General comments

Touvinen et al use data from a year of field measurements to describe the development of intermediate size ions during new particle formation events. The goal is to determine the optimal size of ions to use for the characterization of local intermediate ion formation (LIIF). The data is well presented, and the paper is generally well written and easy to follow. As mentioned in the paper, new particle formation is an important topic and is still not fully understood, so new ways of looking at these kinds of events are welcome.

I am missing a discussion of why a characterization of LIIF is a useful tool for describing new particle formation events, as compared to looking at neutral particles in the same size range. Instrumentation is probably one good argument as it is typically easier to measure small/intermediate ions than neutral particles in that size range. And details of ions can also reveal information about the role of ions in the formation of the particles.

The main result is that ions of d~2.16 nm are best suited for studying local intermediate ion formation. It is perhaps not surprising that the smallest size range where a significant signal is seen is optimal, considering the “local” boundary condition, as made clear by Eq. 1 of the paper. Could it be phrased slightly more general, something like that the optimal size range is “the smallest size bin above the upper size range of small air ions”? And will the cut-off between small and intermediate ions always be around 2 nm or how chemistry dependent is that?

Overall, I think this is a fine contribution to the description of the behavior of ions during new particle formation events that can be published after a few elaborations and corrections.

Specific comments and technical corrections/suggestions

L14: --> “measured by the Neutral cluster and Air Ion Spectrometer”

L17: “Local” is mentioned several times and here described as “close proximity to the measurement site”. Is there a more technical way to describe what is meant by “local”?

L33: Specify the fraction.

L50: --> "contribute with most of the particle production"?

L89: Charged ions --> "charged particles" or simply "ions".

L90: Some neutral particle formation is bound to happen at the same time, at least from neutralization of charged particles.

L148-149: This line is confusing to read, I think it is supposed to say that “Simple linear calculations were made to illustrate the size dependency on how far a growing ion can be transported before being measured” or something like that?

L162: The reasoning for the size limits chosen probably belongs in the Methods section, at least I was wondering about it since line 131. The upper limit of 3.1 nm could be explained more. What is the “desired source area” (L164)? I suppose the 3.1 nm are found based on your eq. 1 using typical growth rates and windspeeds from the area, but these should be listed so the reader can better understand the reasoning behind the 3.1 nm. From the data in figs 1-4 the upper limit seems reasonable.

L253: What is the reason behind the larger positive ion concentration? Typically, small positive ions are larger than small negative ions (as you note in L284) so a shift in the spectrum between the two polarities could be an explanation. This should be possible to check from the NAIS data, I think.

L298: That the signal in negative ions is stronger than in the positive ions can be a sign that the nucleation is due to ion-induced nucleation (as described by some of the authors in Kerminen et al, JGR 112, D21205, 2007 and measured in Enghoff and Svensmark, J. Aerosol Sci 114 p13-20, 2017) which typically has a negative preference. This would in turn suggest that the negative polarity would be the better choice in most cases (with the exception e.g. of the cases mentioned in the beginning of Sect. 4).

L402: Should probably start with “Second” instead of “In addition” to make it clear that this is the second of the cases introduce din L395.

Figs 1-4: Thicker lines would be appreciated; it is hard to tell the difference between the types of dotted/dashed lines when printed.

In "Intermediate ions as indicator for local new particle formation", Santeri Tuovinen and co-authors argue that ambient ions with mobility diameters just above 2 nm are most suitable as indicators of new particle formation initiated locally (within kilometers). Their analysis is based on several years worth of data at a boreal forest site in southern Finland. Concentrations of smaller ions tended to respond to processes not related to sustained growth, whereas larger ions would be more susceptible to sources and processes farther afield.

The study has thus limited scope, but it presents a new perspective for ambient ion mobility size spectrometry data. And although only based on measurements at a single site, it is a prominent site in the field of atmospheric sciences and has ample statistics available. With that, I believe the study will be of interest to the expected readership of "Aerosol Research".

Overall, it is not too badly written either.

My main concern evolves around the often repeated formulation "characterization of LIIF" (and variations thereof). Pivoting around those tend to be unclear lines of reasoning -- increasingly so the farther the analysis proceeds. Similarly, I find the "locality" aspect insufficiently defined.

The analysis overall and general conclusions (if I understand them correctly) are interesting and worth reporting, but they are also really quite straightforward, given the convenient (yet likely justified) assumptions made. In light of that, the paper is considerably more elaborate than it needs to be, leading to some confusion to this reviewer (and increasing irritation) with increasingly more frequent vague formulations as the paper carries on.

My major comments are all related to that main concern:

The Abstract is generally concise yet informative. However it makes three uses of that rather vague term "characterizing LIIF" ... What is meant? ("identifying"?)

The Introduction is well structured, but there is some lack of clarity in the goals of the study. In particular at line 100 (L100), plus the rest of paragraph, I wonder again, what is meant by "characterizing LIIF"?  Differentiating LIIF from IIF? The latter being affected by "transported ions" (related to my minor comment on L96+, see below), whereas the former is not?

Results and discussion:

L157: "characterization of LIIF" again

L248, 249: "investigating evening cluster formation" and "characterization of IIF" ... As above, what do the authors mean by these? Identifying periods? Measuring rates of formation?

I also get the feeling here that we are just restating definitions. (The remainder of the paragraph further strengthens that feeling.) In this case <2nm meaning clusters, >2nm meaning II. The relation of either to onward growth (no for clusters, yes for II) requires looking at the concentration evolutions of multiple size bins. (As the authors are doing in this study, including references to previous works that made use of size distributions.) But why is it beneficial to examine, in which size bin on its own would concentration increases most likely relate to that onward growth? That is something that could be explained when motivating the study. Hence, this comment again relates to the hazy initial objectives.

L304: another example of those vague formulations: "used to characterize IIF" ... Would "indicative of IIF that relates to continuous growth traditionally associated with NPF events" work, for example?

L332-333 (and the remainder of the paragraph): To judge how "probable it is that [ions] can be attributed to LIIF", one would need to define LIIF. However, that has not been done. If I understand correctly, the goal is merely to associate ions with a start of growth as close ("local") as possible. Which of course, at least in the spirit of the transport analysis in this paper/section, is trivially the case for the smallest ions. The paragraph thus appears as an attempt to oversell that trivial circumstance.

The Section "Atmospheric relevance and applicability" is quite useful, as it raises awareness of possible spatial heterogeneities in processes that could affect ion size distributions. But similar instances of unclarity are encountered.

L378: I do not understand the statement "ions could also have been transported during their growth". Is not the whole premise of assessing the locality of IIF based on the transport of these ions during their growth (Section 3.2)? Please clarify. (The latter part of the paragraph makes me believe the suggestion is to consider effects of heterogeneities and possible direct emission sources on apparent growth rates?)

L407: "represent" is unclear. "estimate"?

The "Conclusions" are pleasantly concise, yet rather a "summary". The first sentence emphasizes my concern about the poorly posed objective.

Minor comments:

Abstract:

Could add reason for why negative ions better suited (~L22).

Introduction:

L38: growth to which sizes is considered required for "NPF"?

L44: What are "quiet NPF" and "traditional NPF event times"? Should be elaborated on or the sentence skipped.

L96 + paragraph around it: I find this part difficult to follow. Not sure I understand correctly what are considered "transported ions" vs. non-transported ions. Or there might be a mistake here. Please double-check, and ideally clarify.

Methods:

2.1, 2nd paragraph: could mention that the NAIS measured ions of both polarities simultaneously.

L128: Does that mean that new classification methods differ from old methods, and that old method was used here to define "NPF day"? If so, is there a reference for that specific classification method? That should be clarified if the classification or definition of "NPF day" will matter for this paper.

2.2, last paragraph: presumes/implies it is known when IIF takes (not) place from visual inspections ... What did that "visual analysis" consist of? Why was the statistical analysis performed if visual analysis was sufficient for identifying IIF? Or was it so that the "visual analysis" was performed on the statistical results? Also, if so, that "visual analysis" needs to be elaborated on.

Results and discussion:

L180: Could be added that the coinciding of statistically observed IIF with the times of NPF during "event days" will retroactively vindicate that assumption. (Or if later text states so, could refer to that later text here.)

L245: "cannot get an accurate view of the periods" is vague and confusing. Replace with clear and specific wording.

L340: missing "to" ... however: "included" in addition to what else? (Also, in a revised manuscript, I hope "characterization of LIIF" would be well defined by now or a different formulation be used.)

Atmospheric relevance and applicability:

The 2nd paragraph could be shortened considerably.

L405-407: This sentence ("Kulmala et al. (2020) recently ... particles and NPF.") is not useful. It does not seem to relate to the other parts of the text, while introducing new terms that are not explained. It is not necessary either. I suggest simply leaving it out.

Technical comments:

L14: missing "a"

L109, 110: missing "the", "The"

L204: missing word?

L209: "photogenic" should probably read "photosynthetic"

Fig. 1 caption: missing "a" ("a Neutral...") and "the" ("the SMEAR")

Also Figs. 2-5 and S1-S2. (And the missing "a" also in Fig. 7.)

For all these figures I would also consider thicker lines and possibly different line styles. At the moment, the different styles are difficult to tell apart from each other.

Fig. 6 caption: "The growth rate"

Fig. 6: suggest horizontal grid lines for better orientation