The manuscript entitled 'Importance of hydrated aerosol particles for aerosol-fog relationships in the Italian Po Valley', submitted by Neuberger et al. focuses on investigating the relationship between ambient air pollution (namely, aerosols) and fog. The study was conducted in the Italian Po Valley, which is infamous for being one of the most polluted regions in Europe, with frequent occurrences of radiation fog. The authors attempt to quantify the impact of hydrated aerosol particles on fog microphysical properties and visibility.

This is an interesting study with a well-designed approach that brings new important results to our understanding of fog behaviour. The scientific outcomes are interesting, the topic is within the scope of ACP and in interest of this journal readership, and as such deserves to be published.

However, I strongly feel that the basic terms used in this study should be properly defined, ideally in the introduction, as they can currently be perceived differently by the audience. I would like to see the difference between hydrated and activated aerosols clarified, as well as the difference between droplet activation and aerosol activation.  Furthermore, it should be made clear that the authors are talking about radiation fog. This is specified in the title, but should also be stated explicitly in the text. For example, the statement 'the fog develops in calm and stable air, where supersaturation remains very low' (line 43) is not generally valid for fog formation, but is only valid for radiation fog.

Line 73 - The authors declare that 'pollution levels have declined in the last decade'. It would be very useful to present at least some indicators of ambient air pollution in the Po Valley to support this general statement.

Line 95 – The measurements were carried out between November 2021 and May 2022. The authors should explain why they selected this specific time period. For example, is it the period when the highest fog frequency can be expected?

Line 375 – It should also be noted what the shortest observed fog event was.

Line 476 - 'These past results from the Po Valley are generally consistent with our observations, although the supersaturation levels in particular seem to have decreased over the years.' Could the authors please discuss this in more detail, explaining the likely reasons for the suggested decrease in supersaturation? If possible, could they support this statement with numbers?

This manuscript investigates the link between air pollution and fog by emphasizing the role of hydrated aerosol particles in fog microphysics. Using an extensive measurement dataset from a field campaign in the Po Valley together with large‑eddy‑simulations (LES), the authors quantify how hydrated aerosols affect fog effective diameter (ED), cloud‑droplet number concentration (CDNC), liquid‑water content (LWC), and visibility. Their results demonstrate that incorporating hydrated particles is essential for interpreting observations in particular in polluted areas and must also be represented in numerical models. The study is well presented, provides novel insights into the influence of hydrated aerosols on fog, and falls within the scope of ACP.   I recommend publication after minor revisions. 

R1: Add a definition section for key terminology

I would suggest having a clear definition of the key terms in one place, very early in the manuscript: hydrated vs activated aerosols, activated/unactivated fog, activated droplets. the terms are defined, in different locations in the manuscript and it would be easier for the reader to have it in one terminology section

R2: Clarify the use of GFAS data

In section 2.3.3. it was shown and mentioned that the GFAS is most likely underestimating LWC, I would clearly state in line 319 that in the following LWC measurement from the PVM are used for quantitative LWC assessment and GFAS data are only used for relative trends and patterns. Otherwise readers might wonder, why GFAS data is shown in later figures.

Minor suggestions:

- L 211:  typo: “normalized root mean square error (normalized root mean squared error (NRMSE))”

- L 238: how is droplet activation defined?

- L340:  “until 14 UTC” on the consecutive day

- L355: S10 comes before other supplementary Figures in the text. E.g. S12 mentioned before S11. Please reorganize supplement figures to correct order

- L360: more discussion and explanation of Fig. S11 would be helpful here

- L374: It would be nice to also have fog duration in the comparison plot of Fig 3

- Figure 4a: it is hard to make out the interstitial k distribution in yellow. Maybe another colour works better