Review of López-Cayuela et al. 2024: FINE AND COARSE DUST RADIATIVE IMPACT DURING AN INTENSE SAHARAN DUST OUTBREAK OVER THE IBERIAN PENINSULA. PART I: SHORT-WAVE DIRECT RADIATIVE EFFECT

This study quantifies the dust direct radiative effect (DRE) in the short-wave range (SW) during a springtime dust episode over the Iberian Peninsula using data from five lidar stations. The authors emphasize the comparison of two distinct methodologies for estimating the SW DRE: one that directly calculates the DRE of dust, and another that calculates the DRE separately for fine and coarse dust, which are then summed to provide the total DRE of dust. The study highlights that the fine fraction, which primarily modulates the SW DRE, cannot be disregarded, contributing about 50% of the total DRE at the top of the atmosphere (TOA) and bottom of the atmosphere (BOA). The differences between the two approaches are linked to variations in the assumed asymmetry factor (derived from AERONET inversions) between the fine mode and total dust.

General Comments:

What sets this study apart from previous research is its comparison of the two different methodologies. In previous studies, the authors have used the methodology that separates the fine and coarse components, and in this study, they compare this approach with the more commonly used method based on lidar measurements to estimate the DRE. While the study is interesting—such separation can be very useful for evaluating DRE per size mode in dust climate models—I have several major comments that need to be addressed before considering the paper for publication.

The main novelty of the paper, as highlighted in the abstract, is the comparison of the two methodologies. However, the paper is very descriptive and fails to provide a comprehensive assessment of the causes behind the discrepancies between the two methods. Most of the paper (section 4.2 and associated figures) is devoted to describing the episode in terms of the evolution of the dust properties and DRE across the stations considered and comparing these findings with previous studies and other events in the region. Only a very short section (4.3) is dedicated to exploring the differences between the methods. While the differences are highlighted, they are not explored in detail, leaving the conclusions and implications of these results unclear.

Several aspects need to be considered in the analysis of the results to provide a comprehensive picture:

A key difference between both methods is the assumed asymmetry factor in the fine, coarse and total dust derived directly from AERONET inversions. These asymmetry factors are supposed to be internally consistent with other AERONET inversion products (PSD, fine and coarse mode AOD and SSA). In other words, one should be able to derive the asymmetry factor of the total dust derived from AERONET from the asymmetry factors of the fine and coarse components weighted by the AOD and the SSA of the fine and coarse components, respectively. If that is true, it may not be surprising that the DRE calculated using the asymmetry factors of the fine and coarse components together with the fine and coarse dust extinction from the lidar measurements differs from the DRE calculated directly from the asymmetry factor of the total dust and the overall extinction from the lidar measurements. Understandably, this difference seems to scale with the differences between the asymmetry factor of the fine mode and that of the total dust. All this points towards the lack of consistency between AERONET inversions (which are internally consistent) and the lidar retrievals (fine and coarse extinction). This aspect needs to be explored in much more detail. First, the AOD of the fine and coarse components from the AERONET measurements and the ones from the lidar measurements should be compared. To what extent the differences in the fine to coarse ratio of the AOD between AERONET and the lidars can explain the results? One potential sensitivity test would be to constrain the fine and coarse dust extinction (and the total extinction) of the lidars with the AODs of AERONET and then calculate the associated DREs.

The above is just an example of the multiple analyses that could be done to comprehensively understand the discrepancies between the two methods. In addition, there are other aspects that are not properly discussed: 1) are the coarse components of AERONET and lidar comparable given the potential different sensitivities to coarse and super coarse dust particles between active and passive sensors? This is particularly important in this case given that the differences in the retrieved fine and coarse components of the extinction may at least partly explain the differences between the methods. 2) To what extent the asymmetry factors for the fine component in AERONET are affected by anthropogenic aerosol in the boundary layer? Given this influence, is it wise to assign these asymmetry factors to fine dust?

All in all, my main suggestion is to reduce the more descriptive parts of the paper (section 4.2) and emphasize more on the analysis of the differences between the methods (through hypothesis testing) in section 4.3 along the lines highlighted above. More elaborated conclusions and implications should be considered.

Specific comments:

Line 29: How relevant is to mention this in the abstract? Who has ever said that the fine mode could be disregarded?

Line 45: Given the focus of the study better emphasize on the uncertainties in the SW (which are by the way quite high and very important given the strong SW forcing).

Line 50: Can you provide a reference for the relationship of heatwaves and dust instrusions?

Line 71: In which studies the effect of the fine dust fraction has been ignored? I think this is not true. You may mean that studies may have not separated the fine and coarse contributions. When using the total extinction, one is accounting for fine dust as well.

Line 80 to 84: Can you explain the relevance of this in a broad context. Why is this important?

Line 108: The assumption of vertically constant g in the fine mode could have profound implications given the effect of boundary layer aerosols. This should be highlighted here and in the discussion of the results.

General aspects of section 2.1: a more comprehensive description of the AERONET products is needed. Please consider my general comments here on the internal consistency of the products, the potential limitations for the coarse mode, and the potential effect of anthropogenic aerosols in the fine mode. Also, better describe the assumptions in the lidar retrievals (even if they are provided in other publications) in comparison to AERONET. It is a good moment to talk about the potential inconsistencies when using fine and coarse g from AERONET together with the lidar extinctions.

Line 154: Why it could be considered more precise? I do not understand why. This cannot be shown. For example, the potential inconsistency between the fine and coarse modes in AERONET and the lidars may make the more refine method even more uncertain.

Line 175: dust ageing is typically used for chemical ageing. Also, what do you mean by absences of uniform gravitational settling?

Section 4.1: This may be a section where to introduce as well the fine and coarse mode AOD from AERONET compared to the fine and coarse mode AOD from the lidar measurements. An analysis of the internal consistency of the g for fine, coarse and total dust may be also performed. This is important for further discussion in section 4.3.

Section 4.2: I find this section very long and too descriptive. The benefit of the comparison with other studies is rather limited given the differences in the events, AOD, height of the dust layers, etc. I think a comparison table between studies (a proballby limited to radiative efficiency) and a structure and concise discussion with some key aspect would be more informative. Consider also adding in the table estimates from dust modelling studies in the region.

Section 4.3: Please see my general comments to provide a comprehensive assessment of the differences between the methods. This section needs major rework and additional figures for the analysis.

Conclusions: reconsider the conclusions in view of the new analyses performed, and emphasize much more on the implications on a broader context (for modeling and radiometric measurements). What’s next? How can AERONET measurements and lidar measurements be better combined? How can we assess uncertainties?

Figure2: improve color scale. We cannot see the low values with this space.

The study presents the direct radiative effect of dust particles in the shortwave range over an extended dust intrusion event over the Iberian Peninsula. The study is interesting, in particular regarding the comparisons between a novel approach that quantity size-dependent DRE versus a commonly used approach that quantifies DRE based on total dust. The manuscript is well written, however a bit difficult to follow with all the different abbreviations. The manuscript requires some major revisions in the discussion and presentation of key findings before it can be published in ACP.

General comments:

Do and by how much the AERONET vs lidar AODs differ in terms of fine, coarse, and total dust aerosols? How would the differences between these two networks affect the conclusion of this study?

More critically, it is very descriptive in the presentation of the results, i.e. it presents the results in the tables and figures but does not discuss the potential reasons of these differences, e.g. between stations, between AERONET and Lidars, and especially between DRE calculation approaches, which is the main selling part of the study.  

Although it is very good that the findings are compared with previous estimates but is difficult to follow. Maybe a table could help to improve this?  

Specific comments:

Ignoring the fine fraction in DRE calculations in the previous statements is a strong statement and is misleading.

Line 108: What are the implications of having vertically constant g values, in particular in the BL or over clean vs polluted background sites?

Line 114: Can you explain how the dust extinction coefficients from POLIPHON are degraded?

Line 176: This definition is not used for aging. This is simply gravitational settling as described later in the sentence.  

This study quantifies the dust direct radiative effect (DRE) in the short-wave range (SW) during a prolonged dust episode over the Iberian Peninsula. The analyses were performed over five lidar stations. The study uses two distinct methodologies to simulate the SW DRE. One separately estimates the effect of fine and coarse dust particles, and one estimates the effect of the total dust. The study highlights that the fine fraction cannot be disregarded as it contributes to nearly half of the total DRE at the top of the atmosphere (TOA) and bottom of the atmosphere (BOA). The differences between the two methodologies are attributed to differences in the asymmetry factor between the fine mode and total dust components.

The manuscript requires some major revision before it can be published in ACP.

General Comments:

This study points out the comparison of the two different methodologies as its main highlight. The study is interesting and has potential implications for evaluating size-dependent DRE. The differences between the two methodologies are not explored in detail. While the planned future studies with dust episodes that exhibit higher variability in the fine-to-coarse ratio would provide additional insight, some considerations can be discussed more within this study.

What could be worth considering are the differences in AERONET and POLIPHON fine-to-coarse ratios and their impact on the DRE estimates. Additionally, a comment on the other aerosols present above the stations, particularly the boundary layer aerosols and their possible contribution.

How do the uncertainties of the POLIPHON retrieval affect the fine-to-coarse ratio and therefore the differences in the two DRE estimation methodologies?

Specific Comments:

Line 48: The way this is phrased does not clarify the association between dust intrusions and heat waves. Is it that the particular synoptic conditions are favourable both for the intrusion and the heatwave?

Line 71: The statement that the fine particles have been ignored in previous studies should be referenced. Additionally, in the following sentences, it is stated that they have not been explicitly ignored but considered as a part of the total dust. Perhaps what was meant was that the contribution of the fine particles separately has not been researched as extensively as in this study.

Line 103: What is the impact of using the AERONET retrieved asymmetry parameter, particularly as related to the boundary layer aerosols?