This is an interesting study on a novel variation of one of the most prominent experimental techniques in atmospheric chemistry, especially aerosol-relevant chemistry. The study is written so well, that even a modeller like myself could follow the discussion, and appreciate the numerous small tweaks the authors have made to improve their setup. I can thus recommend publication essentially as is. I have some very minor comments and questions that the authors can address at their discretion, these are described below. 

-line 230, “reagent ions and analyte ions that are very weakly bound to the reagent ions (e.g., water cluster with a binding energy of 42 kcal/mol)”. 42 kcal/mol is not a weak binding energy - even the most strongly bound ELVOC*NO3- clusters are typically bound by less than this. And certainly iodine-formic acids (mentioned as an example of moderate binding a few lines below) is bound by less than this. Perhaps the 42 kcal/mol value refers to the total binding energy of multiple water molecules? Or is there a unit conversion error somewhere? Please explain/elaborate. 

-line 385: “(XX%)”, please update the actual number here (or explain the notation) - this looks like something that a co-author forgot to fill in during finalising of the manuscript. 

-Line 448: something is grammatically wrong with the formulation of this sentence: “the formation of”… “can react”. Maybe reformulate to “The RO2 radicals generated”… “can further react”?

-Line 511: “Differences in the contribution of these compound groups with previous work could be due to different sensitivities of the instruments…”. While certainly true, isn’t it also the case that especially the formation of ELVOC and ULVOC compounds (by a combination of autoxidation and dimer formation) is very sensitive to the experimental conditions (concentrations, residence times, NOx levels, OH scavenger, temperature etc). So probably some of the differences between the present results and earlier studies are just because of subtle differences in the actual ozonolysis experiment, rather than differences between the instruments? Additionally, maybe explain a bit more in detail what specific “differences in contributions” are meant here?

-in figure S4 (in the supplementary), it is presumably values of the subscript x (indicating the number of O atoms in the three composition families) that are varied from 1 to 4. It took me a while to understand what the “O=1” etc texts in the figure meant. Please use “x=1” and so on instead, to be consistent with your own notation in the legend. 

In this manuscript, Riva et al. introduced a newly developed medium-pressure chemical ionization reactor, Vocus AIM, utilizing adduct ionization for the detection of gaseous inorganic and organic compounds. The performance of the Vocus AIM reactor in terms of time response, sensitivity, and selectivity under different ionization schemes was characterized using selected inorganic and organic species, as well as experiments of alpha-pinene ozonolysis. The novel design of Vocus AIM significantly improves the time response of instrument, enables sensitive detection of a range of inorganic and organic species in sub-ppt level, and effectively eliminates the water vapor dependencies of the sensitivity by introducing an appropriate dopant. In addition, by combining multiple ionization schemes with different selectivity, the Vocus AIM is able to measure organic compounds spanning a wide range of volatility and oxygenation level. This work is scientifically sound and the manuscript is nicely written. I recommend its publication in AMT after the following comments are addressed.

L186-188: I would expect that the reagent ion current is affected by the mixing ratio of benzene with other reagent ion precursors. Do the authors have any recommendations for those mixing ratios?

L212: In the protonated acetone dimer ionization chemistry, are the species mainly ionized by adduct formation or proton transfer? How does the relative contribution of these two ionization pathways depend on the molecular properties of analytes?

L261-263: Was the sensitivity of protonated acetone dimers to levoglucosan also measured? If so, the relevant results should be presented and discussed in the manuscript.

L385: Please provide the sensitivity deviation value for formic acid and nitric acid.

L402: What are the typical reagent ion currents for chloride and ammonium ionization schemes? Also, what are their sensitivities and LoDs toward the calibration standards used in this study?

L410: How the limit of the detection is determined should be described in the manuscript.

L455: In the ammonium ionization scheme, protonated acetone dimers are also present. Were any organic compounds ionized by protonated acetone dimers, via adduct formation or proton transfer?  

L476: Eq. 10 is wrongly inserted into line 481. Please modify it.

L733, Figure 5: The measured compounds were labelled as HOM-monomers or HOM-dimers. As substantial amounts of less oxygenated organic species were detected, particularly with ammonium, chloride, and iodide ions, I suggest labelling the compounds with OOM-monomers or dimers.  

The authors present results from a newly designed VOCUS AIM reactor. They evaluate the instrument's performance by examining its time response (focusing on sticky molecules), sensitivity, limits of detection, and selectivity. Additionally, they use dopants to mitigate the influence of humidity on the sensitivity of different molecules, with acetonitrile being the most effective dopant. The authors highlight the strength of the AIM, which can switch within 2Hz between multiple reagent ions to access compounds of varying volatility. The instrument is tested in the oxidation of α-pinene to assess the selectivity of different reagent ions and the broad access to all products of such oxidation with high sensitivity. This paper is well-written and suitable for publication. Only minor comments from my side.

Comments:

Line 399-403: The straightforward normalization using the recorded reagent ions measured at the detector is great. It would be helpful if the authors could clarify the differences in the AIM-VOCUS that make this possible compared to the traditional VOCUS, where such normalization can be challenging.

Line 432-436: Switching chemistries at up to 2 Hz is impressive. However, could there be any interferences from one ion chemistry to the next that were observed? Are the 2Hz switching timescales recommended to ensure no interferences? Considering that the authors have already demonstrated the impact of tailing for sticky molecules, which may take several seconds to stabilize when fluctuations occur, I wonder if such frequent switching is ideal. Discussing potential limitations in this regard would be beneficial, as it could save the community considerable time in determining the optimum operating conditions of the AIM. Additionally, it would be interesting to note how these datasets are handled from a software perspective.

Line 462-464: What are the expected concentrations though? The introduced a-pinene is at high concentrations and I wonder whether these compounds could be observed at such high intensity in ambient air.

Comments on figures and tables:

Figure 1: It would be nice if the authors could indicate the pressures in the different sections of the IMR in A or B. Is the dopant flow included in the CFD calculations? I am sure the effect should be minor but still something to mention.

Figure 3: I would recommend that the authors change the acronym of the color bar caption from “ACN” to “dopant flow”.

Table 1: It would be great if the authors could provide the contribution of any interfering clusters to these calibrations that could complicate the spectra. How clean are the spectra for these calibrants and what is the expected % interference of clustering? Also, more AIM reagent ion chemistries are presented including NH₄⁺, Cl^– , and NO₃^-. Are there separate sensitivity results for these ionization modes?

Figure 5: The yellow color is not consistent in all graphs. There is no indication of what the size is. If it is selectivity, it would be great if the authors define how it is determined in the caption.

Typos:

Line 266: Replace "slpm" with "sccm."

Line 385: Correct to "xx%."