Review

The manuscript present aerosol hygroscopicity measurements performed one year long at the Demokritos station located 8 km to the north east from the city center of Athens. The measurements present important new results, and therefore I suggest the paper for publication in ACP. However, in my opinion still quite a bit of work is to be done on the paper before the publication. Please find my detailed comments below.

Detailed Comments:

1. Line 32-33: not only as CCN but also as IN, and those play a role in the indirect effect as well
2. Line 48-49: as the kappa theory does not perfectly describe the water activity, you cannot report a certain kappa value for salts either. Kappa is dependent on RH and also particle size, so please say at which RH and D are those kappas are valid or give a range or say that it is an approximation
3. Line 59-61: please rephrase the sentence, link between particle hygroscopic growth and what? And you say that there are only a few long-term hygroscopicity studies, there are also quite a few on CCN activity (these also investigate hygroscopicity) please add these as well or say explicitly that you only mean the HTDMA studies here. And are you sure that there are only these long-term HTDMA studies available? Please check again. And by Sellegrini et al. 2014, don’t you mean Holmgren et al. 2014? (https://doi.org/10.5194/acp-14-9537-2014)
4. Line 82-84: is there already a paper on this custom-built HTDMA? If yes, please cite it. If there is not please add more details on the instrument regarding performance and calibration.
5. Line 81-92: please add the different flow values for the HTDMA
6. Figure 2: please double check the figure that you name and explain all parts of it: the first item with a nafion has a name: Aerosol, I guess it should be Aerosol dryer, the two DMAs could be labelled as well for better understanding. After the first humidifier and before the RH/T measurement no line is drawn. Half of the textbox of the CPC is missing.
7. Line 96: “were measured in parallel by the standard SMPS system of the Athens ACTRIS station” was this system moved from the Athens station to the Demokritos station and used there? Or was it operated not exactly where the HTDMA was standing? Not clear from the text.
8. Line 100: RH lower than 45%, is that enough? I always have at least lower than 40% in my mind, but even better if lower than 30%. Please comment on it.
9. Line 120: “an inversion algorithm applied to” -> “an inversion algorithm is/was applied to”
10. Line 122-125: for readers not familiar with the HTDMA inversion, it might be not well understandable. The readers who are familiar with the inversion, it is not necessary. So either explain it better with more details, mentioning at least that when you select a certain size in the first DMA, particles with other sizes go through the DMA as well, that you have multiply charged particles as well and so on… Or leave the whole thing and only refer to the inversion paper, where it is explained in details, and mention that.
11. Line 125: “x2” is it not usually “Chi” and not x?
12. Line 133-134: The two sentences after each other are repetitions with similar meanings, do you really want both to be there?
13. Line 135-137: Typo, bit of too many brackets here?
14. Line 182: some boardening? Or the complete boardening? Based on the average you just cannot tell anything about the mixing state. You could have always perfectly internally mixed aerosol and a changing GF with time which would result in a broad GF-PDF as well. You should clearly state this.
15. Line 187-189: Or another possibility is that BC is simply bigger than 30nm, and therefore whatever is in the nucleation mode is already little bit hygroscopic, and at the bigger sizes one could have the more hygroscopic material condensed on BC cores or even the pure BC particles as well.
16. Figure 4: the vertical black line is not defined.
17. Figure 4: D=250nm, there is a small peak at high GF of 1.9 or so. Please comment on it, what that could be, if that is a real peak with something highly hygroscopic or just measurement noise?
18. Line 218: “(fig. 5. Panel A).” Where does this reference belong to? If it belongs to the sentence before: the annual mean is not shown in the figure.
19. Line 218-219: this sentence is strange: distinct month-to-month variability, but no seasonal variability? What do you mean here? I do see a seasonal variability, For D>30nm higher GFs in spring/late spring, minimum in August then again higher towards the end of the year, and January is again low. 30 looks a bit different with not that pronounced and bit shifted minimum in summer but therefore maybe a higher amplitude of GF change.
20. Figure 5 and 6: I do not really see the reason to show both figures. To my opinion figure 6 is better suitable to discuss the seasonal changing of the hygroscopicity. And you could add the average values without any problem to the boxplot as well additionally. And make a third column for the kappa boxplot. Here you even see better that there is a seasonal variability in the hygroscopicity to my opinion.
21. Line 223-224: monthly average kappa? Are these not the yearly averages?
22. Line 225: “standard deviation” please change it to GF-PDF standard deviation
23. Line 231-232: sigma for the 250nm particles is as low in September as in August
24. Line 241-242: do you have an idea why February is so much different from January? Why only that month?
25. Line 245-246: “Aitken particles and the particles in the accumulation mode (D0 > 30 nm) and the particles in the accumulation mode” too many accumulation modes
26. Line 248-257: about the separation of the non- to slightly hygroscopic fraction from the moderately hygroscopic fraction. Selecting a constant GF of 1.12 as a limit means that for the different dry diameters you define a different hygroscopicity limit: for D=30nm GF=1.12 means a kappa of approx. 0.075, the same GF for a 250nm is approx. 0.048. I would suggest to define a kappa limit and calculate a GF limit for each dry diameter. Even if that this would not make a big difference in the results.
27. Figure 7, label for f: typo “>/<1.12” should be in subscript as well, not only GF
28. Line 280-281: “Specifically, for dry particle diameters D0 > 30 nm, the contribution of the non- and/or slightly hygroscopic mode was maximum in spring and minimum in winter.” ??? fGF<1.12 (Fig 7B) shows something completely different: minimum in spring, higher values in winter, maximum in August.
29. Line 282-283: “In the case of Aitken particles, the non-hygroscopic particles almost equal contributed to aerosol hygroscopicity with the slightly hygroscopic particles in all seasons except for spring.” sorry, I do not understand this sentence, or as I can interpret it, that is not seen in the graph, please clarify.
30. Line 285-286: “Specifically, the average number fraction of the slightly hygroscopic particles were 0.62, 0.80, 0.67 and 0.70 in winter, spring, summer and autumn, respectively.” Do you mean here the fraction of the moderately hygroscopic particles? Please check the naming in the complete discussion on Figure 7, it is very hard to follow this discussion. Maybe it is only coming from the confusion with the names of the different fractions.
31. Section 3.2: you only show average values for the diurnal variations. A box plot would include much more information here as well, e.g. for the mean GF, or at least add the standard deviations to the plots.
32. Section 3.2.: check again the naming of the GF<1.12 and GF>1.12 fractions! It is mixed up again at a lot of places, naming the fraction with GF<1.2 sometimes non-hygroscopic, sometimes non- or slightly hygroscopic and naming the fraction with GF>1.2 slightly or moderately hygroscopic. Hard to follow this section again.
33. Line 324: “whereas the minimum appeared at noon (GF < 1.3) (21:00)” ?? At noon or at 21:00?
34. Figure 9: how was the time period of the particles being externally or internally mixed exactly defined? And is this plot then showing really only the particles that were externally mixed (A) or internally mixed (B)? Or is it showing just the average for the mentioned time period, when you say, that mostly the particles were externally/internally mixed? Please be more specific here! And why do you show different things with the white circles in panel A and B?
35. Figure 10: Is there a reason why you only show the seasonally separated diurnal variation of the moderately hygroscopic fraction? And not for the average GF or for both fractions? If yes, please clarify!
36. Line 351: "The shape of the diurnal pattern of the larger particles (30, 80 and 250 nm)” only 80 and 250nm??
37. Figure 10: is the difference for both 80 and 250nm particles in the different seasons really significant? And also the diurnal variation? For me these curves look quite flat and similar in each season. Showing not only the average but rather a boxplot or standard deviations or doing some statistical test would help to decide on that.
38. Section 3.3: the same question which I have asked in the methods part (comment 7), were the SMPS measurements performed at the same station or in Athens? If they originate from the same place (the SMPS measured there where the HTDMA), then please ignore this comment, only state that clear, if not, then you cannot use the hygroscopicity data to describe the different size distribution peaks from another place, and with that this complete section is not valid to my opinion. But only then.
39. Figure 11: why not to include the GF values in this figure instead of having them only in the supplementary?
40. Figure 11: it looks for me that the different GF values are quite stable for all clusters, and even the number fractions of the two hygroscopic modes does not vary too much. Is there really a significant difference between the hygroscopicity of the different size distribution clusters? Like GF_50_2 changes between 1.19 and 1.23 if you look at the different clusters. Please provide some analysis, tests there instead of mentioning some selected GF values in the text. An idea would be also to show the average GF-PDFs for the different clusters and compare them. One should see there better if there are differences or not.
41. Supplementary tables: some description is missing there. Like what the different abbreviations mean? Like GF_50_1 and GF_50_2. Please add an exact definition to each value.
42. Line 458: something went wrong with the formatting of this reference
43. Overall, quite a few sentences are a little bit hard to follow in the manuscript, I was not always sure what the authors meant. It would be nice if a language edit could be done prior to publication.
44. You present kappa values in the manuscript but do not discuss them a lot. I would suggest to add a more detailed discussion on kappa values, maybe compare it to what other studies found as well.

 Recommendation

The paper presents very valuable and rare 1-year data of hygroscopic growth factors of aerosol particles as function of size in a new environment, namely an urban background area in southern Europe. I recommend to accept the paper upon major revisions.

Summary of recommendations for major revision:

The paper misses GF calculations for 100-150 nm diameter. Please give reason for this. I acknowledge the difficulty in these kinds of measurements, but I require an honest explanation of the missing size range, which is very important for cloud condensation nuclei. Otherwise, the reader might think there was a scientific reason to leave these measurements out.

The analysis of GF is much too long and should be substantially shortened in number of figures and in the analysis, which requires a substantial new layout of the written text. The paper should be shortened by at least 1/3 of its current size in the number of words, and at least a half of the figures and tables should be removed in the paper and supplementary information. The readability is very hard at the current state and contains repeatability of similar messages (although shown with new types of results and analysis approach).

Some of the analysis of the results contains rather speculative discussions on the reason for high or low GFs. A more detailed analysis with trajectory data, wind speed data, and other meteorological data and a detailed look on individual days with particle number size distribution data is needed to reach firmer conclusions. However, without compromising the obligatory shortening of the paper.

Detailed revision and comments

Length of paper:

Several of the figures could be removed (as a very good example Figure 10), which also goes for the figures in the supplementary information. For the analysis, one could for example mention the GF for the first time for all diameters at the same time. Then, one could focus on each individual dry particle diameter and summarize the findings around this particle size, and not mention all the different parameters and circumstances around it (GF, GF PDFs, sigma-values, diurnal variation, seasonal variation, less, intermediate, more hygroscopic modes, relative number fraction of the different hygroscopic modes, meteorological influence, and so on) if it doesn’t give new substantial information. Alternatively, the authors can choose another strategy as well for the shortening of the text and removal of figures.

Due to the length of all information, it is very difficult for me as reviewer to make a decision on which figures and analyses to shorten. The authors are more familiar with their own data sets and results, and hence I leave it to the authors to make this prioritization.

Analysis of GFs:

The reason for some of the interpretation of GFs is sometimes speculative. How is it even theoretically possible that the 30 nm diameter particles are more hygroscopic than the 250 nm particles if they come from traffic exhaust? Normally one would expect very high number fraction of hydrophobic particles from relatively fresh fossil fuel combustion at around 30 nm in an urban area (e.g. Guo et al., 2020, https://www.pnas.org/doi/full/10.1073/pnas.1916366117; Titta et al., 2010, doi:10.1016/j.atmosenv.2009.06.021, Kristensson et al., 2013, https://aaqr.org/articles/aaqr-12-07-oa-0194 and many others). You have provided some context to this, explaining that some of the 30 nm hygroscopic particles might come from new particle formation events and that the traffic exhaust particles are aged. But you have to provide more detailed analysis to be able to come to this conclusion: Trajectory analysis (trajectories can be downloaded for free from the Hysplit site) if the air really comes from Athens and under what weather conditions and how long time it took for the air to arrive to the site from Athens and the phochemistry activity with meteorological parameters (for ageing purposes), and closer look at individual size distributions on individual days to see if it resembles a traffic exhaust particle size distribution, or new particle formation or something else. It is not enough to look at the average size distribution of clusters like in Figure 11, since the averaging of several size distributions might mask the shape of the individual size distributions. Besides, a look on an individual day would reveal if it is a new particle formation event day or not.

A closer look on all of the clusters in Figure 11 for individual days is also necessary to make correct conclusions. To me it seems that the interpretation of the sources of different clusters and their typical sizes is not correct or highly speculative. Based on the size distribution shape, the diurnal variation and the wind direction doesn’t lead to the conclusions about the origin of the clusters. For example, the second cluster is not at all nocturnal, and even seems to be more of a traffic exhaust related cluster than cluster 1 due to the association with morning and evening hours, which could be representative of morning and evening traffic. Maybe, the clustering does not even give a valid representation of different representative aerosol types. Maybe you should consider to abandon this analysis and make a manual analysis instead of how air masses influence the size distributions and in turn the GFs as suggested previously?

Another example of inconclusive interpretation is the sub-10 nm diameter particles log-normal modes that you present in Figure 11. You have to take a closer look on the possible sources of this mode: Is it particles from traffic exhaust that have nucleated some time after the emissions? It is probably not primary emitted traffic particles in Athens, because the maximum for such a mode should be significantly higher than 20 nm diameter at the time it reaches the site after 1 hour of ageing or similar. Again, a closer look at size distributions in connection with trajectories could reveal the reason for their appearance. Please also explain cluster 5 in a clearer way, it seems to contain contradictory information about the sub-10 nm diameter mode when speaking about cooling of exhaust gas.

Grammar:

Some grammatical improvements can be made, for example: “As a direct effect, aerosol particles interact with solar radiation through light absorption and scattering, inducing a positive or negative radiation forcing”. “As a direct effect” sounds strange. Another example: “The hygroscopic properties of atmospheric particles are strongly related to particle chemical composition (Gunthe et al., 2009; Gysel et al., 2007), while they undergo continuous changes over particle lifetime”. Why do you write “while” in this sentence? These sentences sound a bit strange, and such are found throughout the paper. This needs to be corrected.

Comprehension:

Chapter 2.3.3 is hard to understand. I know what you mean, since I have been doing similar things. But, not sure that people who haven’t done this before will understand your method approach. Please describe it in a few more sentences to make it clearer.