This is a short review as all aspects of the paper were not evaluated: it is too long, thus the Fair rating on Presentation.  I gave it Fair on Scientific quality for reasons detailed below.  I gave it Good for Significance as the loss of sticky species on inlets is important to get correct.  I think with a rewrite and a careful paring down of the text along with addressing the science quality issues below, it could be a pretty good report. 

1) Wall loss is a tricky thing to model.  How is it incorporated in Fluent?  OF course no loss (set species to have no flux at the surface) and diffusion limited loss (set species to have zero concentration at the surface) are conceptually easy to understand and to implement in Fluent.  How does one address mass acc. coefficients other than 0 and 1 in Fluent? 

2) Related to that issue,  the mass acc. results do not make sense in the laminar realm.  At 298 K and using 98 for molar mass:  (i) At 1 atm (or 0.85 atm? as in Colorado Springs), the diffusion limited loss rate in a 1cm ID tube is about 1.5 s-1.  (ii) The kinetic limit (for a mass acc. = 1) is about 2.3x10^4 s-1.  What this means is that mass acc. values greater than about 0.001 are at the diffusion limit and there should be very little to no dependence on mass acc. for the throughput of the sampling tube. 

3) Another related issue is the diffusion coefficient.  It looks like the authors are using the pressure independent value for all pressures: it actually has units of atm.cm2/s.  Diffusivity at altitude will be some 6 or 7 times that at sea level due to the pressure change.  Yet it is colder so apply a typical T^1.75 factor.  What temperature is the sampling tube at altitude? 

General Comments 

Overall a good paper but some sections could be decreased as there is a long lead up to the main results on the gas transmission efficiency of the inlet. Saying that, I would encourage the authors to expand the methods section to include more information on the operation and setup of the CIMS as these measurements are fundamental to the paper. Furthermore, it is not clear to me how widely applicable the results are to other aircrafts/instruments based on the experimental conditions and assumptions used throughout the study. For example, some CIMS instruments would sub-sample a smaller flow from the sample line or some sample lines may experience a temperature gradient due to differences between the ambient and cabin temperature. It would be useful to clarify the broader applicability of the findings.  

Specific Comments

Introduction paragraph one – some statements are repeated multiple times and disrupts the flow of the paragraph (e.g. importance of condensable vapours for aerosol growth and hence health). This could be rewritten so that it is clearer.

Line 43 – The sentence on the relevance of trace gases currently reads as this is an exhaustive list. Should be made clear that these are examples.

Line 45 – Understanding the formation and growth of short-lived reactive gases. Suggest change word growth, this feels more appropriate to describe aerosols.

Line 110 – What is the material of the sampling tube?

Line 114 – What is the range of flow rates sampled by the CIMS?

Fig1d – What does the dashed line represent?

Line 135-136 – What concentrations are used for each of the reagents? And what is the resulting concentration of H2SO4?

Line 174 – Later on in the paper you mention the different humidity conditions in the wind tunnel across the experiment period. Is it correct that H2SO4 is diffusing in dry air?

Line 176 - In addition, as the temperature gradient in the transmission line is insignificant, we neglect thermal diffusion loss. Does this remain true for ambient sampling where there can be larges differences between the cabin and ambient temperatures?

Line 229 - These ion concentrations were recorded under different operating conditions by CIMS. Different inlet or CIMS operating conditions? If CIMS what are these different conditions and what is the rationale for this?

Line 314 – Can you include a schematic of the NO3 CIMS in the methods that highlights the IMR region you are describing here.

Line 315 – I would be explicit here that the lower signal response at 16 SLM is specific to the instrument used in this study and you cannot be certain that this holds true for other CIMS instruments that are operated under different conditions.

Fig 6 – it would be helpful to the reader to define Q in the caption as this is defined later in the paper.

Technical Comments

Line 17 – remove using

Line 40 – composition-dependent, . remove comma

Line 56 – replace aboard with onboard

Line 109 – replace aboard with onboard

Line 161 - sampling tube designs use the commercial code – needs rewording

Line 174 – replace refer with referred  

Line 237 - (𝐻2𝑆𝑂4 𝑁𝐶𝑃𝑆,) – remove comma

Line 338 – Hanson et all., remove et al as Hanson only author

Fig5 caption – description of chapter 2.4. Change to section 2.4

Line 397 - This is due to sample flow is more turbulent. Typo