In this manuscript an interesting new approach for particle counting is presented. Two techniques, condensation particle counting and electrical measurement based on diffusion charging are combined. Particles are first grown to micrometer sizes by condensation and are then charged in a corona charger and measured in a Faraday cup electrometer. This could result in a lower detection limit for size and concentration compared to DC-Devices and to higher measurable concentrations compared to CPC’s. The paper is in most parts clearly written and easy to read.

Here are some remarks, which tom my opinion would be helpful for the reader:

It would be nice to have a real quantitative comparison between the new device and  in particular CPC’s, including the photometric mode offered by several CPC’s. Now many statements are more qualitative, the photometric mode is not mentioned. What are the quantitative advantages?

Lines 55ff: A slightly more detailed description of the Saturator/Condenser would be helpful.

Information, how the diffusion charger is influenced by the working fluid should be given (may significantly influence ion mobility)

Equation (2): as written here is for strictly monodisperse aerosol and the exponent (here 1.1) depends on the charger design, this should at least me mentioned or the integral form could be shown.

Equation (3): As the detector is operated in the mode where the induced current is measured, there is no stationary current while the precipitator is on or off (the stationary current is zero in both cases, this should be described better.

Line 161: an explanation why DEG can condense on sub 3 nm particles without homogeneous nucleation would be interesting

Chapter 4.1, 4.2 and 4.3: effects are discussed qualitatively, but not the specific impact on the device.

5.1: Did I correctly understand that in a first step a COMSOL simulation treats the flow ant heat transfer, determining the space resolved supersaturation. Then the particle growth is calculated in Matlab? If I am right, how is the change in supersaturation by the condensation process considered?

Experiment: Some experiments are done in pure N2, some in air, some with influence of the WF. This will result in different charging efficiencies, which are not mentioned or did I misunderstand something?

Summary of review

The authors present a novel approach for measuring particle number concentrations of aerosols with a lower particle-cut-off diameter of 2.5 nm at concentrations exceeding 10⁶ cm ^-3 using a condensation diffusion charging approach.  The work is sound, well-presented and draws appropriate conclusions. The work would be a valuable addition to the literature.  I would suggest that the article is accepted for publication provided that the comments below are address appropriately.

Technical comments for the authors

1. Figure 1: The caption should be improve to clearly indicated where the diffusion charger is located and, explain what DCEM is, and explain the meaning of the symbols used
2. Section 3: This theoretical background section is very long and should be made shorter by either / both: (a) summarising just the main point and including references to other sources where the information can be found; (b) moving most of the text to the supplementary information.
3. Section 4.3: The authors state that a 2021 paper by Hoa et al, discusses 46 possible working fluids. This section should also describe why only two of those working fluids (DEG and glycerol) were considered for in this work.
4. Lines 267-273: Please include a discussion as to whether it would be beneficial to use a larger delta_T which would result in an even smaller d₅₀?
5. Section 6.1: The calibration of the CPC and electrometer are not explained. Please include a summary of how the instruments were calibrated.
6. Lines 318: Please comment on whether the performance of the instrument would be different for different types of polydisperse particles other than NaCl (most importantly real world soot).
7. Line 334: Including a table of the measured droplet sizes at each particle seed diameter would be useful, perhaps in the Supplementary Information.
8. Figure 5: The authors should include error bars on each of the data points in order to allow the reader to assess any significance in differences between the plotted points and lines. Please also make the two shapes used for data points much more distinct and different colours.
9. Line 388: The type of polydisperse aerosol used should be re-stated at this point in the text
10. Figure 6: The authors should include error bars on each of the data points in order to allow the reader to assess any significance in the trend shown
11. Figure 6: Please comment on the scatter of the data below 10⁴ cm^-3, as this is larger than would be expected for a CPC.
12. Figure 7(c): The gradient of the line looks to < 0.9. Please explain why this gradient is not nearer to 1, as appears to be the case for Figure S5(c)
13. Figure S5 is very important as it is fundamental to demonstrate the claim that the device is suitable for operating at concentrations over 10⁶ cm^-3, so should be brought into the main paper as Figure 8 (or combined with Figure 7).

Minor comments for the authors

1. Line 21-23: The discussion should be updated to describe the new European air quality directive (2024/2881).
2. Line 29: Include a reference to international standard ISO 27891 after ‘CPC calibrations’
3. Line 30: Suggest changing ‘doubly’ to ‘multiply’
4. Line 41: missing word: change to ‘however, they come’
5. Line 71: Should the unit be mm, not mm²?
6. Figure 2: The line styles are very similar so would be very difficult to distinguish for readers with colour-blindness. Please consider making the dashes and dotted lines much more distinct.
7. Figure 4: same comment for figure 2. In addition, the symbol shapes are very small and therefore difficult to distinguish. Please increase symbol size.

Comment for the editor

1. Page 18: Is the data availability statement acceptable?

The work presents more or less, a CPC without an optical head. It’s growth system where the optical head is substituted by a detector baser on an electrical charger and an aerosol electrometer. The authors pretend that the instrument can then measure high concentrations of particles (up to 10⁶p/cm) without dilution. Of course this instrument cannot be used for low and very low concentration. It should appear in the text. Indeed the maximum concentration measured by the CPCs sensitive to 3 nm, are limited 10⁵p/cm³.  

These CPCs (called ultrafine CPCs) don’t use the photometric mode method (10⁴-10⁷p/cm3). They use only the counting mode. The reason is the risk of agglomeration because of the high diffusion of the sub 10 nm particles. High concentrations will ‘probably’ induce agglomeration which mill induce a shift in the diameters and a decay in the number concentration.

The ultrafine CPCs can activate and detect particles down to 2,5 nm because use a sheathed condenser where the particles are confined in the center of the condenser. Indeed the maximum sursaturation profile is locate in the center of the condenser. On the other-hand the sheath decrease the diffusion of the particles to the walls of the condenser where they will be lost. This method has been introduced by Stolzenburg and Mc Murry in 1991. The TSI ultrafine CPC is based on this design.

The inner diameter of the condenser is missed. The temperature of the saturator is between 55°C and 40°C depending on the working fluid I guess?

The flowrate in the saturator and the condenser is not well defined although its value is very important in terms of saturation of the air (or N2) at the outlet of the saturator. It’s critical in the condenser too since the profile of the sursaturation is function of the flowrate (residence tie in the condenser). The reader can find that the flowrate in the AEM is 0,54 lpm

Schmidt-Ott and Burtscher (2006) is it a paper? A patent? It’s hard to find this work. The authors could cite many other works including other works of Andreas Schmidt Ott and Heinz Burtcher.

In the Figure 2 the working fluid is missing. Is it diethylene glycol or butanol?  In the same figure,  40/05 couple of temperatures gives a better results 55/20°C. 50/15 and 45/10 would have been interesting couples to simulate  to confirm the explanation and the behavior given by the authors.  

The authors have used a TSI CPC model 3775 with a detection limit of 4 nm. I would suggest to limit the paper to 3 nm rather than 2,5 nm although the simulated results in the figure 3 for example are limited to 5 nm.

I have a deep question about the paper and the results. It’s well know that DEG as a working fluid in the ultrafine CPC design of TSI, cannot grow particles to more than 100 nm (0.1 µm). That is the reason why they use a a CPC as a booster to count the droplets. The present design seems to detect and grow particles of  2,5 nm up to 1.8 µm without booster nor a sheathed condenser. The authors must give the reason or the secret of their method in the paper. The inner diameter of the condenser is not given nor the profile of the supersaturation at different temperatures must be given too..

The second question I have is about the final diameter of the droplets of sub 10 nm seed particles. It’s well known in the pass that butanol droplets are size dependent in the sub 10 nm. People has used that in the method called Pulse Height Analyser (PHA) to size particles in the sub 10 nm. Is that not the case with di ethylene glycol?