General:

The manuscript "Development, Characterization and Rapid Diagnostics of an Aircraft

Aerosol Mass Spectrometer Inlet System" by Kim et al. is a very detailed study on the focussing properties and the transmission efficiency of aerodynamic lenses used for the Aerodyne AMS. Additionally, it describes the aircraft inlet system required to maintain a constant pressure in the aerodynamic lens (pressure-controlled inlet, PCI) and the transmission efficieny of the combination of CPI and aerodynamic lens. An automated procedure to scan the lens position and thereby the particle beam over the vaporizer was designed and is presented.

The title therefore is not fully adequate, to my opinion it should also reflect the findings on the lens focussing properties, the particle beam width, and the transmission properties of the lens types. Also the consequences for the analyis (especially ion ratios for organic nitrates) are important.

Thus, the paper is more on "Characterization of focussing and transmission efficiencies of aerodynamic lenses and aircraft inlet systems used for an Aerosol Mass Spectrometer, and development of a rapid diagnostics tool"

I suggest you refine the title along these lines.

As the study is very detailed, the paper is very long, such that it took me much longer than expected to read it. Overall, I find it very good and have only minor comments, which are listed below. 

It fits very well into the scope of Aerosol Research and will find many interested readers.

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General remark:

Given the importance of the correct positioning and alignment of the lens, would it be useful to design a better fixing mechanism such that the lens is adjusted once and then remains in its optimal position?

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Specific remarks:

line 46-48: Sentence not correct, maybe skip "that" in line 46?

line 54: Maybe add reference to DeCarlo et al 2004/5 for d_va? That reference also explains how to convert d_mob into d_va.

line 75 "the SV"? 

line 170: 0.05 M ?

line 175: Fig S7.1 shows the size distributions. From the text I had expeced to see the nebulizer itself.

Line 191: In recent years, it was recommended (at AMS user meetings) to use the mass based IE calibration. Did you do that and did you compare it to the ET mode IE calibration results?

Line 219: Should read Table S9.3. What do you mean by "air signal delay"? The "PToF"-signal of the m/z 28?

Line 239, Equ. 6: The unit conversion factor is only needed if V_chem is given in µg sm-3, right? The equation would be correct w/o the conversion factor if you replace all densities by "rho_species" (as you did for "rho_OA"). The values for rho can be explained in the following sentence, as you did for seasalt. Where does the value of 1.52 g cm-3 for for Cl come from? 

Line 243-245: You did not take into account rBC, because no mesurement was available. But was a measurement for seasalt available? 

Line 357: better: 800 hPa (SI unit)

Line 370 ff/Fig.5/Table 1: what is the inner diameter of the EV-D_up and EV-D_down? 

Line 398: is "informed" the right word here? Does it mean "based on"? Or "The cone angle and dimensions... were chosen based on ..."?

Line 429: HIMIL has already been introduced earlier.

Line 446: -> "are reported"

Line 627: I thought m/z 57 is for HOA; not m/z 55?

line 646: May it also be that no particles larger than 1.5 µm d_va were procuded by the particle generator?

line 732: What is "ruby orifice"?

FIg 12b: Above 150 nm, model and measurement agree perfectly? That's too good to be believed.

lines 835-843: Knote et al. (2011) do not mention explicitely a d_50, the use a linear slope for the transmission between 0 and 100% vs log d_va from 40 to 100 nm (and then the same for 550 - 2000). Please explain this, otherwise it's hard for the reader to find it in the Knote paper. Furthermore, in line 842 I would not say "recommended", I would say "used by". 

Fig 14, line 884: Molleker et al. (2020) used the ERICA mass spectrometer with an Aerodyne PM2.5 lens, not the ALABAMA which has the custom designed lens as described in Clemen et al., 2020. Pleace check and correct.

line 861: dva,50,high ~ 120 nm, -> should read dva,50,low 

line 909-910: "Due to unknown reasons, PCI-D required EVup downstream of COup to enhance the transmission efficiency of large particles." I don't understand the meansing of this sentence. I understood that was the reason for adding the expansion volume EV.

 

Fig 15 c), d): legend: please correct Mollecker -> Molleker

Review of "Development, Characterization and Rapid Diagnostics of an Aircraft Aerosol Mass Spectrometer Inlet System" by Kim et al.

This paper describes a new pressure-controlled inlet, designed for use with the Aerodyne AMS instrument, that allows both for operation across a range of tropospheric to lower stratospheric pressures and across a wide range of particles assizes as well as a computer-controlled alignments system. With numerous airborne AMS instruments, the community needs a highly capable inlet of this sort.

Unfortunately, this paper is not publishable in its current form.

The paper is far too long. The authors’ stated goal in the title and abstract is to characterize and describe a new inlet system and include performance from a set of flights; reading the paper, it is evident this could be accomplished by a paper cut by a factor of 5 or more in length. The main paper is 44 pages before the references with 17 figures. The supplement has a further 20 figures, many highly repetitive, and is another 58 pages of length. I will state this for clarity - there are currently 102 pages of manuscript with 37 figures to describe only a new inlet design. 

I recommend rejection. The authors should cut this paper to be both impactful and readable; this journal is not meant to be a vast repository of all information they collected during their development process. If the focus, as stated, is the design and development of the inlet then please maintain that but eliminate all the extraneous information and repeated figures. I suspect their objectives could be met with a paper length of something like 15 pages and 5 or 6 figures. At that time a normal review cycle could take place.