Report to the manuscript "Multiphysical description of atmospheric pressure interface chemical ionisation in MION2 and Eisele type inlets”

This manuscript presents multiphysics simulations of two atmospheric pressure chemical ionization sources frequently used in analyzing atmospheric samples with a mass spectrometer. The authors validate the theoretical results with current measurements at specified electrodes and also compare them with general observations having made while working with these sources. In general, this manuscript provides interesting and valuable insights into the physical and chemical working principles of these devices. Hands-on visualizations of parameters impacting the operation are given.  

In accordance with AMTs referee guideline, following aspects are addressed:

1. Does the paper address relevant scientific questions within the scope of AMT?

--- To users of these sources the presented simulations might be relevant, also in view of further improvements.    

1. Does the paper present novel concepts, ideas, tools, or data?

--- According to the authors it is the first time that a multiphysics simulation tool has been applied to describe and visualize the physical and chemical principles of these devices.

1. Are substantial conclusions reached?

--- In my opinion, no further substantial conclusions are reached despite the insightful visualizations.

1. Are the scientific methods and assumptions valid and clearly outlined?

--- In principal yes, specific points are discussed further below.

1. Are the results sufficient to support the interpretations and conclusions?

--- In principal yes, specific points are addressed further below.

1. Is the description of experiments and calculations sufficiently complete and precise to allow their reproduction by fellow scientists (traceability of results)?

--- Yes.

1. Do the authors give proper credit to related work and clearly indicate their own new/original contribution?

--- Yes.

1. Does the title clearly reflect the contents of the paper?

--- Yes.

1. Does the abstract provide a concise and complete summary?

--- Yes.

1. Is the overall presentation well-structured and clear?

--- Appropriate.

1. Is the language fluent and precise?

--- Appropriate, specific points are addressed further below.

1. Are mathematical formulae, symbols, abbreviations, and units correctly defined and used?

--- Yes.

1. Should any parts of the paper (text, formulae, figures, tables) be clarified, reduced, combined, or eliminated?

--- Generally okay, specific clarification is addressed further below.

1. Are the number and quality of references appropriate?

--- Yes.

1. Is the amount and quality of supplementary material appropriate?

--- Not applicable.

In compliance with the AMT referee guideline I do recommend publishing this article, however, with some minor changes/additions and requested comments presented in the following:

(i) lines 147 and 148: Please comment on the measured ion currents in the order of 10^-11 A. The used Tenma (72-2595) can only measure in the µA range according to its specification.  

(ii) line 60: “…by 40…” please revise

(iii) line 76: “a narrow…(A)…” please revise

(iv) fig 4d and e: U_A = -3000 V instead of U_A = 3000 V

(v) line 202: “The pinhole current is measured at a higher voltage than predicted.” Please clarify.

(vi) line 204: “This could lead to a softening of the voltage sensitivity.” Please clarify.

(vii) Please clarify the argumentation for the significantly differing slope of the measured pinhole current in contrast to the simulated ion concentration in figure 5.

(viii) line 288: “cm^-3” instead of “cm^-1”

This paper compares two atmospheric pressure chemical ionization geometries using a detailed model simulations. Results from ion current measurements are presented and compared to simulations of the two different geometries. The overall goal of this manuscript is interesting and the results would be useful to the community. However I find the paper inconsistent or confusing at the least.

From my understanding, the advective velocities used in the model (Figures 1 and 7) are too large for the typical operation of either case of the CIMS. This is especially true for the Eisele design. For the MION described in Wang et al. [2021] with a 22 mm flow tube, a velocity of 2 m s^-1 equates to a total flow of ~45 slpm which is on the higher end of the 20-30 slpm typically described (Wang et al. [2021] used 32 slpm for example). For the Eisele design using the 44 mm diameter flow tube described in the model, 1.7 m s^-1 equates to a total flow of ~160 slpm. Well above the 20-45 slpm typically described (Tanner and Eisele [1995] and Sipila et al. [2018] used 45 and 30 slpm respectively). Later however, in Table 1, values more consistent with previously published operating parameters are presented for both systems.

The authors state that a reaction time of 113 ms is assumed for the Eisele configuration. Using a flow of 30 slpm as stated in Table 1 and the 44 mm diameter, a flow velocity of ~30 cm s^-1 is obtained, yielding a reaction distance of only 3-4 cm? The physical length of the actual IMR cylinder is ~15 cm. The 113 ms reaction time IS greater than the calculated ~88 ms geometric reaction.  

This inconsistency/confusion furthered in the topic of turbulence. The authors state that a Reynolds number, R_e, of ~1600 was assumed. Using 45 slpm in a 22 mm dia. flow tube, as in the MION model, yields a R_e of ~2800, well into the transition zone towards turbulent flow. The 160 slpm flow in a 44 mm flow tube, as in the Eisele model, yields a R_e >4000, well into the turbulent flow regime. If, however, the flows from Table 1 are used laminar conditions are maintained.

Overall, as a person familiar with both systems, I find this work inconsistant and/or confusing and hard to follow. The manuscript should be rewritten with an eye towards addressing these inconsistencies or making things more understandable. The work is interesting and will provide insight for users of either technique. The visualizations are quite useful. However, it needs more work before I would recommend publication.