The manuscript investigates cloud droplet activation properties of urban aerosol particles based on a rather long measurement data set. The theoretical approach and used methods are scientifically sound, and the text is well organized and relatively clearly written. The study is not particularly original, but the results bring some new insight into this topic and are therefore worth to be published. I have a few, relatively minor issues that should be addressed before accepting this paper for publication.

General comments:

The manuscript investigates cloud droplet activation properties of aerosol particles, not really cloud droplet activation (there are studies that differentiate the real cloud droplet population from cloud interstitial particles, and studies that aim to related the cloud droplet population to below-cloud aerosol population) . To avoid any potential confusion, it is important to make this difference in the paper. I therefore strongly suggest modifying the title of the paper into something like "Cloud activation properties of aerosol particles in…". The same concerns wording on lines 9, 67 and 78.

I appreciate the detailed description of methods used in this investigation. However, there are couple of minor issues related to this section. First, the motivation for the criterion introduced on 198-203 should be improved. What is the real purpose of selecting this ratio, and why to select the value of 70% for this ratio? Second, section 2.3 is not really about modeling, but about using existing mathematical formula. Therefore, the title of that section should be modified into something like "2.3 Calculation of particle hygroscopicity".

Detailed comments:

The statement on lines 19-20 (They were related to the size-dependent chemical composition and external mixtures of aerosol particles) gives the wrong impression that this paper measured the aerosol composition and mixing state. It is very true the size-dependent chemical composition and external mixtures of aerosol particles are the most likely explanation for the observations made in this paper. So it should more clearly stated that this is the most likely explanation rather than a real finding of this paper.

Lines 30-32: Written like it is now, it is not clear what is the result from this particular work (low kappa values in an urban site) and what is based on studies made by others (lower kappa values in regional or remote locations). Please modify.

Lines 43-47: The text is not quite consistent with itself. Since S is affected not only by the updraft velocity but also by sink of water vapor (existing cloud droplet population determined by CCN), I would recommend writing "Different updraft velocities in clouds, together with existing cloud droplet population that depend on CCN concentrations, result in different Ss…".

Line 62: This is unclearly written. Maybe one could write "…interactions at S values typical for atmospheric conditions and…"?

Lines 79-81: I would modify the writing a bit: "Specifically, we will report, … various Ss, in order to determine…".

Lines 426-427: One of the very first studies showing that the minimum diameters of aerosol particles able activate into a cloud droplet is typically well below 100 nm in a remote environment was that of Komppula et al. (2005, J. Geophys. Res., 110, D06204, doi:10.1029/2004JD005200). It might be worth mentioning that paper here.

Line 460: Please explicitly write what dependency you refer to here. I assume this refers to the observed slope of the S vs. particle diameter relation in Figure 3 that is different from the theoretical slope for a particle population with a size-independent chemical composition. A reader might not catch this because it requires returning to the information given in the previous paragraph.

Lines 515-516: I do not fully understand this statement. Does it refer to different seasonal behavior of the S=0.1% curve as compared to those of other values of S? The following text (lines 555-558) is also somewhat difficult to understand.

Line 630: ..at lower sizes?

Lines 630-632: I do not understand how particle hygroscopic properties could depend on the particle number concentration. I suppose the authors mean something else here, but it is written in a confusion manner.

Table 3: The table caption should explicitly tell that the unit of the numbers given in the table is "nm".

The manuscript “Cloud droplet activation in a continental Central European urban environment” by Salma et al. presents a comprehensive study of CCN characteristics of aerosol particles in the urban environment of a continental European city. The paper discusses on-line particle measurements, including CCN number concentrations, aerosol particle size distributions and meteorological data, as well as the CCN parameters derived from these measurements.

At the moment, the presented study is of rather limited scientific relevance and novelty as the potential for CCNC in aerosol-cloud interactions studies has been exhausted. The authors state that our knowledge about aerosol–water vapour interactions in the atmosphere is insufficient and use this as the motivation for their study; however, the authors do not acknowledge that the presented study does not result in significant improvements in our understanding of the aerosol-cloud interactions in the ambient atmosphere. At this point CCNC can no longer help us understand aerosol-cloud interactions and their effects on radiation and climate. The manuscript is a description of the aerosol CCN properties in an urban environment. This notion is not meant to change or take away from the paper; it is more meant as the direction of potential future work for the authors.

Having said that, the paper is of very good quality and written very well, and the authors do an excellent job at interpreting the data, presenting the results and discussing them in detail. The objectives and conclusions are clearly stated, and the paper makes great use of existing literature and puts its results in perspective. The paper is well-structured and provides the interested reader with a lot of information about CCN characteristics in an urban environment. Some of the results can be put more in perspective, and conclusions can be expanded a bit. However, I definitely recommend the manuscript to be published after the minor revisions suggested below are incorporated.

General comments

1. The authors should put their measurements and results in a bigger perspective. The manuscript presents long-term measurements conducted at a single point. How representative are these measurements? Can they tell us anything about the aerosol effects on the cloud formation over urban environments? Considering the size or urban environments compared to, let’s say, much bigger marine or forested environments, do we expect any effects of the urban aerosol population and its CCN properties on the actual ambient cloud formation?
2. Conclusions section is fairly short and needs to be expanded. It would be particularly useful to focus on those Ss found in a typical ambient atmosphere and summarise if and how urban emissions are expected to, at least theoretically, affect the cloud forming potential in urban environments. The authors could elaborate more on how their study compares to similar previously published literature about CCN properties in urban environments and draw conclusions about how their study complemented or added to the existing knowledge. I think it would also be important to notify the reader what else could be done in the future studies to increase the representativeness of single-point measurements and our knowledge of aerosol-cloud interactions.

Minor comments

1. Introduction – please, give examples of Ss that can be found in the ambient atmosphere.
2. Lines 42-43 – the particle’s ability to act as a CCN is primarily and overwhelmingly controlled by its size and to a much lesser degree by its chemical composition and the mixing state. Please, rephrase.
3. Line 49 – it’s not the CCN, but the droplets that alter the radiation. Aerosol particles also interact with solar radiation, but I think here the discussion is about the droplets.
4. Lines 63-64 – “Long-term studies (of at least 1 full year) are required to understand [aerosol–water vapour interactions in the atmosphere under S conditions] and their consequences.” Has your study done that? I think the statement is rather strong and not supported by your conclusions.
5. Line 183 – could also be added? Or could also add what?
6. Lines 187-203 – these paragraphs are slightly confusing, and I am not sure about their purpose. Why do you want to exclude nucleation mode particles? How does that affect the robustness of your statistics? The measurement location is in an urban and polluted environment, so I am not sure why there is a need to define such conditions based on the fraction of nucleation mode particles.
7. Line 277 – what is meant by orientation?
8. Lines 277-278 – we are not able to see from the table whether there were any extraordinary situations because the table shows values averaged over one year. I think the entire sentence would only make sense if seasonal meteorological values would be shown.
9. Lines 294-297 – what would cause such rapid concentration changes in an urban environment?
10. I am not sure if SD is defined anywhere in the manuscript. Is it standard deviation? Or size distribution? What is RSD?
11. Lines 388-411 and Figure 2 – how do the data points and the curve in Figure 2 compare to the curves and the parameterisation presented in Figure 4 of Paramonov et al. (2015)? Lines 394-396 present a very good argument. Is it known whether the considerable time variability of N_6–1000 is driven by particular size bins? Or does it vary across all size bins? What drives such changes in an urban environment for nucleation mode particles and for those over 100 nm in diameter? This could be of interest since, indeed, both N and CCN seem to show a lot of variability.
12. Line 414 – median dc decreased with increasing S
13. Line 428 – “This confirmed that the water activation properties depend on the aerosol type.” – yes, this is already well-known. If your “average diameters and CCN concentrations were larger than for” other sites, this means that a) your total particle numbers were higher and b) your particle population was, on the whole, less hygroscopic.
14. Line 431 – the mean fraction
15. Lines 460-461 – I believe chemical composition is size-dependent for all natural environments, not just urban ones.
16. Lines 503-504 – “The changes were pronounced mainly for the laRger Ss”, which are the least relevant for an ambient atmosphere. This means that these seasonal changes are not very likely to have any effect on the aerosol-cloud interactions.
17. Lines 503-516 – I don’t think it is reasonable to talk about monthly changes when there are no error bars with the data points. It doesn’t seem like anything is really changing much throughout the year, and there is no need to try and look for such changes (lines 504-509).
18. Line 558 – should say “March and April” as the campaign ended in April 2020.
19. Figure 5 – your measurements were form April 2019 to April 2020. Why is the x-axis from March to February? The only March you measured was in 2020, towards the end of the campaign, but in the figure it appears as the first data point. Please, correct this. Having proper time series shown can also better demonstrate whether any changes occurred during the first COVID outbreak.
20. Lines 558-561 – this could be expanded a bit more. In previous sections you talked about changes in total particle numbers during the first COVID wave. Please, elaborate more here on what happened to N_CCN, Dc and AF during the last weeks of the campaign.
21. Line 564 – with increasing S
    
    

References

• Paramonov, M., Kerminen, V.-M., Gysel, M., Aalto, P. P., Andreae, M. O., Asmi, E., Baltensperger, U., Bougiatioti, A., Brus, D., Frank, G. P., Good, N., Gunthe, S. S., Hao, L., Irwin, M., Jaatinen, A., Jurányi, Z., King, S. M., Kortelainen, A., Kristensson, A., Lihavainen, H., Kulmala, M., Lohmann, U., Martin, S. T., McFiggans, G., Mihalopoulos, N., Nenes, A., O'Dowd, C. D., Ovadnevaite, J., Petäjä, T., Pöschl, U., Roberts, G. C., Rose, D., Svenningsson, B., Swietlicki, E., Weingartner, E., Whitehead, J., Wiedensohler, A., Wittbom, C., and Sierau, B.: A synthesis of cloud condensation nuclei counter (CCNC) measurements within the EUCAARI network, Atmos. Chem. Phys., 15, 12211–12229, https://doi.org/10.5194/acp-15-12211-2015, 2015.

Thank you for an excellent paper and best of luck with the review process!