This manuscript presents a unique and innovative year-long dataset of particle number size distributions (1.2–480 nm) collected in the urban background of Milan using advanced instrumentation. Through statistical analysis of these data combined with meteorological and pollution conditions, the authors show that new particle formation (NPF) events are favoured under relatively clean atmospheric conditions—with lower pollutant concentrations, reduced condensation sink, and stronger ventilation—whereas stagnant conditions within the Po Valley inhibit NPF. The study is highly relevant and I recommend it for publication in ACP after considering the following comments.

Major comments

* My main concern is that the manuscript combines data from very different atmospheric conditions (e.g., varying levels of pollution, ventilation, and meteorological regimes) without sufficiently distinguishing between them in the analysis. By aggregating these diverse situations, the results risk being biased or leading to misleading conclusions, as the mechanisms controlling NPF occurrence and growth are strongly dependent on background conditions. For example, clean-air episodes driven by strong northwesterly winds are fundamentally different from stagnant periods within the Po Valley in terms of condensation sink, precursor availability, and atmospheric dynamics, but are these conditions more frequent in winter than summer when NPF is expected to be more frequent? I strongly recommend that the authors stratify their dataset according to representative regimes (e.g., different seasonal contexts or maybe clean vs. polluted and ventilated vs. stagnant,) and assess NPF occurrence separately. This would not only reduce potential biases but also strengthen the scientific insights and policy relevance of the study. Additionally, a more explicit discussion of the limitations and uncertainties associated with mixing these conditions would help clarify the robustness of the conclusions.

Minor comments

Section 2.2 – The manuscript states that different PNSDs are combined, but it is not specified which size ranges from each instrument are ultimately considered after corrections. For example, does the 15 nm data come from the NAIS or the SMPS? Similarly, is the 2.2 or 3 nm range taken from the NAIS or the nCNC? Figure 3 shows the median PNSD, but was this calculated only for periods when all instruments were operating simultaneously? Finally, since the NAIS was installed in a different building, the authors should discuss whether this could introduce uncertainties. Have inlet losses been quantified and corrected?

L146-148 – what about polystyrene latex particles (PSL) calibration and in situ intercomparison with a total CPC?

L176 – I recommend using the terminology “eBC – equivalent black carbon” rather than “BC – black carbon” (Savadkoohi et al., 2024).

L177-179 – The BLH is estimated using two different models depending on conditions, but the methods are insufficiently explained. Given that the ventilation index is a key parameter throughout the manuscript, further explanation of these methods and their limitations is required.

L195 – CET time is UTC+1 or UTC+2 depending on the period of the year?

L226 – “condensation sink (CS).”

L247-251 – “growth rate (GR)”. What is meant by “the days above the 80th percentile rank,” and why is this metric used instead of the daily GR? If this choice is motivated by uncertainty in GR, how does the uncertainty compare with that introduced by selecting only the 80th percentile?

Section 3.1 – Is the amount of data available sufficient to be representative of each season? Does it make sense to combine the size distributions when not all three instruments were operating?

Figure 4 – Is there a physical explanation for the decrease in concentrations in the 1–3 nm size range? In addition, figures should include the minimum and maximum values of the axes.

L309-314 – I recommend including particle number concentration values in Milan compared with other southern European cities. Additionally, consider including the N/BC ratio (as an indicator of the contribution of primary and secondary particles) to strengthen this section and compare with other locations.

L319-322 – Please use the term “total particle number concentration” consistently, and add “number” at L322.

L315-324 – From a reviewer’s perspective, this paragraph could be removed, as the discussion on whether the new air quality directive is appropriate is not sufficiently developed and does not fit within the scope of this manuscript. I think is not the place to open the question if the new air quality directive is appropriate or not.

L355 – GR values are means or medians?

L357 to Fig. 8 – Could J₃ be lower than J₇ because of the decrease in PNSD previously mentioned for Figure 4?

L375-377 – While CS is lower in summer than in winter, precursor concentrations and chemistry also vary between seasons. The conclusions drawn here are too strong given that different factors are not isolated (look major comment).

Section 3.4 – The anticorrelation between SO₂ and H₂SO₄ may be influenced not only by CS but also by radiation. Likely SO₂ is higher in winter, when nano ranking is lower(?). How much do the observed H₂SO₄ concentrations contribute to the calculated growth rates?

References

Savadkoohi, M., Pandolfi, M., Favez, O., Putaud, J.-P., Eleftheriadis, K., Fiebig, M., Hopke, P. K., Laj, P., Wiedensohler, A., Alados-Arboledas, L., Bastian, S., Chazeau, B., María, Á. C., Colombi, C., Costabile, F., Green, D. C., Hueglin, C., Liakakou, E., Luoma, K., Listrani, S., Mihalopoulos, N., Marchand, N., Močnik, G., Niemi, J. V., Ondráček, J., Petit, J.-E., Rattigan, O. V., Reche, C., Timonen, H., Titos, G., Tremper, A. H., Vratolis, S., Vodička, P., Funes, E. Y., Zíková, N., Harrison, R. M., Petäjä, T., Alastuey, A., and Querol, X.: Recommendations for reporting equivalent black carbon (eBC) mass concentrations based on long-term pan-European in-situ observations, Environ. Int., 185, 108553, https://doi.org/10.1016/j.envint.2024.108553, 2024. 

The manuscript “Ventilation and low pollution enhancing new particle formation in Milan, Italy” describes the characteristics of the particle number size distribution in an urban setting. The aerosol measurements are combined with meteorological measurements and modeling to provide context for the observed variations in particle number and size. Given that the measurements are made in an urban environment, there are implications for air quality management. I think that the manuscript is acceptable for publication following consideration of my comments below.

Comments:

• Overall: The manuscript provides a high-level overview of the measurements with most of the analysis centered on the percentile of NPF rank. This choice was probably made to keep the analysis and complexity at a reasonable level. However, given the seasonal variation that occurs in the physical drivers of NPF (precursor concentration, boundary layer height, potentially transport direction and/or clean vs stagnant conditions, etc.) and the fact that many of these drivers co-vary, this overview analysis potentially hides important details that would further advance “…our understanding of the state and behaviour of the atmosphere and climate” (ACP scope). I encourage the authors to consider if it would be more appropriate to consider the rank analysis after already segmenting the data based on a given condition (season, transport regime, or something similar). This would increase the scientific contribution and policy relevance of the results. In the absence of such an analysis, the manuscript must include a more comprehensive discussion about the potential biases/complicating factors of doing the analysis on the complete data set.
• Figure 2: Given the lack of data in the summer months, how robust are the conclusions for this time period? While this would be challenging to evaluate, the manuscript should discuss this limitation in greater detail and should consider if it is appropriate to “sell” this as a year of measurements or if only fall, winter, and spring should be considered.

Technical:

• Sect 2.2: Please describe the inlets (material, length, flows, etc.) that were used for the particle instruments. It would also be good to comment on the magnitude of the correction required in different size ranges for the inlet losses. This provides the reader with important details to understand associated uncertainty.
• Sect 2.2: Inlet cleaning is mentioned several times. Did the cleaning have any noticeable effect on the measurements? If so, is there any drift/bias associated with the measurements (due to time since cleaning)?
• Sect 2.4.3: Please specify if the condensation sink was calculated using dry particles or if assumptions about water were made.
• Line 262: Please provide additional information on the CHIMERE model (reference, version, etc.).
• Figure 4: Please include a color bar scale. Please consider changing the number of data points to equivalent days of data (or hours). This would be easier for the reader to interpret.
• Lines 315-324: This conclusion that measuring at a smaller cutoff size increases particle number is expected. While there is an important policy discussion to be had regarding the appropriate cutoff diameter, in the absence of further analysis, this section (and the associated figure) does not add to the analysis and could be deleted.
• Figure 7 and lines 348-349: That the sub 2.5 nm particles increase first followed by larger sizes, is not apparent from the figure. I suggest adding an inset or a second panel so that zooms in on the few hours of interest so that the reader can better visualize this trend.
• Figure 7: Please include details about the gaussian filter so that the processing steps taken on the data are more transparent.
• Figure 12: Please include a marker for the study site.
• Lines 451-452 and Fig. 14: The seasonal change in SO2 to H2SO4 conversion (i.e. due to increased OH) and not just CS could potentially explain these results. This analysis should be considered with greater nuance or should be removed as it does not add much beyond what has been presented previously for CS.
• I encourage the authors to consider depositing their data in a repository. Given the variability of NPF in urban environments that is discussed in the manuscript, it is clear that measurements in a single location are inadequate to understand the variability and controlling factors of NPF. Accessibility of data from various environments will aid in improving our understanding of NPF.