Review for Ding et al. “The Importance of Aerosol and Droplet Microphysics for the Properties and Life Cycle of Radiation Fog in the Po Valley”

Overall Evaluation:

This study uses large-eddy simulation to investigate the effects of aerosol and droplet microphysical processes on the life cycle and properties of radiation fog in the Po Valley, Italy. A suite of sensitivity experiments is conducted to systematically evaluate the impacts of aerosol physical and chemical properties, alongside droplet microphysics, on key fog characteristics—including fog formation and dissipation, atmospheric visibility, droplet number concentration, and droplet sedimentation velocity. Overall, the manuscript is well-structured, logically organized, and comprehensive in content, exhibiting good general quality. Nevertheless, the physical mechanisms underpinning several of the reported results lack sufficient in-depth discussion. I therefore recommend a Major revision, with a specific focus on strengthening the physical interpretation of the findings to further enhance the rigor and logical coherence of the manuscript.

Specific Comments:

1. Line 113, “Savre et al. 2014” should be “Savre et al., 2014”.

Line 184, the unit of the “standard deviation to 1.9”

2. Section 2.1:

1)        It is recommended that the authors add an introduction to the observational instruments. In particular, the measurements of aerosol size distribution, aerosol chemical composition, and fog droplet spectrum in the experiments should at least be described.

2)        The method for calculating hygroscopicity from aerosol chemical components should be stated (ZSR?).

3)        The term "hydrated particles" cannot intuitively represent unactivated haze particles, and it is recommended to use "unactivated particles" or “non-activated particles” directly.

4)        Lines 105 to 108: The authors regard the first peak with diameters less than 10 μm in the droplet size distribution as unactivated particles, which I find hard to agree with. Based on Figures 1 and 3, I am more inclined to treat the peak of the first bin (possibly the 2–4 μm bin) as unactivated particles, given the magnitude of water vapor supersaturation estimated in previous studies on radiation fog (e.g., Shen et al., 2018, https://doi.org/10.1029/2018JD028315; Wang et al., 2021, https://doi.org/10.1007/s11430-020-9766-4; Mazoyer et al., 2019, https://doi.org/10.5194/acp-19-4323-2019). Alternatively, the authors are requested to provide a more robust justification for regarding particles with diameters less than 10 μm in the droplet size distribution as unactivated particles.

5)        Lines 108–110: The authors states that there were two brief interruptions during the fog event, due to the rapid decrease in large droplets and LWC. However, the identification of the second interruption is confusing to me. It is not obvious from Figure 1a that it satisfies the author’s criteria of “rapid decrease in large droplets and LWC”. Could the authors please further clarify the criteria for identifying the second interruption, or provide a unified quantitative definition/indicator for “interruption”?

3. Lines 317 to 319 & Line 343:

When Na = 8000 cm⁻³, the simulated fog formation and dissipation times do not exhibit consistent characteristics. If the authors could further explore the possible reasons for this behavior, it would help make the analysis more rigorous.

A similar issue exists for the exception case of 𝑁𝑎 = 200 cm⁻³ mentioned in Line 343, and further explanation is also recommended.

4. Lines 328 to 329:

It is recommended that the authors further discuss the possible physical mechanisms behind this result to strengthen the interpretation of the findings.

5. Figure 9 & Figure 13:

Two sets of curves are shown in panel (a), but the legend does not currently distinguish between the fog top and fog base. It is recommended that the authors clearly indicate this information in the figure caption or legend to avoid ambiguity.

6. Lines 359 to 362:

The sentence attributes the asymmetry to changes in the hygroscopic growth factor associated with variations in 𝜅. However, the explanation mainly describes why increasing 𝜅 leads to lower visibility (and vice versa when 𝜅 decreases), without clearly addressing how this mechanism results in the asymmetric behavior highlighted in the text.

The authors should clarify how this mechanism specifically leads to the asymmetric response and provide a more detailed explanation to support the statement more effectively.

7. Lines 52 to 65:

Regarding the shape of the cloud/fog droplet size distribution, the following studies could serve as valuable references:

Wang et al. (2023, https://doi.org/10.1029/2022JD037514) and Zhang et al. (2025, https://doi.org/10.1029/2024GL111643; 2026, https://doi.org/10.1029/2025MS005410) developed a novel parameterization for cloud/fog droplet size spectra, and further investigated its impacts on fog microphysical processes, sedimentation characteristics and optical properties via WRF-LES simulations.

Review of The importance of aerosol and droplet microphysics for the properties and life cycle of radiation fog in the Po Valley, by Ding et al.

The paper investigates the importance of representing aerosol growth, activation and interaction with clouds for radiation fog. To do so, a set of Large Eddy Simulations (LES) are performed, with varying conditions, for a case study of a real fog event from the FAIRARI campaign, on February 18-19, 2022 in the Po Valley (Italy). Although studies of the impact of aerosols on radiation fog are not new, this one stands out because detailed observations of aerosols and droplets were available to constrain the model initialization and for its evaluation, and because it focuses on the importance of accounting for non-activated aerosols. The experiment design and results are clearly presented, and the paper is easy to read and follow. However, I have one major and some minor concerns that need to be addressed before publication.

Major comment

My most important concern is about the distinction between the non-activated aerosols and the cloud droplets. To me, this is the most interesting part of the paper, so I would like some more precision/discussion on the method and its quality and uncertainty.

In the model (lines ~135), it is clearly stated that particles with a wet diameter above 2 microns are classified as fog droplets, with a criterion on the activation size to differentiate between non-activated and activated aerosol.

In the observations (lines ~107), all particles below 10µm are counted as non-activated aerosols.

It seems to me that the two approaches are very different. If I made no mistake with the Köhler theory, a 10µm activation diameter corresponds to rather large dry aerosols, over 500nm, that are almost absent from the dry aerosol population (fig. 1b)? Could you elaborate maybe on the validity of this assumption, and the uncertainty it introduces when comparing observations and simulations in the next sections?

Does that change the possible interpretation of fig 3a, where the bimodal PSD is linked to the presence of non-activated aerosols? Could such bimodal PSDs also result from collision-coalescence starting from a population of small droplets, even in schemes that do not account for hydrated non-activated aerosols?

Minor comments

- Introduction: In the first scientific question, meteorological conditions are not really the topic here?

- The paper of Schwenkel and Maronga (2019) is very interesting and complementary to the work presented here. It is only briefly cited here, but probably could be discussed in more depth. First, the choice of the supersaturation treatment in the model may have an impact on the results. The Morrison and Grabowski approach is chosen here (line 123), do you think that the results would be different with another method? The comment about mixing ratio overestimation could also be expanded, as Schwenkel and Maronga show that very different results are obtained in their sensitivity tests, so is that really a common plague of bulk schemes?

- Case description lines 96-97: are 13.3-792 nm and 2-60 µm the observation limits for aerosols and droplets? And is that why a 2µm threshold is chosen for fog droplets in the model?

- Visibility calculation: why use the parametrization by Gultepe, instead of computing explicitly the visibility from the Koschmieder formula? Do you think this can be an explanation for the difference between the observed visibility and the visibility computed from the observed act acd+hyd particles in fig. 5d? Or is there an important effect on visibility of hydrated aerosols that are too small to be observed?

- Simulation set-up: it is very nice that you checked the impact of the time step on supersaturation. But I am also concerned about the domain size, is it large enough to resolve small-scale circulations that happen inside the fog layer, even in radiative cases? Have you checked if the reference simulation on a larger domain behaves similarly? The fog evolution on fig. 4 shows nothing wrong, so this might not be an issue.

- Is cloud droplet deposition at the ground parametrized? As for example in:

Katata G. Fogwater deposition modeling for terrestrial ecosystems: A review of developments and measurements. Journal of Geophysical Research: Atmospheres 2014;119(13):8137–8159.

Could that explain overestimation of liquid water content and droplet number concentration at the ground (lines 277-278)?

- Fog top height (and vertical structure): At lines 330+, the very similar fog top height in all simulations is discussed. But can there be an impact of the temperature nudging that represents the warm advection? This nudging also should be more precisely presented in sect. 2.3 (only adding one sentence, mostly about the altitudes where nudging is applied, and keep the figure in the supplemental material).

- Fig 7: clarify in the caption that Nc, Qc and Dc are for act+hyd particles?

- Fig. 13: have a colour scheme that depends on narrow / wide spectrum following the classification in fig. 11? A2N1 could be an orange shade, A1N8 a green shade?