The only thing which is missing in this interesting work is the response time vers the diameters of the tube.

Few questions and comments on the other hand:

The details given by the authors about the changes made in the ‘modified commercial water CPC’ are not clear or not enough. Indeed the authors are talking about changes in a ‘commercial’ version of the CPC. That could be taken to mean by the reader as some thing doable by any body who has a TSI 3789 CPC.

Few details about the ‘modified commercial water CPC’ will be very helpful for the reader. What was the change made in the CPC compared to the commercial version? What are the benefits or advantages provided by these changes?

It will help the readers to have an exhaustive description of the modified commercial instrument.

Why this particular temperature of the initiator 59°C  (default value) rather than 60°C for example?

Is their a large difference if  one takes 60°C. What is the precision on the temperature measurement?

The authors should cite the previous  work of Ahn & Liu

Ahn, Kang-Ho and Liu, B. Y. H. (1990) Particle activation and droplet growth processes in condensation nucleus

counter--I. Theoretical background J. Aerosol Sci. 21, 249-261.

Ahn, Kang-Ho and Liu, B. Y. H. (1990): Particle activation and droplet growth processes in condensation nucleus counter—II. Experimental study. J. Aerosol Sci. 21, 263-275.

The manuscript describes simulation results for modified water-based condensation particle counters operated with different instrumental parameters. The study is useful to guide the development of particle instruments targeting non-standard applications such as airborne deployments and investigation of very small nucleation mode particles. The study is overall presented well although the manuscript would benefit from careful language editing. Some sentences are very long and lack clarity. The part discussing the experimental verification is very short and could be extended.

Given the discussion of very small differences in activation diameters and other quantities - is there any way to assess the uncertainty of the derived quantities from numerical algorithms within COMSOL? Are there any aspects in the model where assumptions might break down or are questionable? Even though the derived values show physically reasonable trends a discussion of practical relevance of those small variations should be included.

l. 181: How significant is the limitation of this assumption? It might be good to discuss at least qualitatively how the results might be affected by partially soluble particles - especially since the experiments are done with AS.

l. 203: "can be governed by..." sounds very vague - please state the conditions where this formula is valid.

l. 234: This sentence is unclear, please rephrase.

l. 246: What temperature and flow rate was the furnace operated at?

l. 249: Figure S2 could be incorporated into the main paper since the setup is central to the experiment.

        Does the validation hold towards lower pressures? For tropospheric airborne research pressures down to ~200hPa might be relevant.

l. 252/253. What figure does this sentence refer to?

l. 275ff: this sentence is long and confusing.

l. 331: heating flow --> heat flow

l.339ff: The correlations with inlet pressure are already shown in he previous subsection - I would recommend limiting the discussion in 3.2 to the T-effects and discussing all pressure-related results in 3.3. Alternatively, merge and overall shorten the two subsections.

l. 381: Is this change in D in any way of practical relevance? See comment above.

Fig 6 a/b: if the y-axis would be scaled as in Fig 5a/b would there be dependence on initiator length for L>20mm visible? 

l.429ff.: is it obvious what causes plateau-like shape of the curves shown in Fig. S3? What happens between 0.7 and 1s? Is there a better phrase for "Allowed particle growth time"?

l.438 the phrasing "...for two configurations of 2nm and 7nm..." is unclear - maybe better use "..vWCPC set in the 2nm and the 7nm-configuration..." or something along those lines.

l. 441/Fig 7.: The reference to Mei et al 2021 as source of Fig 7a should also be given in the figure caption. The 30C TSI curve in Fig 7a either requires an explanation or should be left out if not relevant for this study.

Sec. 3.5:

In this section a mix of pressure units atm and hPa is used, this should straightened out for consistency, ideally the authors should resort to SI units throughout the paper unless there is a good reason not to.

The comparison of model and measurement results should really be shown in a single plot for each of the CPC settings. The different axis scales for the different panels make it difficult to see the key message.

l. 466: Please clarify, what does "commercially modified vWCPC" mean? Is this instrument commercially available in this form or has it been modified from its standard configuration for the purpose of those experiments?

l. 471 It should be discussed in how far this improvement is practically relevant. 

I don't quite understand the statement "... without enhancing the droplet growth...". Doesn't Fig 5 show a reduction in growth for smaller tube diameters?

References should be listed with DOI numbers to ensure proper linking in online documents