Review Reports - Distinguishing Saharan Dust Plume Sources in the Tropical Atlantic Using Elemental Indicators

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This paper reports the analysis of dust plume sources from Sahara Desert. Samples collected in a 25 day AEROSE campaign were analyzed by ICP-MS. The measured concentrations of different metal isotopes were used to identify the sources of dusts. The reported results are new and useful for future study of climate change and air quality monitoring. It is a good paper.

Author Response

Thank you very much for taking the time to review this manuscript. We value your feedback and have carefully considered your comments in the revised manuscript. Please find the corresponding revisions in track changes in the re-submitted file.

Reviewer 2 Report

Comments and Suggestions for Authors

his paper titled, "Distinguishing Saharan Dust Plume Sources in the Tropical Atlantic Using Elemental Indicators" by Yeager et al utilized Inductively Coupled Plasma Mass Spectrometry to determine metal isotope concentrations in airborne Saharan dust particulate using cascade impactors size-dependently sampler. They studied elemental indicators of Saharan dust plume sources during the 2015 Aerosols Ocean Sciences Expedition (AEROSE) in the tropical Atlantic. Moreover, back-trajectory analysis and NOAA satellite aerosol optical depth retrievals were used to confirm the dust plume sources. This study is in accordance with the aims and objectives of the journal "Atmosphere". It will aid the scientific community and policymakers related to Saharan dust plume sources to explore further avenues and adopt specific recommendations in the context of this case study considering dust plume sources. Therefore, I recommend this publication. However, there are certain minor comments which are very important to address which can further add value to the manuscript. These comments are as below:

1- Introduction. What are the traditional elemental indicators of Saharan dust plume sources? The authors should review the prior studies of elemental indicators about Saharan dust plume sources. What are the new take-home messages compared to prior studies?

2. What internal standard solution is used for the calibration quality in Inductively Coupled Plasma Mass Spectrometry (ICP-MS) analysis? What methods are used to eliminate the influence of background for determining metal isotope concentration?

Author Response

Thanks for your review. Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

MAJOR COMMENTS

The authors obtained a lot of promising and original experimental data,
but the methods of their analysis should be adjusted.

1. The authors used "10μm and 2.5μm pre-impactors" "to discriminate against larger non-dust aerosols", while it is known that long-range transport involves dust in the range of aerodynamic diameters of 3-7 µm [1]. Thus, perhaps the most interesting range of particles from the point of view of studies of natural dust sources is not used. Note that the data from the 6-stage impactor are more interesting from the point of view of studying anthropogenic dust sources.

2. The authors attempted to identify African dust sources by VIIRS-AOT coupled with corresponding 5-day backward trajectories. The method (“potential source area frequency”) proposed by the authors cannot reliably localize regions of dust sources, because simple weighting of backward trajectory residence time with VIIRS-AOT is applied. Over the past 40 years, a number of statistical trajectory methods have been developed for reliable (by sufficient long data series) localization of atmospheric pollutant sources.
If an atmospheric pollutant parameter related to one point (concentration and so on), the PSCF (Potential Source Contribution Function; see [2]) and CWT (Concentration Weighted Trajectory; see [3]) methods are most often used. If a parameter of atmospheric pollutant related to whole atmospheric column or its part (MODIS-AOT, “volume concentration” by AERONET and so on), the reversible method described in [4] was recently proposed. The PSCF method is most suitable for identifying sources using marker isotopes.
For example this method was used in [5] to determine long-range aerosol transport based on 2-stage impactor data and 5-day backward trajectories.

3. To localize dust sources on the base of VIIRS-AOT data, backward trajectories were calculated for the single height of 5 m a.s.l., while VIIRS-AOT refers to the entire atmosphere. If we take into account that dust is transported close to its sources within the atmospheric boundary
layer (ABL), the height of which can reach several km, it is necessary to calculate backward trajectories for several levels, or better for several dozen levels. When calculating sources from measurements in the ocean near the west coast of Africa, the dust can be assumed to move within the ABL.
Far from sources, for example in the mid-Atlantic or near the American coast, the entire lower troposphere (up to about 5 km a.s.l.) should be taken into account. The ABL height is very close to MLD (Mixed Layer Depth), which the HYSPLIT_4 outputs for each point of a backward trajectory and could be used for selecting the trajectories propagating within ABL.

The lack of consideration of overlying atmospheric layers is one of the reasons for the weak correlation of columnar M-AERI data and near-surface AL-27 data as it is seen from Figure 3.

Thus, it is necessary to calculate the sources using mentioned trajectory methods and then adjust the corresponding results, discussion and conclusions.

REFERENCES

1. Reid, J.S.; Jonsson, H.H.; Maring, H.B.; Smirnov, A.; Savoie, D.L.; Cliff, S.S.; Reid, E.A.; Livingston, J.M.; Meier, M.M.; Dubovik, O.; et al. Comparison of size and morphological measurements of coarse mode dust particles from Africa. J. Geophys.Res. 2003, 108, 8593. https://doi.org/10.1029/2002JD002485.
2. Y. Zeng; P.K. Hopke (1989). A study of the sources of acid precipitation in Ontario, Canada. Atmos Environ, 23(7), 1499–1509. doi:10.1016/0004-6981(89)90409-5
3. Hsu, Y.‐K.; Holsen, T.; Hopke, P. Comparison of hybrid receptor models to locate PCB sources in Chicago. Atmos. Environ. 2003, 37, 545–562. https://doi.org/10.1016/S1352‐231000886‐5.
4. Shukurov, K.A.,Simonenkov, D.V., Nevzorov, A.V., Rashki, A., Hamzeh, N.H.,
Abdullaev, S.F.; Shukurova, L.M., Chkhetiani, O.C. CALIOP‐Based Evaluation of Dust Emissions and Long‐Range Transport of the Dust from the Aral−Caspian Arid Region by 3D‐Source Potential Impact (3D‐SPI) Method. Remote Sens. 2023, 15, 2819. https://doi.org/10.3390/rs15112819
5. Shukurov, K.A., Shukurova, L.M. Source regions of ammonium nitrate, ammonium sulfate, and natural silicates in the surface aerosols of Moscow oblast. Izv. Atmos. Ocean. Phys. 53, 316–325 (2017). https://doi.org/10.1134/S0001433817030136

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MINOR NOTES
Since manuscript lines were not numbered I used descriptive references to manuscript's sections, paragraphs and lines.

Title

It seems it is useful to end the title with "...during AEROSE-2015 campaign".

Abstract

Line 10 from above: please change "AEROSE ‘15" to "AEROSE‘15".
Line 3 from below: please change "Sahara desert" to "Sahara Desert".

Introduction

The end of the first paragraph: it seems it is need to add references to
trajectory analysis of aerosol transportation as well as ones to investigations of Sahara Desert dust outbreaks, for example:

- Reid, J.S.; Jonsson, H.H.; Maring, H.B.; Smirnov, A.; Savoie, D.L.; Cliff, S.S.; Reid, E.A.; Livingston, J.M.; Meier, M.M.; Dubovik, O.; et al. Comparison of size and morphological measurements of coarse mode dust particles from Africa. J. Geophys. Res. 2003, 108, 8593. https://doi.org/10.1029/2002JD002485
- van der Does, M.; Brummer, G.‐J.A.; Korte, L.F.; Stuut, J.‐B.W. Seasonality in Saharan dust across the Atlantic Ocean: From atmospheric transport to seafloor deposition. J. Geophys. Res.: Atmos. 2021, 126, e2021JD034614.
https://doi.org/10.1029/2021JD034614.
- Das, R.; Evan, A.; Lawrence, D. Contributions of long‐distance dust transport to atmospheric inputs in the Yucatan Peninsula. Glob. Biogeochem. Cycl. 2013, 27, 167–175. https://doi.org/10.1029/2012GB004420.
- Reid, E.A.; Reid, J.S.; Meier, M.M.; Dunlap, M.R.; Cliff, S.S.; Broumas, A.; Perry, K.; Maring, H. Characterization of African dust transported to Puerto Rico by individual particle and size segregated bulk analysis. J. Geophys. Res. 2003, 108, 8591–8612. https://doi.org/10.1029/2002JD002935.
- van der Does, M.; Korte, L.F.; Munday, C.I.; Brummer, G.‐J.A.; Stuut, J.‐B.W. Particle size traces modern Saharan dust transport and deposition across the equatorial North Atlantic. Atmos. Chem. Phys. 2016, 16, 13697–13710.
- Gläser, G.; Wernli, H.; Kerkweg, A.; Teubler, F. The transatlantic dust transport from North Africa to the Americas—Its characteristics and source regions. J. Geophys. Res. Atmos. 2015, 120, 231–252. https://doi.org/10.1002/2015JD023792.
- Gutleben, M.; Groß, S.; Heske, C.; Wirth, M. Wintertime Saharan dust transport towards the Caribbean: An airborne lidar case study during EUREC4A. Atmos. Chem. Phys. 2022, 22, 7319–7330. https://doi.org/10.5194/acp‐22‐7319‐2022.

2.1. AEROSE Campaign Overview

The first section, line 2 from below: please indicate French and Brasilia scientific organization that also response for maintenance of the PNE experiment.

Figure 1's caption: please add names of the regions (El Djouf Desert, Sahara Desert, Bodele depression and so on) where the numbered PSA regions are located.

2.2.1. Cascade Impactor

The first section (and after), line 6 from above: please divide value and unit of aerosol size (correct "10μm" to "10 μm"). The duration and frequency of sampling, as well as air flow (volume of pumped air for one sample), and inaccuracy in sampling and particle size.

2.2.2. Isotope Analysis of Dust Samples

The first section, line 8 from above: please replace "18 mega Ω distilled water" with "ultrapure (18 MOhm) water".

2.3.1. Dust Event Classification

The third section: please indicate model and manufacturer (Cimel C-318?) of the used sun-photometer.

2.3.2. Source Region Quantification

The second section, line 3 from above: please remove "back-trajectory" because HYSPLIT-4 model is used for both backward and forward trajectory analysis, not only for backward one.
Please add a link to the VIIRS retrievals archive. Please indicate what meteorology (NCEP/NCAR Reanalysis, GDAS or others) was used.
The proposed PSAF metric is duration (since its unit is time) rather frequency (unit is per time). The second section, line 2 from above: please add a reference to HYSPLIT_4 model.

3.2.2. Correlational Analysis of Dust Sample Isotopes and Source Region Emissions

Figure 6: please add significance level (p value) for each determination coefficient (DC) in the sub-figures. Low DCs couldn't have high p in view of shortness of isotope data series. It is better do not consider DCs with low p in a signature.

References

9. Please correct "Atmosphere (Basel)" to "Atmosphere".

27. It seems by the journal's rules list of authors of this article should be provided fully (without "et al.").