In this manuscript, the authors investigated the number size distribution of atmospheric ions observed in Leipzig, Germany. The authors showed that ions classified into different size ranges have different diel behaviors and explained such behaviors in association with other atmospheric parameters. 

In general, I see nothing wrong in this manuscript, where most of the explanations are scientifically sound. However, I am a little concerned about the significance of the findings presented in this manuscript. I didn't see enough new insights, except for the new locations. Since the authors deployed a nitrate CIMS during the measurement, if they can associate the concentration and composition changes of gaseous species with the variability of ions, it is possible to bring this study to a new level. 

Technical issues:

1. there are several places (mostly at the beginning of paragraphs) show format issues. 

2. I think the nighttime high concentration of small ions is due to its connection with boundary layer dynamics, as well as the competition between particles of different sizes in taking up the ions. Without solid proof, it is not convincing to say the diel pattern of smallest ions are due to radioactive decay. (Line 255).

General comments

Rowell et al. have studied the concentrations, growth rates and formation rates of small ions, intermediate ions, and large ions in Leipzig, Germany, during a month long campaign in summer 2022. They paid special attention to the charged particles and air ions during NPF event days.

One issue becomes apparent immediately. The size ranges, which are used in this study to classify ions into small ions, intermediate ions and large ions are, to my understanding, based on mass diameters. However, the diameters used in this study are mobility diameters. It is very possible that the impacts of this on the results themselves are minor. However, the concentrations of sub-2 nm ions can be considerably higher than those of above 2 nm ions, which might impact the intermediate ion concentrations used in this study.

Some factors, such as missing information from the methods section and multiple errors with the references to tables/figures, give an impression of a rushed work. Quite a few relatively minor issues could also be identified. In addition, I found it a bit difficult to understand some of what had been done, i.e., how the formation rates and growth rates had actually been determined for the ions.

The writing itself mostly does it job, although at places I found the analysis and argumentation difficult to understand and follow. The amount of references to other studies was a bit lacking at some parts. In addition, I find both the results and analysis in general a bit lacking in depth and novelty. There is potential for more, even with the data the authors likely possess already, some suggestions to which I give below in the more detailed comments.

Despite these issues, I have no doubt many readers of ACP, myself included, would find these results (with the issues addressed) interesting. Therefore, after the comments below have been sufficiently addressed, I would consider the study worth publishing. However, I find that with the current contents, the study might be more suitable as a measurement report than as a research article.

Specific comments

Abstract

1. Line 23: I have concerns regarding this size classification. The limits of the size classification used by Tammet (2006), were based on mass diameters (see J. Aerosol Sci., 26 (1995), pp. 459-475). Here, mobility diameters are used instead, while the diameter ranges are the same. While unlikely to have a major effect on the main results, the diameter limits should be reconsidered.

In addition, here, and later in the manuscript, 0.8-1.6 nm ions are referred to as small charged particles. This is not accurate, as some of the ions in this size range could be large, charged molecules. As such, referring to them as small (air) ions instead of small charged particles would be more accurate. Alternatively, it should be defined that small charged particles can include also large charged molecules. Please revise accordingly.

2. Line 30: the reason why small ion concentrations are lower and intermediate/large ion concentrations higher on NPF event days could be mentioned.

3. Line 32: here, and also many times later in the manuscript, the phrase charged (or neutral species) is used. I am not sure if it is an accurate phrasing to use for charged (neutral) particles, with varying, non-uniform chemical makeup. I suggest using particles instead to avoid any confusion.

Introduction

4. Line 43-44, line 51-52: a reference is needed.

5. Line 58: I do not understand what “persist as a source of charge” means.

6. Line 74, line 76: vague phrasing. The wording “In other remote locations” suggest that the locations studied by Manninen et al. (2010) were all remote and that there was no overlap between the two studies (accuracy of the latter I cannot confirm, see comment below). “In other urban locations” is similarly unclear.

7. Line 74: Kulmala et al. (2010) is not in the reference list, or at least I cannot find it.

8. Line 82-83, the following paragraph: The aims of this paper are a bit unclear and phrased in a vague manner. I would suggest using a more precise phrasing. It could be explained what is meant by behaviour of charged particles. In addition, more details (i.e., formation and growth rates of charged particles are investigated) on what is actually done in the paper should be added.

9. Line 90-92: see my comments for the abstract. The classification used by Tammet (2006) was based on mass diameters, not mobility as is used here.

Materials and methods

10. Line 100: please also state at what height from the ground the measurements are taken from.

11. Line 101: Potentially inaccurate phrasing. What does ‘… in excess of 100 nm from a number of highly-trafficked roads …’ mean?

12. Line 123: The NAIS measures air ion and total (neutral+charged), not neutral, particle number size distributions. In addition, the total particle concentrations are measured based on both the negative and positive polarity columns of the instrument. It could be mentioned, which data is used for the total particle concentration.

13. Line 168-169: is the classification done based on the total PNSD or charged PNSD? This should be specified.

14. Line 170: “… neutral particles ...” should read total.

15. Line 171: It is not clear what combined means. I am assuming the particle number size distributions were combined using data below some diameter from NAIS and above it from MPSS. Please clarify, and specify the connecting diameter.

16. Line 171-172: Details of these plots such as color scale used are presented, yet none of these plots are shown anywhere. I would either suggest removing the last sentence as it is not necessary, or including contour plots in the analysis for added depth to the analysis.

17. Line 174 and the following paragraph: it should be mentioned from which data CS is calculated from. I am assuming from the MPSS data.

18. Line 186-187: Misleading phrasing “When calculating the formation rate …”. This sentence makes it seems like the formation rate is the formation rate of particles with sizes in the size range, i.e., formation rate of 3 to 7.5 nm particles.

19. Line 193, Line 194: As aforementioned, I do not believe “charged species” is an entirely correct phrase to use in this context. Charged particles or ions would be better.

20. There is absolutely no mention of black carbon (BC) anywhere in the methods section. No mention of such data being used, or how it was measured. This information should be added to this section.

Results and discussion

21. Line 200: Table reference showing an error. There are some figure references later in the manuscript, which are faulty too. Luckily, I was able to figure out what the tables and figures referred to were.

22. Line 200, Line 202: Small ions can also include large charged molecules (see my comment for the abstract section).

23. Line 210-212: Some more recent studies could be referenced here too.

24. Line 212-214: The electrode effect depends on the heights and is strongest near ground. As the measurements are taken from the fourth floor, this should be addressed before making any conclusions on the disparity of positive and negative small ion concentrations.

25. Figure 2: are these the mean size distributions of charged particles? This should be stated both in the figure caption, and in the text while referring to this figure for the first time.

26. Line 224: This sentence “However, they were present in substantially larger concentrations …” seems unnecessary and separate at this point, as the differences between NPF and non-event days are not discussed yet. I would suggest leaving it out.

27. Lime 227-231: The size classification diameter limits in these studies are not exactly the same. For small ions, Dos Santos et al. use 0.8-2 nm (in mobility diameters) while Tammet et al. use <1.6 nm (in mass diameters), while this study uses 0.8-1.6 nm (in mobility diameters). To some extent, this can have an effect on the ion concentrations, especially as the sub-2 nm ion concentrations are typically higher than above 2 nm ion concentrations.

28. Line 231: Poor choice of words. “Observed variability …” indicates more to something observed within this study, not to the differences between different studies. Perhaps “The differences between these studies ...” would work better.

29. Line 236-238: see my 26. comment.

30. Line 238-243: Intermediate ion diameter range used in this study is from 1.6 to 7.5 nm and large ion range is from 7.5-22 nm. The large ion diameter range is wider by over 8 nanometers. I do not see how the comparison of the concentrations in these two size classes of very different widths is meaningful. Considering this, attributing the differences in the concentrations of large and intermediate ions to impact of air pollution does not seem justified if no other argument is given than the concentrations of large ions being higher.

31. Section 3.1 in general, Table 1: In addition to the mean values, median values and 5-95% spread of the charged particle concentrations is given, yet these are not discussed anywhere. Looking at them, we can for example notice that the 5% value of positive intermediate ions is larger than for negative intermediate ions. However, 95% value of negative intermediate ions is larger than for positive intermediate ions, Discussing the values aside from the mean concentrations would add depth to the analysis.

32. Line 254-257: “Diurnal cycles suggest … ”. I do not follow the reasoning here.

33. Line 262: Please specify what time midday corresponds to.

34. Line 276-282: I suggest including this part in the methods section instead. Also, it is still unclear whether charged particle or total particle concentrations, or both, were considered when identifying NPF events.

35. Line 287: Faulty reference again.

36. Line 288: Unclear phrasing, I suggest “… variables and concentrations of charged particles in different mobility classifications …” or similar.

37. Line 292-294, the following paragraph: “These trends align with expectations …”. I believe a reference should be added here. Rest of the discussion, i.e., the sentence starting “The parameter is habitually related ...”, in this paragraph could use some references too.

38. Line 302: I have doubts about photoionization having a significant contribution to intermediate or large ion concentrations. Previous studies suggest that in the lower troposphere photoionization should not have a significant impact on the ionization rates (see e.g., Harrison and Carslaw (2003) https://doi.org/10.1029/2002RG000114), which is also stated in the references study by Jiang et al. (2018). In addition, if photoionization contributed to ion concentrations, it should also do so for small ions. I would argue that the observed correlation of solar radiation with intermediate and large ion concentrations is attributable to photochemistry and NPF.

39. Figure 5 and the discussion starting from Line 312: I find the connection of Figure 5 and the discussion in this paragraph with air ions unclear. The role of the discussion here for the manuscript and its aims should be clarified.

40. Line 344-346: Are there any potential explanations for the observation of lower small ion concentrations on NPF event days?

41. Line 361: Is it a coincidence that BC concentrations were higher on nighttime on days, which NPF event occurred compared to non-event days, or are there some potential explanations for it?

42. Line 376: “Observed similarities … “ I found it difficult to understand what this sentence means.

43. Line 387: It is not clear what “combined” means here. How I understood it is that the formation rate of charged particles is determined as a sum of the formation rates of negative and positive ions. Please clarify.

44. Line 388: What does “combined” mean this context? Does this imply that the formation rate is just the formation rate determined based on the total particle number size distributions, which are measured by the NAIS. If so, the use of “combined” is unnecessary and confusing.

45. Line 389: This is a very interesting observation, which it implies that more particles are forming at larger diameters than smaller diameters and that the survival probability of growing particles appears to be over 1. Therefore, if accurate, something else aside from NPF such as traffic has a significant contribution on the observed formation rate values. Some discussion on this and what are its implications for the results of this study, such as on the contribution of ion-induced nucleation on NPF, should be included.

46. Line 396-405: Some references to previous studies would be appreciated.

47. Line 405-406: “It would be reasonable to view …” I do not understand/follow the reasoning here. Please clarify.

48. Section 3.6 in general: I would suggest also including the formation rates of negative and positive ions separately (and not just the combined value) in the analysis/discussion.

49. Line 421: It should be clarified how the GRs of charged particles have been determined. As only one GR per size range is presented, I would assume that the number size distributions of negative and positive ions have been summed and from those a single GR value was derived. Similarly to Section 3.6, I suggest also including GRs of positive and negative ions in the analysis/discussion separately.

50. Line 427-428: “Contrary to …”. A reference is needed here.

51. Figure 3, 5, 6, 7: Please specify also that the lines are mean number concentrations for each hour.

Conclusions

52. Line 443: it could be stated here in the beginning of the conclusions what diameter ranges small, intermediate and large ions correspond to.

53. Line 442: I still do not understand this direct comparison of the concentrations in the different size classifications as the diameter range widths are completely different. I do not find the observation of large ions (7.5-22 nm) and small ions (0.8-1.6 nm) having similar concentrations meaningful as the former covers so much larger range of ion sizes compared to the latter.

54. Line 443: Unclear phrasing “Variable concentrations were observed …” Variable concentrations as compared to what? Additionally, observed suggest that something is observed in this study. A better phrasing would be “The concentrations of intermediate/large ions in this study were observed to be lower/higher than in some previously published studies, possibly linked to ..”

55. Please mention the measurement period in the conclusions section, for example in the beginning of the section. In addition, mention at least that a NAIS was used to measure the charged particle/air ion concentrations.

Technical comments

Line 54: … in the atmosphere, which …

Line 55: missing word. These ions can be …

Line 122: It should read “a NAIS”.

Line 171: Missing word after “Each”.

Line 178: “assumed to be sulphuric acid”

Section 2.4: the symbols denoting parameters, such as D, β, etc should be in italics in the text.

Line 241, 347, 465: “depending on polarity” does not clarify which value corresponds to which polarity.

Line 376: suggest replacing “source strengths” with “concentrations”.

Line 473: A missing word.

Table legends and figure legends (Line 723-): The table and figure numbers are wrong.

The manuscript by Rowell et al. studied the role of air ions during atmospheric new particle formation in urban Leipzig based on data collected from a one-month campaign. The authors investigated the features in air ions in relation to selected meteorological parameters, CS, BC and H2SO4 dimer on NPF days compared with those on non-NPF and undefined days as well as characterised their formation rates and growth rates. Although the work is based only on a short campaign, it is a valuable dataset worth publication contributing to the urban studies. However, the current manuscript has several defects that cause confusion regarding especially size range classification and neutral fraction definition. Also the one-month dataset cannot support the conclusion that ’ ion–induced processes play a minor role compared to neutral species in NPF at Leipzig–TROPOS’. Such general conclusion requires long-term measurements. I would also like to suggest that the authors take a closer look at the CI data, which could possibly help the elucidation of the precursor differences between undefined and non-NPNF days.  Adding further discussion on the impact of urban pollution on NPF at the site will make the manuscript more valuable. 

1. First of all, the authors stated that ’the air ion/charged particle population was mobility classified …’ but then gave size ranges in nanometers. It is confusing. Also size classification in Tammet (2006) is based on mass diameter. NAIS measures mobility diameter in the range of 0.8-42 nm. The authors stated that they followed the classification used by Tammet (2006). A mobility diameter of 0.8 nm is around 0.4 nm in mass diameter. So have the authors omitted the smallest ions? 
2. On P5 L123-124, the authors wrote ’neutral PNSD from 3–42 nm by their mobilities (3.2 to 0.0013 cm2 V-1 s-1)’. The mobility range and the size range don’t match. A mobility of 3.2 cm2/Vs is approximately 0.8 nm in mobility diameter. Also the sentence is confusing. NAIS measures in the mobility size range of 0.8-42 nm, which applies in both air ion and total particle modes. However, since corona charging is used in the total particle mode, data below approximately 2.5-3 nm are contaminated by the charger ions and therefore are not usable. 
3. On P6 L170, the authors stated that they used combined data from NAIS and MPSS to get neutral particles in the range of 3-800 nm. How was the neutral fraction obtained?Is there an ion filter in the MPSS?
4. P7 L202: ’small charged particles (0.8-1.6 nm)’, these are rather clusters.
5. P8 L214: the earth electrode effect is typically only pronounced at ground surface level. The data in this study was obtained from 4th floor. At this height, the earth electric field effect is small. The building’s wall may have an influence. How was the NAIS inlet constructed? The high mobility channel of NAIS may also suffer from electric noise. Are the concentrations comparable between polarities in indoor environment?
6. L283-285: NPF days have strong seasonal dependence. It is better to make comparison with studies in summer from other sites.
7. L321-322: ’charged particles may play a significant role in stabilising clusters’. It is confusing that particles could stabilise clusters. Please change charged particles to charges.
8. L374-375: Fig. 7 shows that BC on non-event days is comparable to that on undefined days.
9. L375-376: ’ Non-NPF days are possibly observed on these days due to low source strengths of precursors.’ The authors have access to the CI data which should be able to provide more details. 
10. L387: ’…and neutral particles (charged and neutral particles, combined; J3–7.5neutral and J7.5–22neutral) ’. The authors wrote ’neutral particles’ but in the bracket ’charged and neutral particles, combined’ . Are they charged or not? Or total particles?

     Also on P19, the authors sometimes discussed about total particles and sometimes neutral particles. Very confusing.

1. The study is based only on a one-month campaign. It is not evident enough to reach the conclusion that ’ion–induced processes play a minor role compared to neutral species in NPF at Leipzig–TROPOS’. The generalisation requires studies from long-term measurement. 
2. The reference list is messy. Please follow the alphabetic order and use the format of surname followed by abbreviation of given name. 

Other issues:

L111: change ’city’s weather’ to ’the weather of the city’

L200, L208,L286, etc.: Error! Reference source not found. Please check figures and tables.

L200-201: ’large’ is split. L287: ’variables’ is split.