Air ions promote new particle formation through ion-induced nucleation, so measuring air ions, especially small ones, is crucial. This manuscript describes a newly designed instrument, Cluster Ion Counter (CIC), which measures the number concentrations of air ions below 5 nm. Such a device could complement the family of instruments for studying the new particle formation. This manuscript is well written. I believe this manuscript could be published in AMT after addressing the following comments.

Major comments:

1. Neutral cluster and Air ion Spectrometer (NAIS), which was designed by same research group of this study, is widely used for the observation of air ions. I’m curious to know what differences or advantages the CIC has over NAIS. This study appears to have done a parallel comparison experiment of CIC and NAIS (Figure 5), but the results are not mentioned in the manuscript.
2. CIC looks quite small compared to the NAIS. What is the weight of the CIC? Can mobile observation be an advantage for CIC?
3. The authors state that CIC is capable of making precise and robust long-term measurements. Did the CIC operate under ambient conditions before? How does CIC perform in field measurements? I think it is important to make it clear that CIC can be used not only for chamber experiments but also for long-term field measurements.

Minor comments:

Abstract: I suggest adding a description of CIC application prospects to the abstract.

Line 146: The full name of NAIS needs to be given here.

Review of:  Design and performance of the Cluster Ion Counter (CIC)

by Sander Mirme, Rima Balbaaki, Hanna Elina Manninen, Paap Koemets, Eva Sommer, Birte R.rup, Yusheng W3, Joao Almeida, Sebastian Ehrhart, Stefan Karl Weber, Joschka Pfeifer, Juha Kangasluoma, Markku Kulmala, and Jasper Kirkby

This article describes a study of the response of the CIC instrument. The contents has little scientific interest. Nevertheless, the work is carried out competently. Furthermore, since this instrument is used by various groups for basic research, an article such as this one is needed as basic reference. The article is also brief, so publication would be justified once various technical issues are resolved.

The title includes the term design. However, the article contains few design considerations. Why the relatively small flow rates used? Why 3 size ranges? Why the sizes selected? These and many other general issues related to atmospheric studies would presumably have guided the final choice of operational parameters. For instance, the entry filter appears to be a sphere, and the flow past it is likely to separate and become turbulent. No details are given on how this sphere is supported, but its support surely would have an effect on the flow. If the sphere is supported on the inner electrode, then there would be a boundary layer on this support, with a decelerating region near the ogive of the inner electrode, where the boundary layer would separate and increase the level of turbulence. Why would a turbulent flow field be preferable to a laminar one at such low flow rates. How are the flow calculations shown in Figure 3 executed? Either these various key issues are discussed, or the term design should be removed from the title. Even so, the reliability of the mobility response of the device depends on the flow field, so some discussion of this seems inevitable.

There are important design considerations besides the flow field. For instance, why an integrating electrometer rather than a direct current measurement with an inverting amplifier? The one developed by the Burtscher group features a noise level of about 0.1 fA at 1 Hz, and responds in less than 100 ms (i.e. Aerosol Science and Tech., 51(6), 724 – 734, 2017). The fact that zero current level measurement need to be taken every 1-5 minutes suggests that there is a considerable drift, which is not the case in the Burtscher circuit.

The generation of ions with such high mobilities in air (2.5 cm²/V/s) is not a simple matter, and requires some more explanation. We are told they are produced by a hot W wire in N₂ and selected by a Herrmann DMA. Would the authors please include a Herrmann DMA mobility spectrum with the ion calibration peaks and some discussion. Is this high mobility cluster a bare metal cation? What other comparably high mobilities are produced by this source? Is the ultradry environment essential for this? Does the neutralizer need to be cleaned specially to yield such high mobilities? Most neutralizers are contaminated and would tend to transform such high mobility ions into larger solvated particles.

On the same subject of design, I was especially puzzled by the broad remarks on the difficulty to detect the low atmospheric levels of ions, combined with a low flow rate instrument. Given that the Tartu group has previously developed ion detectors with much higher sampling flow rates, what special advantage does the current device offer to compensate for its greatly reduced sensitivity?