Review of: The export of African mineral dust across the Atlantic and its impact over the Amazon Basin

By:

Wang et al.

General Comments:

This is a very well written paper that discusses several aspects of the Saharan dust transport towards the Amazon Basin, from its origin to the impacts over the South American rainforest. The use of the chemical transport model GEOS-Chem, constrained by observations, is a very interesting approach, since it allows the assessment of regions and features not possible by real world observations, while validation against real world observations assure the general accuracy of the simulations. This paper represents a valid effort to better understand the transport and impacts of the Saharan dust.

Some aspects about methodology and results of the comparison between AERONET observations and PMSD schemes might need some clarification. Between lines 185-187, you say that only observations dominated by coarse aerosols are used [(contribution of fine aerosol to total aerosol volume < 3%)]. But figure 3 also shows box-plots of the mass fractions of column integrated aerosols in the 0.1-1.0 micrometers size bin. In addition to that, according to figure 1, you also use one AERONET site on an island and at least two sites (in Marrocco and in Tunisia) not far from the coast. I wonder if sea salt contribution to coarse size aerosols will significantly affect the observations. And if yes, to what extent. If what the observations show are significantly affected by sea salt, extra care should be taken while drawing conclusions from the comparisons. Some clarification on this aspect of the methodology and how this could affect the results could be helpful.

The discussion about the dust emissions (section 3.1) feels incomplete. The winds being a major driver of the emissions is an important and interesting aspect, but it was already pointed out in several previous papers. The potential relevance of soil moisture for all regions except for region D, suggested by the significant negative correlations, is another important and interesting aspect but also a more novel one, which should be more highlighted and/or discussed (e.g. in the conclusions). Regarding the winds, I expected a wider discussion on the local and synoptical meteorological aspects which result in those winds. This is briefly discussed around line 268, where the emissions from central Sahel and west Sahel are mentioned. But Region A (west Sahara), referred to as the biggest dust source, is not even mentioned. 

The dust lifetime is presented in section 4, and the differences are justified mostly by dry deposition near the source and by wet deposition along the transport path. That is another interesting result, but I also feel it could have a wider discussion, especially regarding the aspects involving dry deposition. Different seasons will obviously have different meteorological and thermodynamic conditions and these different conditions will result in different structures of the dust plumes. I would expect this to be of big relevance for the dust lifetime. I would recommend the reading of “The Three-Dimensional Structure of Transatlantic African Dust Transport: A New Perspective from CALIPSO LIDAR Measurements” by Liu et al. (2012), and/or related papers. 

Specific Comments:

l. 36: Pg a^-1 is an unusual notation, I would recommend using (in this line but also in the rest of the manuscript) a more common notation like Pg yr^-1

l. 54: You wrote downwards, but I think you meant downwind?

l. 96: It would be nice if you included more information about where El Djouf is located. Either “El Djouf, between Mauritania and Mali” or “El Djouf, in western Sahara”.

l. 203: I think there should be a comma after “Amazon Basin”.

l. 300: Please include units.

l. 509: I think you meant “exists” and not “exits”.

l. 534: Units for the first values are missing.

l. 595: Maybe substitute “consistent” with “significant”.

General Comments:

The paper discusses the export of African dust across the Atlantic and its impact on the Amazon Basin, using mainly the GEOS-Chem model results and a few observations.

This paper discussed several aspects about the export of African dust across the Atlantic and its impact on the Amazon Basin. It provides many results and statistical numbers, which are mainly based on the GEOS-Chem model simulation and a few observations. Though it raised some interesting topics with a lot of analysis, the important scientific points are not focused and highlighted enough with strong evidence. In some places, the descriptions are unclear and not accurate enough. I suggest focusing on fewer aspects and providing stronger evidence through more observations or model sensitivity experiments. This would allow for a more interesting and well-focused study, rather than trying to cover too many aspects. In my opinion, further study of 1-2 sections in this paper can be a very interesting study and well-focused paper.

Furthermore, the study heavily relies on the GEOS-Chem model results and only uses a few MODIS or AERONET observations for model evaluation. Especially, the AERONET sites and their available data are not good enough in spatial and temporal coverages, the major results or those statistical results/values are mainly calculated from model results (GEOS-Chem), which means most of these conclusions are model-dependent. It is important to note that changing or switching to another global transport model or using other dust schemes can significantly alter the results, and the study's major conclusions could be affected. Therefore, instead of focusing on the exact model values, the study should provide more accurate estimates about the relative values. (e.g. how much of the contribution from dust compare with other aerosols on the Amazon Basin; It there any interannual variability about the contribution, how significant it is? Which are the major factors impacting on the interannual variability, Met. conditions in transport or in emission flux). Also, better to highlight the points and conclusions from the study, not just describe the values and figures.  

For the PMSD/PVSD, the paper mainly considers the coarse aerosols, see L186: what is the paper r definition of coarse aerosol here, diameter >1 um? If it is, I don’t think it can derive the sea salt aerosols, and how much of the impact from sea salt during this long-range transport has not been discussed. Therefore, the paper should clarify the definition of coarse aerosols and address the impact of sea salt aerosols during long-range transport.

I would suggest making substantially modifications before submitting it again based on following comments.

Specific comments:

1. The Abstract is not concise enough, somehow looks like introduction. I recommend revising the abstract to make it more concise and focused on summarizing the key points of the paper.
2. Section 2.1, this study is using GEOS-Chem to simulate dust, the descriptions about dust scheme and the major factor controlling the emission (from the formula of dust emission flux) need to be discussed in the section.
3. P6, L177: Why did the paper choose the 675 nm AOD from AERONET, not 550 nm, which is normally used for AOD comparison with observation?
4. 1: Which month?
5. 2: Is it surface wind or 10-m wind?
6. 3 and L343: The V12 looks quite comparable as the observation for bin 2 and bin3, while V12_C is much better for bin1 and bin4, how did the paper conclude that the v12_C agree better with the observation?
7. 4: The quality of this figure is not clear enough, which is difficult to distinguish each experiment, the lines are almost overlapped by each other.
8. L261: the impact of Met. Fields on dust long-range transport need to be separated as two aspects: 1) the impact on dust emission flux, which is mainly related to the Met. Fields associated with the dust scheme used in GEOS-Chem (The standard dust scheme in GEOS-Chem is the dust entrainment and deposition (DEAD) mobilization scheme of Zender et al.[2003], combined with the source function used in the Global Ozone Chemistry Aerosol Radiation and Transport(GOCART) model [Ginoux et al., 2001;Chin et al., 2004]as described by Fairlie et al. [2007]). So the paper need to get the real Met. Fields in the emission formula to determine the correlations, I think here it is 10-m wind and soil moisture, please make it accurate and clearly. 2) The impact on dust transport (include deposition), especially long-range transport, including vertical velocity, precipitation, and wind.
9. L272: what is the major Met. factor contributing to the significant emission decrease in 2013. The paper discussed the impact of the precipitation and other climate factors in the following descriptions by referring to some previous studies, but I would like to remind, for dust emission flux, only the 10-m wind and soil moisture are the major factors to impact on the emission flux (please double check the scheme formula), while the other Met. Fields or climate event are not directly impact on the emission flux in the emission formula, however, transport. If the paper would like to discuss the climate impact on those Met. Fields and transport, please use sensitivity experiments and provide more climate evidence from the model to validate it.
10. L359 and Fig. 5: The total AOD includes all the aerosol species, how can the paper get accurate estimate about dust AOD biases and its PMSD? I do agree that the modeled AOD is much lower than that of the MODIS observation, especially over the downwind areas, dust may be one of the reasons, but it cannot conclude how much of the contribution is coming dust. Also, V12 shows low biases over the dust source region, while the V12_C shows much larger AOD over the dust source region of western Africa, but it also underestimates the AOD in the downwind areas between 40-60W, why? I am not sure why did the paper use log scale here for figure e and f, can the paper explain that, zoom in the differences? It is difficult for me to quantify the exact values between these different PMSD on AOD.  How important is the small AOD differences (less than 0.02-0.03) over the ocean?
11. 6: Again, better to derive the contributions from other aerosols when the paper compares the dust concentration with the observed PM10.
12. Section 4, which PMSD scheme did the paper use in the analysis of this section?
13. Table S2, this is dry deposition or wet deposition?
14. Figure S4: I don’t think one seasonal average figure of column burden can clearly describe the accurate transport path. If the paper would like to discuss the transport path, please use a more accurate analysis of aerosol horizontal/vertical fluxes or divergence analysis, cross-section analysis with temporal evolution. Also, the transport path at different layers would be quite different (a lot of previous studies have shown that), please don’t get the conclusions without providing enough support. If the paper cannot provide evidence to support the conclusions, I would suggest not to include them in the paper. The paper has included many of these descriptions/conclusions without providing strong support from both model and observation analysis.
15. L428: Better to show the formula about the way to calculate the life time.
16. L438: How did the paper define/calculate the dust deposition flux and wet deposition ratio? I saw that the wet deposition ratio is not the largest in winter, which is different to the descriptions in L435.
17. Section 5: which PMSD scheme did the paper use in the analysis of this section?
18. Figure 9: I am confused about this figure, better to describe it with more details. For dust and sea salt, is it total concentration or coarse part? Also, what is the major points about this figure, I saw a lot of descriptions in the section with numbers/values based on the model results, I am wondering how much we can trust them and how did the paper use the observation to validate its performance since I did saw many biases between the observation and model in this figure? What is “Other” meaning in the figure? I can get the information about the “good performance” from the figure showing here.
19. Section 5.2 and Fig.10: can the paper describe how to calculate the dust events frequency and the interannual variation of dust events? Also, I really don’t know what is the point about analyzing them? Any interesting points that the paper would like to highlight here? This is dust intrusion pattern due to long-range transport. Also, for figure (c) and (d), which season, please clarify these details clearly in several places?
20. Section 5.3 and Figure 12: I agree this is a useful and interesting application to estimate the nutrient input based on previous studies or measurements. But I would like to emphasize, the accuracy of these conclusions should be based on how much we can trust the model results. In the other words, the model performance of dust needs to be validated as pretty good performance from different aspects, but I don’t think this part has been done well in the paper to provide enough evaluations.