Elucidating the mechanisms of atmospheric new particle formation in the highly polluted Po Valley, Italy. New particle formation (NPF) is a significant source of aerosol particles and cloud condensation nuclei in the troposphere, playing an essential role in air quality and climate. Frequent NPF events have been observed in heavily polluted urban environments, contributing to the aerosol number concentration by a significant amount. This topic is suitable for publication in this journal. However, the manuscript needs an improvement before publication. Therefore, I would like to recommend publication with 'major revision.'

1. Line 34: What exactly does organic refer to? The scope of organic is too large and needs to be clarified.

2. Line 30-41: More quantitative results need to be given. In lines 30-41, the quantitative results of this part are less, and the highlighted conclusion of this article is not given. Need to clarify the characteristics of NPF in the highly polluted environment? What are the mechanisms? For example, in lines 35-37, what was obtained using ion cluster measurements and kinetic model results? This statement is currently too general.

3. Introduction: This section needs to be reorganized. The summary of global NPF researches in the second paragraph is insufficient, especially the summary of NPF-related researches in polluted atmospheric environments. The importance and uniqueness of Po Valley is not adequately explained in the third paragraph. What makes Po Valley unique compared to other polluted areas? What are the differences, especially compared with relevant researches in China and India? In addition, what is special about the so-called highly polluted environment selected for this study? How does the highly polluted environment compare to studies in China and India? What are the existing NPF formation and growth mechanisms in polluted environments? How does this study differ from these previous studies?

4. In Section 2.3.2, Condensation sink, nucleation and growth rate calculations should give the calculation formula.

5. Sections 3.1 and 3.2 are merged.

6. Section 3.3: It is necessary to add some in-depth analysis of mechanisms. For example, what are the commonalities compared with research results in Shanghai, Beijing and New Delhi? What are the differences in the Po Valley's meteorological conditions, chemical compositions and aerosol background concentrations compared with these polluted areas? What impact do these differences have on the NPF incident? Is the growth mechanism of new particles in the NPF event in Po Valley similar to that in other polluted environments? What are the effects of being highly polluted?

7. The discussion in Section 3.4 is too simple and requires in-depth analysis to compare the similarities and differences in the generation and growth mechanisms under different atmospheric environments. In particular, the unique results of Po Valley need to be highlighted instead of simply comparing the differences in data values as currently done.

8. Line 268, "concentrations" should be "concentration." Pay attention to other similar singular and plural expressions in the manuscript.

9. Line 259, Please add citations for Beijing and Shanghai (59 cm-3 s-1 − 225 cm-3 s-1),

10. Line 344, The unit expressions in “GR=6.1 nm h-1” and “(1.0– 2.4) nm/h, 4.6 (2.9 – 5.8) nm/h, and 5.1 (3.8 – 8.8) nm/h” in line 323 should be consistent, which should also be noted in other parts of the article.

11. Conclusion. The authors should give unique conclusions on the formation and growth mechanism of NPF in the highly polluted environment of Po Valley, especially compared with other polluted environments. Needs to focus on giving quantitative results.

New particle formation (NPF) can play an important role in urban smog formation, and thus has a considerable impact on air quality and public health. However, it is still unclear when and where sulfuric acid nucleation will cease to dominate urban particle formation as sulfur emissions continue to decrease. This study investigated the characteristics and mechanisms of frequently occurring NPF events in the polluted Po Valley region through a 2-month measurement using a suite of mass specs and particle sizers. The authors first calculated particle formation and growth rates of NPF events based on the measured particle number size distributions. In addition, the authors discussed the role of measured gas-phase sulfuric acid, highly oxygenated organics, and base compounds (ammonia and amines) in particle nucleation growth, through comparisons with results from the CLOUD chamber experiments. Finally, the author concluded that sulfuric acid and base compounds remain the primary driver of the frequent NPF, while oxygenated organics may play a more important in particle growth.

The research topic of this paper is novel and has important implications of interest to a broad range of atmospheric scientists, the measurement techniques are state-of-the-art, and the results are comprehensive. Overall, this is a relevant study that fits well within the scope of the ACP. However, the way the results are interpreted and discussed needs minor revision to improve the clarity for non-specialist readers. Here are my comments:

1. The key point: My understanding is that the lower levels of condensation sink and amine concentration are the two main differences in the Po Valley, compared to previous urban measurements, such as those taken in China. And interestingly, these two factors offset each other, resulting in somewhat comparable particle formation rates. This is actually a very important message that emphasizes the need to further reduce amine emissions as we reduce aerosol pollution in both Europe and China. These differences were discussed in the text, but were less clear in the abstract. I would suggest that the author modify the abstract and conclusions extensively to highlight the key point of this work.
2. Other amines: In Fig. 3(b), most of the data points fall between DMA concentrations of 0.5 -1.8 pptv, which is close to the measured DMA concentration in Fig. 5(d). Is there any other evidence to support the conclusion that DMA is not sufficient? Or are there other amines measured by the H3O+ CIMS? If there is no strong evidence, I would suggest that the authors tone down this assertion.
3. Line 344: “…the higher formation rates at 1.7 nm (87 cm-3 s-1) may be the decisive factor to overcome the CS and determine if a growing mode can be observed leading to a classification of the day as an NPF with growth day.” This is more speculation than fact – other factors such as meteorological conditions may also play an important role. I would suggest that the authors tone down this assertion.
4. Line 352: “Acid-base clusters were not observed in monomer (SA1), dimer (SA2), or trimers (SA3), likely due to declustering effects in the APi-TOF instrument”. This is not true – pure SA3 is energetically more stable than SA-base clusters.