The manuscript needs revision.

The comments are categorized as general comments, specific comments and technical comments and can be read in the pdf attached

The manuscript by L. Dada and Coauthors provides a comprehensive analysis of the phenomenology and sources of ultrafine particles at a rural site in Switzerland. The study is supported by an in-depth analysis of the seasonality and diurnal cycles of meteo parameters, major trace gases, black carbon concentrations, as well as about new-particle formation events frequency and classification. All is nicely described in the main text and in the supplementary material (containing 26 additional plots).

The main science objectives is the assessment of the primary and secondary source fractions of total ultrafine (UFP), Aitken and nucleation particles. To this aim, the traditional BC method is employed, and compared to an innovative methodology based on thermo-denuded SMPS measurements, although the latter was deployed only for a short period of time. The Authors claim that consistent results were obtained by the two methodologies. They also acknowledge that both tend to produce low estimates for the primary fraction of the aerosol (lines 419 – 420). This is a little bit in contradiction with the Authors’ statement at lines 414-415: “we do not expect large uncertainties in our estimation of the primary particle contribution”. In this reviewer’s opinion, several low biases can arise from the BC method. The assumption about a constant N-to-BC ratio cannot hold in an environment where the contributions of different primary sources (biomass burning, traffic) vary during the day, as well as the contributions from transport vs fresh emissions. While BC mass is conserved during transport, N is not, as a consequence of coagulation processes. The choice of selecting a small N-to-BC ratio (Fig. S14A) as characteristic for primary particles guarantees non-negative fractions for the derived secondary particle concentrations, however it can lead to greatly underestimated concentrations of primary particles in conditions when N is high per unit of BC mass emitted. Fig. 2 shows that nucleation mode particle concentrations increase from 1000 cm-3 at nighttime to 5000-6000 cm-3 during the morning rush hour. According to the BC-method analysis, ca. 80 - 90% of such growth is accounted for by secondary particles (Fig. 5F), which is somewhat counterintuitive. It is true, as noticed by the Authors, that the peak in the secondary particles is delayed with respect to that in the primary fraction witnessing the occurrence of traffic-related secondary aerosols. Nevertheless, the very dominant contribution of secondary particles with respect to the primary one during the full evolution of the rush hour peak (Fig. 5F) is unexpected: not necessarily wrong, certainly quite noticeable. The BC method seems to do a better job in attributing the rush hour peak concentrations to primary particles in the Aitken mode fraction, leaving a flat diurnal profile for the secondary particle concentrations. However, the similar apportionment into primary and secondary fractions in Aitken mode aerosols between the cold and the summer season is also unexpected, because NPF is certainly much more prominent in spring-summer and should have some effect also in the large background particles range, and especially because biomass burning is largely reduced in the summer. Again, also this result is unexpected but necessarily wrong. In general, I think that Abstract and Conclusions do a bad job in highlighting the most controversial and innovative findings of this study and should be improved.

I suggest including a figure and a brief discussion about the diurnal trends of primary vs secondary UFP fractions during an average winter day, based on the results of the non-volatile particle method.

Minor comments:

The x axis is missing in Fig S13.

The title of the x axis is missing in Fig S14(A).

Fig. S18 (A) lack of both x and y axes.

The manuscript of L. Dada and coauthors sheds light on the contribution, seasonality and diurnal behavior of the sources of ultrafine particles in a rural area in Switzerland. They use novel methods to estimate the contribution of the primary sources to the total UFP and discuss adequately their limitations. They also provide a detailed analysis of the observed NPF events and their characteristics, and compare them to other similar studies, giving insights into the potential driving NPF mechanisms in the area. However, confirming these mechanisms would require incorporating future VOC measurements. Therefore, I recommend the manuscript for publication after the authors address the minor comments outlined below.

Specific comments:

Lines 42-44: Since you discuss the source apportionment studies focusing on PM composition, I believe it is worth mentioning that there are also valuable studies focusing on UFP source apportionment methods, which utilize the different patterns and shapes of the measured UFP size distributions. Some examples include the works of Rivas et al., 2020; Garcia-Marlès et al., 2024; Kalkavouras et al., 2024; Vörösmarty et al., 2024.

Section 3.3: It is not very clear how the event classification is being performed. Although the new tailored method (Fig. S3) is detailed and accounts for days difficult to interpret, it does not account for regional event days (since only local events are included). Do you first use the other methods mentioned (Dada et al., 2018; Dal Maso et al., 2005) for the initial classification, mainly of the regional events (banana plots), and then apply the new method for a more detailed analysis of the rest of the days?

Lines 312-321: The ammonia’s diurnal profile (Fig. 1D) during spring is very interesting. I guess the different behavior compared to the other seasons (earlier morning peak, as well as an increase during nighttime) is related to the fertilization process. It would be nice to comment on this a bit more. Why is the peak observed earlier compared to the other seasons? Why does the ammonia concentration increase during nighttime in spring and not during the other seasons? Does it have to do with the boundary layer development?

Section 4.2: What was the concentration of PM_2.5 (and/or PM₁) on average during the campaign? It is important to have an idea about the mass concentration when studying UFP because they can affect the particle number (for example by increasing the CS).

Section 4.3.1: The authors address adequately the limitations of the method. However, some studies have found that particles from NPF may contain non-volatile material in the examined temperature (300-350 ℃). For example Wu et al. (2017) reported that although a significant mass had evaporated after the NPF particles were exposed to a temperature of 300 ℃, they still had a non-volatile core that was measured by their instrument. I believe the authors should also acknowledge that this method may potentially confuse the non-volatile fraction of the secondary material with primary material (currently only the opposite is mentioned which is also valid).

Lines 450-452: This is interesting and implies that there are higher traffic emissions during spring and summer compared to the winter and autumn. However, this behavior is not observed in the NO₂ diurnal profiles (Fig. 1B) where summer and spring have smaller concentrations compared to winter and autumn. Could you elaborate?

Lines 497-499: Does the term “local events” include both regional events and events taking place on a smaller spatial scale according to your classification? I understand that by “local” you refer to NPF occurring on-site, with small ions appearing and growing to larger sizes. However, the word “local” is mainly used to describe NPF events taking place in a rather small spatial scale (Kerminen et al., 2018).

Lines 537-539: I agree with this observation and it’s interesting to see that ammonia (or amines) in this area is not the limiting factor. Could you elaborate on the statistical testing methods you used to confirm that sulfuric acid levels were significantly higher on NPF days compared to non-event days?

Line 626: In line 421 you mention that about 25% of the UFP during the winter originates from primary sources (I assume that this fraction is even smaller for the other seasons), leaving the rest 75% to be of secondary origin. However, you find (line 626) that the overall NPF contribution to the UFP was only 5.07%. How is the rest 60-70% of UFPs explained? Are they also of secondary origin but are transported from other areas (background)? What areas/nearby cities influence the site? Are they a result of chemical processing of background aerosol particles (both primary and secondary)? I believe that this remaining fraction of UFPs is significant and deserves some additional discussion.

Technical comments:

Line 36: Is this the correct citation of the Seinfeld and Pandis book? Maybe you mean 2016 (3^rd edition) or 2006 (2^nd edition)?

Line 101: Please replace “are” with “is”.

Line 137: Was this factor stable with time? What was its variation?

Lines 178-180: The authors should consider replacing the terms N₁ and N₂ with alternatives such as N_primary and N_secondary, respectively, to avoid confusion (both here and elsewhere), as these terms also represent the number of particles larger than 1 nm or 2 nm. Also, the total number of particles (N) refers to the total (combined NAIS corrected and SMPS) in the size range of 2.5 – 470 nm?

Fig. S10: Correct legend, the “x” is missing on the equation

Fig. S18A: The plot does not have axis labels and numbers.

Lines 520-521: I guess the CS values reported are missing an “ ×10^-3 ”.

References:

Dada, L., Chellapermal, R., Mazon, S. B., Paasonen, P., Lampilahti, J., Manninen, H. E., Junninen, H., Petäjä, T., Kerminen, V. M., and Kulmala, M.: Refined classification and characterization of atmospheric new-particle formation events using air ions, Atmospheric Chemistry and Physics, 18, 17883–17893, https://doi.org/10.5194/acp-18-17883-2018, 2018.

Dal Maso, M., Kulmala, M., Riipinen, I., and Wagner, R.: Formation and growth of fresh atmospheric aerosols: Eight years of aerosol size distribution data from SMEAR II, Hyytiälä, Finland, Boreal Environment Research, 10, 323–336, 2005.

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Kalkavouras, P., Grivas, G., Stavroulas, I., Petrinoli, K., Bougiatioti, A., Liakakou, E., Gerasopoulos, E., and Mihalopoulos, N.: Source apportionment of fine and ultrafine particle number concentrations in a major city of the Eastern Mediterranean, Science of The Total Environment, 915, 170042, https://doi.org/10.1016/j.scitotenv.2024.170042, 2024.

Kerminen, V.-M., Chen, X., Vakkari, V., Petäjä, T., Kulmala, M., and Bianchi, F.: Atmospheric new particle formation and growth: review of field observations, Environ. Res. Lett., 13, 103003, https://doi.org/10.1088/1748-9326/aadf3c, 2018.

Rivas, I., Beddows, D. C. S., Amato, F., Green, D. C., Järvi, L., Hueglin, C., Reche, C., Timonen, H., Fuller, G. W., Niemi, J. V., Pérez, N., Aurela, M., Hopke, P. K., Alastuey, A., Kulmala, M., Harrison, R. M., Querol, X., and Kelly, F. J.: Source apportionment of particle number size distribution in urban background and traffic stations in four European cities, Environment International, 135, 105345, https://doi.org/10.1016/j.envint.2019.105345, 2020.

Vörösmarty, M., Hopke, P. K., and Salma, I.: Attribution of aerosol particle number size distributions to main sources using an 11-year urban dataset, Atmospheric Chemistry and Physics, 24, 5695–5712, https://doi.org/10.5194/acp-24-5695-2024, 2024.

Wu, Z. J., Ma, N., Größ, J., Kecorius, S., Lu, K. D., Shang, D. J., Wang, Y., Wu, Y. S., Zeng, L. M., Hu, M., Wiedensohler, A., and Zhang, Y. H.: Thermodynamic properties of nanoparticles during new particle formation events in the atmosphere of North China Plain, Atmospheric Research, 188, 55–63, https://doi.org/10.1016/j.atmosres.2017.01.007, 2017.