the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Nov 2025
Late autumn aerosol trace element composition and source tracking over the Southern Mozambique channel
Abstract. The southern Mozambique Channel (20–30° S) receives a range of atmospheric influences, from desert dust and fire emissions through to industrial, mining and agricultural emissions, emitted from both Madagascar and southeastern Africa. Our study characterises the trace element composition of aerosols collected between the south of Madagascar and Durban, South Africa during the low dust season. Dust deposition fluxes (40–263 mg m-2 yr-1) calculated fell within the lower range of modelled fluxes estimates, confirming the absence of major dust or fire events during the study. While prevailing air-masses affecting our samples were modelled to originate from long-range particulate transport over the Southern Ocean, a holistic understanding of our sample composition could only be obtained when accounting for sporadic aeolian inputs from the two local landmasses. Notably, we found surprising high levels of Cr (4 ± 2 ng m-3) and Cd (0.02 ± 0.01 ng m-3) in the atmosphere over the southern Channel which could be, at least in part, attributed to emissions from mining (chromite and gold, respectively) and smelting activities (Cu, Zn and Cd co-emission) on both neighbouring landmasses. Our results emphasise the difficulty to track such specific and overlooked atmospheric sources in the absence of known atmospheric tracers. We also stress the need for multi-elemental studies and encourage the use of detailed (cluster) air-mass transport model analysis in regions dominated by the long-range atmospheric transport as complex atmospheric circulation and minor (sporadic) inputs from terrestrial air-masses may have disproportionate impact on the atmospheric composition.
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Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Preprint
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Supplement
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(929 KB) - Metadata XML
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Supplement
(360 KB) - BibTeX
- EndNote
- Final revised paper
Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2025-4962', Anonymous Referee #2, 12 Dec 2025
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AC1: 'Reply on RC1', Morgane Perron, 03 Feb 2026
We wish to thank Reviewer 1 for providing a rapid and respectful review of our manuscript. We have revised the manuscript according to the few suggestions made by Reviewer 1. Mainly, Reviewer 1 was suggesting that EF>5 should be described as middle impacted by anthropogenic inputs. As Reimann and de Caritat (2005) suggested, caution should be applied when attributing anthropogenic origins to a sample using the enrichment factor analysis. Indeed, large variability exist between different lithogenic sources across the world and a factor 5 difference in enrichment factor can be the result of such natural variability. However, we agree with Reviewer 1 that the presence of anthropogenic inputs in samples showing a mild (yet not significant) enrichment cannot be ruled out. We therefore suggest modifying lines 300-305 of the revised manuscript as follow:
"Amongst minor elements, V, Ni and Pb showed no significant enrichment, with median EF values of 0.4, 5.2 and 7.2, respectively (Figure 4). This indicated a prevailing crustal origin for the three metals in most aerosol samples as variability in the metal composition between different lithogenic sources can result in noticeable enrichment (5<EF<10) . Minor contribution from anthropogenic sources can, however, not be completely ruled out in the case of Ni and Pb. Sample A4 represent one exception for which Ni enrichment exceed the threshold of 10 (EFNi=14), potentially highlighting non-negligible inputs from anthropogenic sources in this sample."
Citation: https://doi.org/10.5194/egusphere-2025-4962-AC1
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AC1: 'Reply on RC1', Morgane Perron, 03 Feb 2026
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RC2: 'Comment on egusphere-2025-4962', Anonymous Referee #3, 04 Mar 2026
The writing of the paper is excellent. The contribution of the study is novel and fills gaps that were previously absent. The paper was a joy to read from the onset and throughout. That said, please see a very few minor comments in the attached file. First, I agree with RC1 on the EF between 5 and 8. Although the authors have already addressed the comment, I have suggested a discussion to add. A 2002 study, cited in the comment, showed four dominant transport pathways out of the industrialized South African interior. One of them shows air mass transport off the southern South African coast, where the majority of your trajectories originated from.
I would also include a brief discussion on the gradual increase in the resolution of the models, if at all different. Coarse-scale models assume homogeneity within large grid-cells, where the variation might be great, or even very random, as you have shown. Dust sources present at the 'finer scale', like the South African Free State pans, and Namibian ephemeral rivers, were also missed by the older satellites, due to their resolution. I am also wondering how these ESM calculated the grain sizes, their transport and deposition, and which velocity was used.
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AC2: 'Reply on RC2', Morgane Perron, 10 Mar 2026
We thank Reviewer 2 for insightful comments and additional thoughts to improve our discussion.
We have slightly extended the discussion on modelling dust deposition in order to incorporate notions of model parametrization variability and the use of satellite to best predict small scale emission sources. We kept this discussion short as some modelling papers had limited information on the parametrisation used and data used for model validation and we did not want to display erroneous information on data that was not acquired in our study.
Regarding the EF analysis. We have made small changes to the text to suggest minor input of elements with EF>5, suggesting a possible origin from southern Africa country. However, in the absence of substantial proof of origin, the lack of knowledge on mining emission fingerprint in the atmosphere and owing to the existing large variability in soil composition worldwide we did remain confident that a significant anthropogenic influence in these low EF remain questionable while no further evidence is available.
Citation: https://doi.org/10.5194/egusphere-2025-4962-AC2
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AC2: 'Reply on RC2', Morgane Perron, 10 Mar 2026
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2025-4962', Anonymous Referee #2, 12 Dec 2025
Some minor editorial suggestions, in the attached PDF file. The writing is excellent. The references are extensive and this reviewer applauds the effort to find suitable references to support the discussion in the various sections. The only serious complaint is that while EF>10 is often used to discuss "crustal" vs anthropogenic sources, EF between 5-8 are still highly enriched and probably indicate some significant anthropogenic contribution. So, I would go over the EF discussion very carefully to avoid mis-characterizing samples that are probably impacted by anthropogenic input.
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AC1: 'Reply on RC1', Morgane Perron, 03 Feb 2026
We wish to thank Reviewer 1 for providing a rapid and respectful review of our manuscript. We have revised the manuscript according to the few suggestions made by Reviewer 1. Mainly, Reviewer 1 was suggesting that EF>5 should be described as middle impacted by anthropogenic inputs. As Reimann and de Caritat (2005) suggested, caution should be applied when attributing anthropogenic origins to a sample using the enrichment factor analysis. Indeed, large variability exist between different lithogenic sources across the world and a factor 5 difference in enrichment factor can be the result of such natural variability. However, we agree with Reviewer 1 that the presence of anthropogenic inputs in samples showing a mild (yet not significant) enrichment cannot be ruled out. We therefore suggest modifying lines 300-305 of the revised manuscript as follow:
"Amongst minor elements, V, Ni and Pb showed no significant enrichment, with median EF values of 0.4, 5.2 and 7.2, respectively (Figure 4). This indicated a prevailing crustal origin for the three metals in most aerosol samples as variability in the metal composition between different lithogenic sources can result in noticeable enrichment (5<EF<10) . Minor contribution from anthropogenic sources can, however, not be completely ruled out in the case of Ni and Pb. Sample A4 represent one exception for which Ni enrichment exceed the threshold of 10 (EFNi=14), potentially highlighting non-negligible inputs from anthropogenic sources in this sample."
Citation: https://doi.org/10.5194/egusphere-2025-4962-AC1
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AC1: 'Reply on RC1', Morgane Perron, 03 Feb 2026
-
RC2: 'Comment on egusphere-2025-4962', Anonymous Referee #3, 04 Mar 2026
The writing of the paper is excellent. The contribution of the study is novel and fills gaps that were previously absent. The paper was a joy to read from the onset and throughout. That said, please see a very few minor comments in the attached file. First, I agree with RC1 on the EF between 5 and 8. Although the authors have already addressed the comment, I have suggested a discussion to add. A 2002 study, cited in the comment, showed four dominant transport pathways out of the industrialized South African interior. One of them shows air mass transport off the southern South African coast, where the majority of your trajectories originated from.
I would also include a brief discussion on the gradual increase in the resolution of the models, if at all different. Coarse-scale models assume homogeneity within large grid-cells, where the variation might be great, or even very random, as you have shown. Dust sources present at the 'finer scale', like the South African Free State pans, and Namibian ephemeral rivers, were also missed by the older satellites, due to their resolution. I am also wondering how these ESM calculated the grain sizes, their transport and deposition, and which velocity was used.
-
AC2: 'Reply on RC2', Morgane Perron, 10 Mar 2026
We thank Reviewer 2 for insightful comments and additional thoughts to improve our discussion.
We have slightly extended the discussion on modelling dust deposition in order to incorporate notions of model parametrization variability and the use of satellite to best predict small scale emission sources. We kept this discussion short as some modelling papers had limited information on the parametrisation used and data used for model validation and we did not want to display erroneous information on data that was not acquired in our study.
Regarding the EF analysis. We have made small changes to the text to suggest minor input of elements with EF>5, suggesting a possible origin from southern Africa country. However, in the absence of substantial proof of origin, the lack of knowledge on mining emission fingerprint in the atmosphere and owing to the existing large variability in soil composition worldwide we did remain confident that a significant anthropogenic influence in these low EF remain questionable while no further evidence is available.
Citation: https://doi.org/10.5194/egusphere-2025-4962-AC2
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AC2: 'Reply on RC2', Morgane Perron, 10 Mar 2026
Peer review completion
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Morgane M. G. Perron
Eva Bucciarelli
Hélène Planquette
Thomas Holmes
Saumik Samanta
Yoan Germain
Alakendra Roychoudhury
Géraldine Sarthou
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(929 KB) - Metadata XML
-
Supplement
(360 KB) - BibTeX
- EndNote
- Final revised paper
Some minor editorial suggestions, in the attached PDF file. The writing is excellent. The references are extensive and this reviewer applauds the effort to find suitable references to support the discussion in the various sections. The only serious complaint is that while EF>10 is often used to discuss "crustal" vs anthropogenic sources, EF between 5-8 are still highly enriched and probably indicate some significant anthropogenic contribution. So, I would go over the EF discussion very carefully to avoid mis-characterizing samples that are probably impacted by anthropogenic input.