The MS deals with the properties of NPF events and particle number concentrations in various size fractions at an urban background site in Lecce and at a costal location in central Mediterranean, southern Italy. It presents valuable results and conclusions and contributes to the growing knowledge on the atmospheric nucleation and consecutive particle growth phenomenon in this larger region. However, the MS could be and should be improved substantially in several ways before deciding whether it is acceptable or not for publication in the ACP. The corrections can hopefully be accomplished by a very careful and thorough revision of the present version.

Major concerns

1. The MS is too much of the descriptive character. Lots of simple statistical results are just supplied without interpreting them or putting them into appropriate frameworks or formulating clear conclusions or messages from them. Examples could be large parts of Sect. 3.2, lines 221–228 or Table 3. Further possibilities for improved interpretations could involve e.g. explaining and better comparing the seasonality of NPF events and diurnal concentration patterns of various particle number size fractions. Further important references on urban NPF could also be added to this purpose.
2. The SO₂ is often used in the existing interpretations (e.g. in Sect. 3.3). Despite the fact that 1) its photochemical oxidation to the key nucleating vapour of H₂SO₄ was shown to be slow, complex and of less direct influence on the NPF, and 2) the authors possess all necessary properties and variables for deriving the proximity value of H₂SO₄ (which is more directly connected to the process) either by the classical method of Petäjä et al. (ACP, 2009) without the scaling factor or by its improved estimation proposed in Dada et al. (ACP, 2020). The authors may want to amend this part, which could contribute to the improved overall quality of the final MS.
3. It was shown in several publications that the size range below 10 nm is crucial for identifying and characterising NPF events in particular in cities (e.g. Nieminen et al., ACP, 2018). The authors are asked to discuss how they avoided the limitations imposed by their relatively large measurable diameter of 10 nm in Lecce. For instance, how did this fact influence the share of the undefined days.
4. The frequency of missing days were relatively large around 22% at both sites. It is wondered how these days were distributed over the years or the measurement campaign since the NPF occurrence frequency showed a remarkable seasonal dependency, which could possibly impact on the representativity of the remaining days.
5. The reader can have the feeling at several places (e.g. in lines 154–160) that the nucleation or NPF processes and the particle growth process are not clearly distinguished. For instance, it cloud be clarified what the authors meant by “the temporal evolution of the events”.
6. It is not described properly how some important properties were obtained. An example could be the lines 222–224 where the CS is mentioned only very briefly. By the way, this (in a more detailed extent) should be shifted from the section Results and discussion to e.g. Sect. 2.3 since this is not their result. In this respect, it is also mentioned that the original NSF in Salma et al., ACP, 2017 was further developed, and its NSF_GEN and NSF_NUCL are more informative than the original form and should be used or at least mentioned. It is not clear (lines 225–228) how the start and end times of the growth events and more importantly, the geometric diameters Dp1 and Dp2 were derived and whether the latter were modal median diameters or something else.
7. Figure 4 (and possibly some others as well) contains too many lines and it is difficult to follow. In addition, it should be discussed whether a local time involving the daylight-saving time (clock change in the EU) or UTC+1 or else were used as the time scale. This could be related to the shift in the positions of the diurnal peaks in different seasons.
8. Line 239 and further: the phenomenon or process is somewhat more sophisticated. The authors perhaps want to include and discuss the NPF occurrence with respect to the ratio of sources and sinks of low-volatility vapours and not just the amount of CS alone.

Minor comments

9. Line 33: use either primary or emission (source).
10. The references should be ordered chronologically and not alphabetically, e.g. lines 36–37.
11. Some abbreviations are not explained, e.g. line 101: MPSS, or Table 3 W, Sp, S and A.
12. Line 105: is TSI Inc. really based in Rome, Italy?
13. Line 239: replace "discouraged" by not favoured or something similar.
14. What is the advantage of using a CNR4 net radiometer which measures the energy balance between incoming short-wave and long-wave far infrared radiation versus surface-reflected short-wave and outgoing long-wave radiation to measuring global or direct solar radiations.

This work reports long-term (4 years) measurements of particle number size distribution at two locations in south Italy. It aims to grow the knowledge on the regional new particle formation events in a large area. It is a complex and extended dataset and analysis, and the results will fit within the scope of ACP, being of interest for the international research community. However, the manuscript should be improved before it is published in ACP, and I would suggest some aspects to be considered in order to improve the manuscript and/or strengthen its impact.

Major comments

1. The manuscript is mainly descriptive, with some statistical analysis but no conclusions about the differences between both sites. I would suggest the authors to include some additional analysis to identify the factors that affect to the differences, specially on NPF events. The growth rate is not clear how it is retrieved or for which size range (see comment below), I would also suggest the authors to calculate the formation rate and include discussion about it. I expect to see some differences between both site on GR and formation rate, if there are differences, means that the vapours contributing to the formation and the growth are different at these sites. Also, I would suggest to include the analysis of H2SO4 instead of SO2 (solar radiation and CS are available and could estimate the sulfuric acid from proxies as Petäjä et al. 2009, ACP).
2. I recommend the authors to combine sections 3.4 and 3.5, and try to answer in these sections why there is that large differences in the event frequency in this two nearby locations? What promotes the regional NPF events? There is cases when NPF events are observed at both sites?
3. In the introduction the authors focus on the importance of regional NPF events, however, the manuscript lack of results and discussion on this topic. I would suggest to include more analysis on this topic, but if no further analysis, discussion, results are included about this, I would suggest to shorten that part in the introduction.

Minor comments

L12 – change “occurred” by “occurring”?

L89 – which different meteorological dynamics?

L94-95 – please rewrite this sentence

L95-97 – would move this sentence after L89 about dynamics and would add other sentence about meteorological dynamic at ECO site.

L103 – move this sentence with the next paragraph, where the authors present the quality control. Are the instrument routinely calibrated or psl checked or compared with total particle concentrations? Have the instruments been intercompared before?

L109 – multiple charged instead of negatively?

Section 2 – I would recommend to name this section “Measurements and methods”, then section 2.1 “Measurement sites and instrumentation” that unifies sections 2.1 and 2.2, and section 2.3 I would rename it as “Data analysis”, “methodology”, “methods”,… “Evaluation of NPF events”, I think is not the most appropriate. Include in this section the formulas for the CS that is later discussed.

L132 – the authors use frequently paragraphs of just one sentence, please avoid this.

Table 1 includes Events, Undefined and Non-event days, that sum the total number of days. However, line 134 says that there is a 78% of data coverage? How can classify more days than the data coverage (~0.78*TotalNumberDays).

L140 – “confirming what was already observed in Dinoi et al. (2021a)” I would suggest to rephrase, something like “showing similar results than those presented by a shorter measurement period presented by Dinoi et al. (2021a)”.

L146 – where these numbers come from?

L156-160 – GR is a quantity that depends on the diameter. Here the authors don’t define the diameter range where the GR is being retrieved. If the GR changes with time, probably because the diameter range change?

L162 – avoid the term “emission levels”, mainly because the authors are not really measuring emissions, only measuring atmospheric concentrations.

L166 – use the correct significant numbers, the table is correct. Same in the following paragraphs.

L176 – add space before ~3100

Eq2 – use subindex for E and NE.

L213-218 – I would add some references were this method has been previously used at different locations and compare how important NPFs are in other locations compared to those presented in this work (e.g., Bousiotis et al., 2021; Casquero-Vera et al., 2021; Thén et al., 2022). Are the values reported averages for the NPF time of for the whole day?

L240 – I would not say is surprising, if there is less CS, probably there is also less precursor vapors too…  

References

Bousiotis, D., Pope, F. D., Beddows, D. C. S., Dall'Osto, M., Massling, A., Nøjgaard, J. K., Nordstrøm, C., Niemi, J. V., Portin, H., Petäjä, T., Perez, N., Alastuey, A., Querol, X., Kouvarakis, G., Mihalopoulos, N., Vratolis, S., Eleftheriadis, K., Wiedensohler, A., Weinhold, K., Merkel, M., Tuch, T., and Harrison, R. M.: A phenomenology of new particle formation (NPF) at 13 European sites, Atmos. Chem. Phys., 21, 11905–11925, https://doi.org/10.5194/acp-21-11905-2021, 2021.

Casquero-Vera, J.A., Lyamani, H., Titos, G., Minguillón, M.C., Dada, L., Alastuey, A., Querol, X., Petäjä, T., Olmo, F.J., Alados-Arboledas, L.: Quantifying traffic, biomass burning and secondary source contributions to atmospheric particle number concentrations at urban and suburban sites, Science of The Total Environment, 145282, https://doi.org/10.1016/j.scitotenv.2021.145282, 2021.

Thén, W.; Salma, I.: Particle Number Concentration: A Case Study for Air Quality Monitoring. Atmosphere, 13, 570, https://doi.org/10.3390/atmos13040570, 2022.