding the handling of humidity in such applications.
- Overly descriptive text – Several sections (e.g., Sect. 2, description of dimensions and housing) read like a technical manual. While detail is important for reproducibility, the writing could be more concise and structured (perhaps moving some details to the Appendix).
- Axial vs. transverse geometry (discussion point) - Many atmospheric-pressure CIMS instruments (e.g., HOx-CIMS) employ an axial geometry in which the sampled flow is directed straight into the pinhole. By contrast, this study adopts transverse geometry. It would be useful for the authors to briefly comment on the rationale behind this choice and how it may compare in terms of sensitivity, turbulence, or wall effects. This would help contextualize the work for readers familiar with axial designs.
Minor Comments
L216: I understand that it may be challenging to evaluate the dependence of sensitivity on humidity in the laboratory given the high sampling flow rate. However, was the potential variation of sensitivity under different humidity conditions examined during field calibrations? In traditional IMRs, for example, Br2 is known to be significantly affected by humidity within certain ranges. How was the influence of humidity accounted for during the BLEACH observations?
L306: The authors should report explicit limits of detection. Although the calculation formula may be cited, the authors should provide a worked example using Br2 that lists all parameter values used in the computation. Because the LOD is a key criterion for assessing whether the t-IMR outperforms traditional IMRs, presenting the full set of inputs and the resulting LOD for Br2 is essential for a transparent and fair comparison.
L328: The reported Br2 sensitivity of 1.5 counts ppt-1 per 1e6 TRIC is substantially lower than the value shown in Figure 2. Please clarify the source of this discrepancy. If the value of 1.5 counts ppt-1 per 1e6 TRIC is correct, it appears lower than that of traditional IMRs. Please discuss whether the t-IMR design prioritizes a lower LOD at the expense of sensitivity, and explain the underlying factors that could account for this trade-off.L341: The specific sensitivity of HNO3 should be reported. If the background concentration of HNO3 cannot be subtracted, it is unclear why this compound was nevertheless selected for demonstration. I suggest that the authors briefly explain the rationale for choosing HOBr and HNO3 as representative species in their analysis.
Other Comments
The authors are advised to carefully check the manuscript for grammatical issues as well as the correct formatting of superscripts and subscripts. Below, I list only the instances I have identified.
L3: “shows significant improvements in potential measurement interference” it is awkward to say improvements in interference. Mitigation or reduction sounds better.
L4: “reduce”, without “s”.
L7:“… exhibited by alpha-pinene ozonolysis” sounds ungrammatical. “…as demonstrated in α-pinene ozonolysis experiments” sounds better.
L16: “affects” instead of “effects”.
L38:” However, by reducing the pressure of the IMR dilutes sample molecular number concentration by up to several orders of magnitude.” Delete “by”.
L41: Change ”For already low concentration specie” to “For species present at very low concentrations”.
L82: “time series” instead of “timeseries”, in here and the rest of the manuscript.
L159: “its” instead of “it’s”
L200: The “2” in N2 should be formatted as a subscript (N2).
L231: “their” instead of “them”
L238: “factor of 2” instead of “factor 2”.
L254 “from” instead of “off of”.
L293: “… located on the southwestern shores of Bermuda operated by the Bermuda Institute of Ocean Sciences (BIOS).” lack of conjunction “and”.
Citation: https://doi.org/10.5194/egusphere-2025-3709-RC2
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see attached review