The manuscript by Kirchhoff et al. describes a comprehensive study on the performance of a Grimm Sky CPC with DMSO as an alternative working fluid to butanol. DMSO has several advantages over butanol, among others, it is non-toxic, not flammable and is consumed significantly less during CPC operation. It is therefore a very promising candidate for airborne measurements. In line with this potential application, the authors investigated the use of a CPC with DMSO under various pressure levels from atmospheric pressure down to 250 hPa. They found the performance to be comparable to a butanol CPC, but also noticed that the droplet growth is clearly different with DMSO compared to butanol, resulting in a larger number of (optically) smaller droplets. Numerical simulations were conducted to explain the distribution of super-saturation levels in the saturator in order to better understand the differences.

The manuscript is well written and presents a lot of novel information, which is certainly of interest to the readers of AMT. The manuscript merits publication, but I suggest a few minor revisions before it can finally be accepted.

Introduction, general: What I miss is a brief introduction into what type of aircraft measurements the CPC is intended to be used for, i.e. at what altitudes and corresponding pressure levels.

Page 2, second paragraph: I suggest to add the information that CPCs are among the very few aerosol measurement techniques that are metrologically traceable (ISO 15900).

Page 2, line 47: Why is the higher liquid consumption of a water CPC a particular limitation for aircraft measurements? From my personal experience, water CPCs can be operated for days if not weeks before the water reservoir needs to be refilled. One main disadvantage of water CPCs is, however, not mentioned and that is its material dependence, especially of the cut-off diameter.

Page 9, section 3.2: The use of symbols is quite confusing. According to Figure 3, c₀ and c₁ are voltages. In the text of section 3.2, all of a sudden Th appears as a symbol for a threshold voltage. It took my quite some time to realize that c₀ and c₁ are actually concentrations with c₁ being the concentration of particles/droplets deemed valid, based on the fact that the scattered light peak triggered a voltage signal, exceeding a defined threshold Th(c₁). c₀ is apparently the concentration of all droplets that cause a signal above noise level. Figure 3 and section 3.2 should be rewritten to make this clear and not leave the understanding up to the interpretation of the reader. I also wonder if Th is an appropriate symbol. Shouldn’t this rather be something like U_th to make it clearer that it is a voltage threshold?

Page 9/10, section 3.3, including Figure 4: How has this been measured? With monodisperse, DMA-classified particles? If so, for which size(s)? Or was it a polydisperse aerosol? If so, how did you know the charge level to obtain the reference number concentration from the FCE?

Page 12, Figure 6: In the caption of the figure and the x-axis, it should read surface area concentration rather than just surface area. What test aerosol was used for these measurements? How has the surface area calculated? Assuming that this result was achieved for rather compact (NaCl or AS) particles, what would happen if particles are highly agglomerated (e.g. soot), thus with a much higher surface area per particle for the same equivalent diameter?

Page 16, Figure 9: Why does the tube length start at 110 mm? Do you only show a part of the saturator or does the section from 0-110 mm belong to the condenser? How did you derive the saturation profile at the inlet of the saturator?

Page 21, Figure 14: x- and y-axis are reversed. The D₅₀ is a function of the temperature difference and not vice versa.

Page 24, Figure 16: Earlier, you mention that the POPS measurements showed smaller droplet diameters for DMSO than for butanol and speculated that this may be due to the difference in the refractive indices. Could this also be the reason for the lower c₁/c₀ ratios, because the corresponding thresholds are for the voltages, which reflect the peak height of the scattered light in the optical detection of the CPC?

The authors present data and modeling of a condensation particle counter operated with DMSO as its working fluid.  This is an interesting idea, and well-deserving of the further exploration presented here. The experiments are carefully conducted, and apart from a few points listed below, the results are clearly presented.  Yet there are several significant points the authors should address prior to publication:

(1) This Reviewer’s primary concern is that the instrument itself, the DMSO CPC, is not described. Nor is it described in the paper Weber et al (2023) “A new working fluid..,.”, which presumably the authors meant to cite.  The authors do state they have used a commercial instrument with DMSO instead of butanol.  Yet, as this is an archival journal, it is important to describe the pertinent physical characteristics of that instrument.  A model number will not suffice even 10 years from now. The authors might include a sketch, and at a minimum give some of the critical dimensions, such as for the saturator and condenser length, the orientation of the condenser with respect to the saturator. They should give the flow rate, and state how this flow is adjusted as a function of inlet pressure.  Did they use a constant volumetric flow, a constant air mass flow or something else?  Is the instrument sheathed?  One infers from the model results of Figure 9 that this is a laminar flow instrument operated at constant volumetric flow, but the authors should so state.

(2) The cited reference for Weber et al (20203) is to “Characterization of a self-sustained, water-based condensation particle counter for aircraft cruising pressure level operation”, AMT 16:3505-3514).  One assumes the authors meant to cite Weber et al (2023) “A new working fluid..,.” Aerosol Research 1, 1-12, 2023. Please add the correct reference.

(3) Given the overlap in authorship, it is very strange that the results from Weber et al (2023) is “a self-sustained, water-based… for aircraft”  are not included in Table 1.  That paper has results for both butanol and water-based instruments.  This reviewer recommends that these results be included.

(4) The statements about the disadvantages of water as a working fluid are gratuitous and should be struck. First, it is simply not true that the high diffusivity of water leads to greater water consumption.  Water-based condensation particle counters have been designed that consume little to no water -- under typical ambient conditions these instruments operate for weeks to months without consuming any water at all.  Second, organic or biological contamination can be handled, just as manufacturer’s have solved the problem of water-uptake by the wick of butanol instruments that killed the performance in the original designs. (how hard would it be to add an activated carbon filter in the water fill line?). Further, not only is water readily available, nontoxic and non-flammable, note that the water vapor saturation ratio required for the activation of 5nm particles is less than one-half of that required when using butanol or DMSO (by the Kelvin relation the critical saturation ratio depends on the product of molecular weight, surface tension divided by the liquid density, which for water is less than one-half of that of butanol). 

(5) Section 3: This Reviewer appreciates the thoroughness with which the data have been handled. However, this section would be clearer if it were written more concisely – It isn’t until the last sentence of Section 3.2 that the reader knows what is meant by “adjustment to Co threshold” in Figure 2 that is referenced ahead of Section 3.1.

(6) Section 3.3—what particle diameter was used for these tests?  Or, if a polydisperse aerosol, what was its approximate mean diameter?

(7) Section 4.1, Table 2, Figure 6 – First, it would greatly help the reader to state at the beginning of this presentation that these are DMA size selected aerosol. Second, this reviewer is not convinced that the droplet size is limited by the aerosol surface area. The droplet size also increases with the diameter of the test aerosol. If the growth is kinetically limited, we know that the larger seed particles activate sooner in the condenser than those near the cutpoint. Might this be what is happening?  Did the number concentration of the test aerosol play a role?  Finally, the small particle size at 1000hPa was puzzling until the reader reaches the model results.  This modeling indicates that the condenser is simply too short for operation with DMSO at atmospheric pressure, and that is why the droplet size is so small except at the highest temperature difference that creates the highest supersaturation. Do take a look to see if this section can be made a bit more accessible to the reader.

(8) Section 4.2.  It would be good to point out the fundamentals of what happens to the saturation ratio profiles at reduced pressure. In a cylindrical tube, for fixed temperatures and working fluid, the heat and mass transport vary with Dz/Q, where D is the diffusivity, Q the volumetric flow and z the axial coordinate. Both thermal and mass diffusivity are inversely proportional to the pressure.  Were these profiles modeled at fixed air mass flows, they would look nearly the same. Reducing the pressure at fixed volumetric flow increases Dz/Q making the saturation peak further towards the inlet of the condenser, essentially increasing its effective length.

(9) Figure 7 shows a concentration effect, ie less droplet growth at high particle number concentrations, and this is attributed to vapor depletion.  This may be correct, but why have they ruled out condensational heating as a contributor to this effect?

(10) As a practical matter, as the condenser is generally lower than the freezing point of DMSO, what is done to prevent build up of ice on the condenser walls? Might this not be a problem for long-term operation?