see attached review

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

1. 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.
2. 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.
3. 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).
4. 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”.