Preprints
https://doi.org/10.5194/egusphere-2025-2387
© Author(s) 2025. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/egusphere-2025-2387
© Author(s) 2025. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
30 Jul 2025
| 30 Jul 2025
Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).
Ventilation and low pollution enhancing new particle formation in Milan, Italy
Abstract. New Particle Formation (NPF) is a crucial process that significantly affects the number of atmospheric particles, forming a substantial portion of the total aerosol population. Therefore, it has important implications for both human health and climate. While extensive research has been conducted in rural areas of the Po Valley, Italy, there is a substantial lack of continuous measurements with state-of-the-art instruments in Milan, one of the most industrialized and densely populated cities in the region. This study aims to address this gap by analysing one year of detailed particle number size distribution measurements between 1.2 and 480 nm at an urban background site in Milan. These data were used to examine the occurrence and characteristics of NPF and to identify how the meteorological and air pollution conditions affect it. We show that a cleaner atmosphere, meaning lower concentrations of air pollutants and lower condensation sink, and a higher ventilation promote NPF. Detailed modelling of the air masses history further revealed that a longer residence time in the Po Valley and a greater exposure to anthropogenic emission sources inhibit NPF. Furthermore, we show that strong winds, particularly from the northwest sector (e.g., Foehn winds), facilitate NPF, likely by reducing the condensation sink for precursor vapours. This locates Milan among the urban sites where atmospheric cleaning enhances NPF, providing insights for urban air quality management.
How to cite. Agrò, M., Bettineschi, M., Melina, S., Aliaga, D., Bergomi, A., Biffi, B., Bigi, A., Ciarelli, G., Colombi, C., Fermo, P., Grigioni, I., Kerminen, V.-M., Kulmala, M., Lampilahti, J., Marinoni, A., Oliewo, C., Sulo, J., Valli, G., Vecchi, R., Petäjä, T., Lehtipalo, K., and Bianchi, F.: Ventilation and low pollution enhancing new particle formation in Milan, Italy, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2025-2387, 2025.
Competing interests: Some authors are members of the editorial board of journal Atmospheric Chemistry and Physics.
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Institute for Atmospheric and Earth System Research / Physics, Faculty of Science, University of Helsinki, Helsinki, Finland0014,
Manuel Bettineschi
Institute for Atmospheric and Earth System Research / Physics, Faculty of Science, University of Helsinki, Helsinki, Finland0014,
Silvia Melina
Department of Chemistry, University of Milan, 20133, Milan, Italy
now at: Ecogeo s.r.l., 24122, Begamo, Italy
Diego Aliaga
Institute for Atmospheric and Earth System Research / Physics, Faculty of Science, University of Helsinki, Helsinki, Finland0014,
now at: Department of Environmental Science, Stockholm University, 11418 Stockholm, Sweden
now at: Bolin Centre for Climate Research, Stockholm University, 11418 Stockholm, Sweden
Andrea Bergomi
Department of Chemistry, University of Milan, 20133, Milan, Italy
Beatrice Biffi
Air Quality Department, Regional Agency for Environmental Protection of Lombardy, Milan, 20124, Italy
Alessandro Bigi
Department of Engineering, University of Modena and Reggio Emilia, Modena, 41125, Italy
Giancarlo Ciarelli
Institute for Atmospheric and Earth System Research / Physics, Faculty of Science, University of Helsinki, Helsinki, Finland0014,
Cristina Colombi
Air Quality Department, Regional Agency for Environmental Protection of Lombardy, Milan, 20124, Italy
Paola Fermo
Department of Chemistry, University of Milan, 20133, Milan, Italy
Ivan Grigioni
Department of Chemistry, University of Milan, 20133, Milan, Italy
Veli-Matti Kerminen
Institute for Atmospheric and Earth System Research / Physics, Faculty of Science, University of Helsinki, Helsinki, Finland0014,
Markku Kulmala
Institute for Atmospheric and Earth System Research / Physics, Faculty of Science, University of Helsinki, Helsinki, Finland0014,
Janne Lampilahti
Institute for Atmospheric and Earth System Research / Physics, Faculty of Science, University of Helsinki, Helsinki, Finland0014,
Angela Marinoni
Institute of Atmospheric Sciences and Climate, National Research Council of Italy, Bologna, 40129, Italy
Celestine Oliewo
Department of Engineering, University of Modena and Reggio Emilia, Modena, 41125, Italy
Scuola Universitaria Superiore IUSS, 27100, Pavia, Italy
Juha Sulo
Institute for Atmospheric and Earth System Research / Physics, Faculty of Science, University of Helsinki, Helsinki, Finland0014,
now at: School of Earth and Atmospheric Sciences, Queensland University of Technology, Brisbane, 4067, Australia
Gianluigi Valli
Department of Physics, University of Milan, 20133, Milan, Italy
Roberta Vecchi
Department of Physics, University of Milan, 20133, Milan, Italy
Tuukka Petäjä
Institute for Atmospheric and Earth System Research / Physics, Faculty of Science, University of Helsinki, Helsinki, Finland0014,
Katrianne Lehtipalo
Institute for Atmospheric and Earth System Research / Physics, Faculty of Science, University of Helsinki, Helsinki, Finland0014,
Atmospheric Composition Research, Finnish Meteorological Institute, Helsinki, 00101, Finland
Federico Bianchi
Institute for Atmospheric and Earth System Research / Physics, Faculty of Science, University of Helsinki, Helsinki, Finland0014,
Short summary
This study investigates New Particle Formation (NPF) in Milan, the most populated city in the Po Valley (Italy), using one year of particle number size distribution data (1.2–480 nm). NPF is enhanced under cleaner air conditions with lower pollution, reduced condensation sink, stronger ventilation, and stronger northwesterly winds (e.g., Foehn events). In contrast, longer air mass residence time in the Po Valley and higher air mass exposure to anthropogenic emissions suppress it.
This study investigates New Particle Formation (NPF) in Milan, the most populated city in the Po...
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.