Particle size traces modern Saharan dust transport and deposition across the
equatorial North Atlantic

van der Does, Michèlle; Korte, Laura F.; Munday, Chris I.; Brummer, Geert-Jan A.; Stuut, Jan-Berend W.

doi:https://doi.org/10.5194/acp-16-13697-2016

Articles | Volume 16, issue 21

https://doi.org/10.5194/acp-16-13697-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue:

The Saharan Aerosol Long-range Transport and Aerosol-Cloud-interaction...

https://doi.org/10.5194/acp-16-13697-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 16, issue 21

Research article

|

04 Nov 2016

Research article |

| 04 Nov 2016

Particle size traces modern Saharan dust transport and deposition across the equatorial North Atlantic

Michèlle van der Does, Laura F. Korte, Chris I. Munday, Geert-Jan A. Brummer, and Jan-Berend W. Stuut

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Final revised paper (published on 04 Nov 2016)
Supplement to the final revised paper
Preprint (discussion started on 02 May 2016)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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SC1: 'Short comment with questions about size definition and sampling method', Andrew Sayer, 02 May 2016
- AC1: 'Reply to Short Comment by A. Sayer', Michelle van der Does, 18 May 2016
RC1: 'Review', Anonymous Referee #1, 02 Jun 2016
RC2: 'Referee comments on manuscript by M. van der Does et al.', Anonymous Referee #2, 03 Jun 2016
RC3: 'Review of van der Does et al.', Anonymous Referee #3, 08 Jun 2016
AC2: 'Response to reviewer comments', Michelle van der Does, 01 Aug 2016
EC1: 'Editor’s Comment: Post-revision, van der Does et al., 2016 ACPD:', Joshua Schwarz, 01 Aug 2016

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Michelle van der Does on behalf of the Authors (25 Jul 2016)

ED: Referee Nomination & Report Request started (01 Aug 2016) by Joshua Schwarz

RR by Anonymous Referee #2 (03 Aug 2016)

Suggestions for revision or reasons for rejection

The authors provided a revised version of the manuscript and point-to-point responses to the referees' comments. The new manuscript is indeed improved, and the authors answered satisfactorily to most of my previous comments. I think that some of those replies should also be incorporated more directly into the manuscript, to enhance the comprehension for all future readers. Here are my final (minor) comments.

3 / 9. “We used time-series FROM submarine sediment traps ...”

3 / 10. “of 16 days. Here we ...”

3 / 25-27. Not completely accurate (e.g. McGee et al., 2013)

6 / 33-35. How are these two observations (one about a vertical profile, the other about a longitudinal gradient) connected?

9 / 10. I would suggest to explain already here the size distribution metrics that will be shown and discussed. For instance, explain that the modal diameter of particles-volume distributions was chosen to focus on the “main” dust mode, and that this particular metric does not account for the presence of shoulders. On the other hand you will show a complementary metric, i.e. sand%, to discuss the presence of shoulders.

13 / 12-15. This paragraph (together with the comment that deposition fluxes will be assessed separately) addresses my comment about giving at least an idea of the potential amounts of those large/giant particles. In their reply to my previous comment (8 / 11-15) the authors also seem to confirm that overall their counting statistics are high, since they have very concentrated samples (as opposed to ice cores). While in general I must agree with that, as expected, I still wonder if this is also the case for the tails in particular, which were the object of my concern. The microscope images indeed show the presence of those particles. I wish that the authors could provide some additional insight into the procedure to convince us that those are also a genuine signal from the water column (e.g. not contamination from the laboratory or similar). These data are are very promising, and the first of this kind, and many colleagues should be interested in those, therefore I think it's worth assessing potential problems in their interpretation, or more positively assuring their validity.

14 / 10. I would make it more clear in the text that this is one possibility, as the authors point out in their replies. E.g. something like “Assuming a consistent relation in the vertical profiles (as discussed above), we speculate that this difference may be related to a change in conditions in the the source areas.”

19 / 7-9. This sentence is unclear

19 / 19. Didn't you suggest this is unlikely, based on HYSPLIT?

19 / 20. I would suggest “Multiple-year samples from this transect AND COUPLED DEPOSITION FLUX MEASUREMENTS should clarify”

References:
McGee, D., P. B. deMenocal, G. Winckler, J. B. W. Stuut, and L. I. Bradtmiller (2013), The magnitude, timing and abruptness of changes in North African dust deposition over the last 20,000 yr, Earth Planet. Sci. Lett., 371–372, 163–176, doi:10.1016/j.epsl.2013.03.054.

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RR by Anonymous Referee #3 (13 Aug 2016)

Suggestions for revision or reasons for rejection

van der Does et al. present interesting results regarding deposition of dust across the Atlantic Ocean using moored traps. The potential for this study is significant, considering the cross disciplinary effort between sedimentology and atmospheric physics and transport. However, a dearth of information regarding the atmospheric component, even in this version, warrants further revision.

General comments:

I see that many of the authors’ responses to the reviewer comments, although sufficient in providing the necessary explanations, did not result in any additions to the manuscript. As in this case, should the authors need to explain themselves to the reviewers for clarity, the readers will certainly need and appreciate the explanations as well. I suggest the authors take their responses from the first revision and incorporate them into the manuscript more than they currently have.

I continue to question the speculation that all the particles sized were dust based on the information provided. As an aerosol chemist, I observe quite a bit of spatial and vertical heterogeneity in atmospheric aerosols, even when close to a source region. Without chemical analysis, I find it difficult to believe all the particles from this study were dust, even considering the chemical treatment methods used to reduce the contribution from other aerosol types. Considering the work adds another variable to what could influence the aerosol compositon, there could be more room for introducing complexity from additional particulate components from the ocean. With that said, there are a few things the authors could do to alleviate this issue as discussed below.

There are ways to support conclusions regarding aerosol type, such as the authors do with MODIS. However, MODIS also has its limitations. For instance, in Figure 10, AOD is clearly enhanced in a pathway from Africa, but these aerosols could be dust or emissions from biomass burning, which is especially predominant in the summer and is evident by the high AOD propagating off central Africa. Did the authors account for biomass burning aerosol? Have the authors considered adding CALIPSO to their analysis to evaluate aerosol type, at least between the coast of Africa and M1? This would greatly reduce the assumptions made regarding aerosol type.

HYSPLIT is a great tool, but when used cautiously. Typically, the error in a single trajectory results in 30% of its distance, meaning the error in where the air mass traveled can be quite substantial far from its initiation location. This can be alleviated by using an ensemble of HYSPLIT trajectories. However, only 4 trajectories are shown, which are also used to conclude in the abstract that transport occurs at higher altitudes and fast winds. Evidence supporting this conclusion is not provided, thus making It quite speculative. The authors do state in the responses that they did this for the year at M1 and it resulted in a mess of sources, however, these trajectories can be clustered. Showing clusters of trajectories, perhaps at various seasons and locations would increase the statistical significance and reduce the uncertainty in the sources and transport pathways, and better support their arguments.

It is true that sea salt is soluble, but mineral surfaces have been shown to become coated by these soluble species in solution (REFS) and at times can be an irreversible reaction, as been shown in cloud droplets (CZICZO). Considering these particles are much larger than aerosols studied in clouds, there is substantial surface area for these (sometimes) irreversible reactions to take place. How are the authors so sure that the treated dust particles are devoid of soluble components, or other types of particles?

Overall, providing more sound, empirical evidence in addition to more statistics on air mass transport would support the authors’ conclusion that what they are sizing is most likely dust. At the very least the authors should clearly highlight the assumption that these particles are dust throughout the manuscript, and provide some more background on what other particulate components could be contributing to the samples. Providing examples of previous studies evidencing dust as the major component in this region would also help.

Were any of the extracted dust samples weighed prior to and following chemical treatment to remove non-dust components? This would strengthen the argument that what was left was likely dust, i.e., if total mass concentrations decreased after treatment. Would also provide a more quantitative approach as compared to Figure 2.

The authors do not provide any discussion of or show uncertainty in the measurements in the manuscript. Please show error bars in the figures and clearly discuss the possible sources of uncertainty or variability in the measurements. For example, Figure 3 shows several markers close together, but it is unknown if these disparities are significant or simply within error. It would be more difficult to draw the conclusions discussed by the authors should these all be within error of each other.

There is quite a bit of redundancy in some of the figures (namely 7, 8, and 9). The authors should consider combining these, which will serve to eliminate the redundancy and a more direct comparison.

Specific comments:

Page 5, lines 11-12: Elaborate on this. Where was chlorophyll and salinity higher? This should perhaps be shown in a supporting information file, especially considering these can change greatly on a seasonal basis throughout the year.

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ED: Reconsider after major revisions (18 Aug 2016) by Joshua Schwarz

AR by Michelle van der Does on behalf of the Authors (13 Oct 2016) Author's response Manuscript

ED: Publish as is (18 Oct 2016) by Joshua Schwarz

AR by Michelle van der Does on behalf of the Authors (20 Oct 2016)

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Short summary

We studied seasonal and spatial variations in particle size of Saharan dust deposition along a transect in the Atlantic Ocean, using an array of moored submarine sediment traps. We show a downwind decrease in particle size, but seasonal changes are also prominent. In addition, the dust is much coarser than previously suggested and incorporated into climate models.