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the Creative Commons Attribution 4.0 License.
| 08 Apr 2025
Elemental composition, iron mineralogy and solubility of anthropogenic and natural mineral dust aerosols in Namibia: a case study analysis from the AEROCLO-sA campaign
Abstract. This paper presents the results of three weeks of aerosol sampling at the Henties Bay coastal site in Namibia during the Aerosols, Radiation and Clouds in southern Africa (AEROCLO-sA) field campaign in August–September 2017. The campaign coincided with a transition period between two synoptic regimes and corresponded to a significant change in the aerosol composition measured at the site and in particular of that of mineral dust. During August, the dust was natural windblown from the southerly gravel plains with a composition consistent with that previously observed in Namibia. In September, the dust was fugitive from anthropogenic mining and possibly minor contribution of smelting emissions in northern Namibia or as far as the Copper Belt in Zambia, one of the regional hotspot of pollution.
Chemical analysis of filter samples highlights the difference in elemental composition, in particular heavy metals, such as As, Cu, Cd, Pb, and Zn, but also silicon, in the anthropogenic dust. The metal solubility of the natural dust was higher, including that of iron. In addition to the higher content of iron oxides and the larger size of particles in the anthropogenic dust, we found that the iron solubility, and, more in general, the metals’ solubility, correlated to the high concentrations of fluoride ion which are attributed to marine emissions from the Namibian shelf. These results highlight in a renewed manner the importance of ocean-atmosphere exchanges affecting both the atmospheric composition and the marine biogeochemistry in the Benguela region.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.- Preprint
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RC1: 'Comment on egusphere-2025-446', Anonymous Referee #1, 18 Apr 2025
Review of “Elemental composition, iron mineralogy and solubility of anthropogenic and natural mineral dust aerosols in Namibia: a case study analysis from the AEROCLO-sA campaign” by Formenti et al.
This manuscript presents the results of aerosol measurements conducted at Henties Bay, Namibia, with a focus on ionic and elemental composition in total suspended particles. Two regimes were identified, one related to regional dust and the other related to dust from anthropogenic activities. The paper is well written and provides insightful results to a region underrepresented in the literature.
General Comments:
- Much of the discussion of the results relies on the relationship between different elements (i.e. Figure 4), however, it is difficult to tell by eye when a change in the ratio is significant. The authors might consider placing error bars on the time series data, especially on the plots showing the time series of the elemental ratios.
- More discussion in the differences between the PM1 and TSP composition would be useful. Currently, the PM1 results are described (i.e. lines 356-350, throughout section 3.2.3) but additional insight into what the authors think is causing these differences would strengthen the paper. Additionally, it is unclear whether PMF was run on the TSP samples or both, as there is only one sentence (line 189-191) alluding to the PM1 PMF composition. If PMF was included on the PM1 samples, this would be a useful comparison.
Minor Comments:
- Section 3.2.1: What factors are driving the change in the Cl-/Na+ ratio? Are the lower values observed during P1 due to acid displacement of chloride in sea salt, or do you expect non sea salt sources of these ions during different periods.
- Section 3.2.2. The source of fluoride being the marine shelf is intriguing. Can the authors comment on the mechanism of how the aerosol ends up enriched in F? Does the sea water in that region have higher F content?
- Line 414: “…and with the exception of a peak value on 26 August, the Si/Al ratio…” the figure does not show a peak on this day, should this be another date?
- Consider showing the time series of the PMF factors in the main text.
- Consider dividing P3 into two sections in the time series in the main text as is done in the supplemental box plots, especially Figure 4. It is clear that there are two regimes, but this is not discussed in the text until later in the manuscript.
- Figure 3: Please clarify in the figure captions when gaps in the graphs correspond with missing data (as is the case in figure 1) and when the measurements were below the limit of detection (as mentioned in the text for figure 3). Also, please label P1 and P2 for consistency with other figures
- Figure 5: Could these be labeled with the date they were collected?
- The four XANES spectra were chosen because they had the highest Fe loading. Do the authors think the fact that these four appear similar is due to a similar source for these four. If so, it may be more interesting to include different examples in Figure 5, such as the samples with clay/hematite signatures, or Fe(II) signatures mentioned in the text. Overlaying the spectra may also help the readers observe small differences between the spectra.
- Supplemental: In some cases one of the PMF factors is called Si-rich, and others it is Sand.
Typographical
Line 416: Planes should be replaced with plains.
Line 592: Gater should be replaced with Later.
Citation: https://doi.org/10.5194/egusphere-2025-446-RC1 -
AC2: 'Reply on RC1', Paola Formenti, 11 Jul 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-446/egusphere-2025-446-AC2-supplement.pdf
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RC2: 'Comment on egusphere-2025-446', Akinori Ito, 16 Jun 2025
General comments
Model predictions of metal speciation and solubility in aerosols are highly uncertain, especially in Namibia. The authors collected PM1 and total suspended particulate (TSP) samples at Henties Bay during the period from 21 August to 13 September 2017. They combined wavelength-dispersive X ray fluorescence (WD-XRF), Ion chromatography (IC), X-Ray Absorption (XAS), Inductively Coupled Plasma Mass Spectrometry (ICP-MS), a thermo-optical carbon analyser, and high-resolution mass spectrometry (HRMS) to investigate the iron mineralogy and solubility. They found two major sources of iron which are influenced by natural and anthropogenic dust. Previous study at the Henties Bay Aerosol Observatory (HBAO) showed strong link between MSA (methane sulfonic acid) and dust iron solubility in PM10 up to 20%. Conversely, their results showed that dust iron solubility was correlated to the high concentrations of fluoride ion in TSP. They hypothesize that the source of fluoride ion is attributed to marine emissions from the Namibian shelf. The comprehensive measurements performed in this paper may help us to advance our understanding of metal speciation and solubility in aerosols, although more work is needed to confirm their hypothesis. I have some comments and questions to improve this paper.
Specific comments
l.41-44: Please consider separating one sentence to two sentences to explain the differences in solubility between the natural and anthropogenic dust aerosols. As you mention, a clear temporal trend of the higher content of iron oxides cannot be defined from Table 1. Figure S6 shows the coarse particle number concentration. Please show the higher content of iron oxides (see comment on l.611) and the larger size of particles in the anthropogenic dust (see comment on l.447).
l.53: Please describe the global dust emissions by anthropogenic activities quantitatively and add the reference.
l.333: It is helpful for the reader if you refer to MQL in Table S3. If not, please specify major elements and ions.
l.351: Please indicate PM1. If not, please specify the size of OC and NaCl.
l.389 and Figure 3: Please show fluoride and MSA in PM1.
l.401: Please show the figure with statistics for the correlation.
l.326 and l.438: It is helpful for the reader if you show the figure of source apportionment. Please mention that the PMF analysis captured the major factor of TSP at first, possibly the sea salt.
l.447 and Figure S6: Please show the fraction of coarse particles with respect to the total number.
l.477: It is helpful for the reader if you show the figure of source apportionment from the CAMS reanalysis at the site in association with the PMF analysis.
l.481: Please show the comparison of sulfate between measurements and the CAMS reanalysis at the site.
l.532: You mention the higher content of iron oxides in the anthropogenic dust on l.42. Please specify Fe(III) oxide and show the results before grouping in supplement. How could you tell Fe(III) oxide from magnetite?
l.610: Please show the results of the more abundant ferrihydrite in the natural dust during P1.
l.611: Please show the results of the more frequent iron oxides in the fugitive dust.
l.613: Please show higher solubility in PM1 than TSP.
l.619: How do you consider the mechanism of fluoride emission from a carbonate fluorapatite mineral in phosphorite deposits on the Namibian shelf?
l.620: How do you consider the mechanism of mixing of fluoride and mineral dust?
l.621: Please discuss the reasons of zero fluoride concentrations from marine emissions even though the MSA concentrations were higher during P2 and P3 than P1. How could you explain the lower solubility during P2 and P3 than P1 by supplementing the processing by DMS described for iron?
Technical comments
l.65, 72, and 74: Please add the references.
l.592: Please correct a typo.
Citation: https://doi.org/10.5194/egusphere-2025-446-RC2 -
AC1: 'Reply on RC2', Paola Formenti, 11 Jul 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-446/egusphere-2025-446-AC1-supplement.pdf
-
AC1: 'Reply on RC2', Paola Formenti, 11 Jul 2025
Status: closed
-
RC1: 'Comment on egusphere-2025-446', Anonymous Referee #1, 18 Apr 2025
Review of “Elemental composition, iron mineralogy and solubility of anthropogenic and natural mineral dust aerosols in Namibia: a case study analysis from the AEROCLO-sA campaign” by Formenti et al.
This manuscript presents the results of aerosol measurements conducted at Henties Bay, Namibia, with a focus on ionic and elemental composition in total suspended particles. Two regimes were identified, one related to regional dust and the other related to dust from anthropogenic activities. The paper is well written and provides insightful results to a region underrepresented in the literature.
General Comments:
- Much of the discussion of the results relies on the relationship between different elements (i.e. Figure 4), however, it is difficult to tell by eye when a change in the ratio is significant. The authors might consider placing error bars on the time series data, especially on the plots showing the time series of the elemental ratios.
- More discussion in the differences between the PM1 and TSP composition would be useful. Currently, the PM1 results are described (i.e. lines 356-350, throughout section 3.2.3) but additional insight into what the authors think is causing these differences would strengthen the paper. Additionally, it is unclear whether PMF was run on the TSP samples or both, as there is only one sentence (line 189-191) alluding to the PM1 PMF composition. If PMF was included on the PM1 samples, this would be a useful comparison.
Minor Comments:
- Section 3.2.1: What factors are driving the change in the Cl-/Na+ ratio? Are the lower values observed during P1 due to acid displacement of chloride in sea salt, or do you expect non sea salt sources of these ions during different periods.
- Section 3.2.2. The source of fluoride being the marine shelf is intriguing. Can the authors comment on the mechanism of how the aerosol ends up enriched in F? Does the sea water in that region have higher F content?
- Line 414: “…and with the exception of a peak value on 26 August, the Si/Al ratio…” the figure does not show a peak on this day, should this be another date?
- Consider showing the time series of the PMF factors in the main text.
- Consider dividing P3 into two sections in the time series in the main text as is done in the supplemental box plots, especially Figure 4. It is clear that there are two regimes, but this is not discussed in the text until later in the manuscript.
- Figure 3: Please clarify in the figure captions when gaps in the graphs correspond with missing data (as is the case in figure 1) and when the measurements were below the limit of detection (as mentioned in the text for figure 3). Also, please label P1 and P2 for consistency with other figures
- Figure 5: Could these be labeled with the date they were collected?
- The four XANES spectra were chosen because they had the highest Fe loading. Do the authors think the fact that these four appear similar is due to a similar source for these four. If so, it may be more interesting to include different examples in Figure 5, such as the samples with clay/hematite signatures, or Fe(II) signatures mentioned in the text. Overlaying the spectra may also help the readers observe small differences between the spectra.
- Supplemental: In some cases one of the PMF factors is called Si-rich, and others it is Sand.
Typographical
Line 416: Planes should be replaced with plains.
Line 592: Gater should be replaced with Later.
Citation: https://doi.org/10.5194/egusphere-2025-446-RC1 -
AC2: 'Reply on RC1', Paola Formenti, 11 Jul 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-446/egusphere-2025-446-AC2-supplement.pdf
-
RC2: 'Comment on egusphere-2025-446', Akinori Ito, 16 Jun 2025
General comments
Model predictions of metal speciation and solubility in aerosols are highly uncertain, especially in Namibia. The authors collected PM1 and total suspended particulate (TSP) samples at Henties Bay during the period from 21 August to 13 September 2017. They combined wavelength-dispersive X ray fluorescence (WD-XRF), Ion chromatography (IC), X-Ray Absorption (XAS), Inductively Coupled Plasma Mass Spectrometry (ICP-MS), a thermo-optical carbon analyser, and high-resolution mass spectrometry (HRMS) to investigate the iron mineralogy and solubility. They found two major sources of iron which are influenced by natural and anthropogenic dust. Previous study at the Henties Bay Aerosol Observatory (HBAO) showed strong link between MSA (methane sulfonic acid) and dust iron solubility in PM10 up to 20%. Conversely, their results showed that dust iron solubility was correlated to the high concentrations of fluoride ion in TSP. They hypothesize that the source of fluoride ion is attributed to marine emissions from the Namibian shelf. The comprehensive measurements performed in this paper may help us to advance our understanding of metal speciation and solubility in aerosols, although more work is needed to confirm their hypothesis. I have some comments and questions to improve this paper.
Specific comments
l.41-44: Please consider separating one sentence to two sentences to explain the differences in solubility between the natural and anthropogenic dust aerosols. As you mention, a clear temporal trend of the higher content of iron oxides cannot be defined from Table 1. Figure S6 shows the coarse particle number concentration. Please show the higher content of iron oxides (see comment on l.611) and the larger size of particles in the anthropogenic dust (see comment on l.447).
l.53: Please describe the global dust emissions by anthropogenic activities quantitatively and add the reference.
l.333: It is helpful for the reader if you refer to MQL in Table S3. If not, please specify major elements and ions.
l.351: Please indicate PM1. If not, please specify the size of OC and NaCl.
l.389 and Figure 3: Please show fluoride and MSA in PM1.
l.401: Please show the figure with statistics for the correlation.
l.326 and l.438: It is helpful for the reader if you show the figure of source apportionment. Please mention that the PMF analysis captured the major factor of TSP at first, possibly the sea salt.
l.447 and Figure S6: Please show the fraction of coarse particles with respect to the total number.
l.477: It is helpful for the reader if you show the figure of source apportionment from the CAMS reanalysis at the site in association with the PMF analysis.
l.481: Please show the comparison of sulfate between measurements and the CAMS reanalysis at the site.
l.532: You mention the higher content of iron oxides in the anthropogenic dust on l.42. Please specify Fe(III) oxide and show the results before grouping in supplement. How could you tell Fe(III) oxide from magnetite?
l.610: Please show the results of the more abundant ferrihydrite in the natural dust during P1.
l.611: Please show the results of the more frequent iron oxides in the fugitive dust.
l.613: Please show higher solubility in PM1 than TSP.
l.619: How do you consider the mechanism of fluoride emission from a carbonate fluorapatite mineral in phosphorite deposits on the Namibian shelf?
l.620: How do you consider the mechanism of mixing of fluoride and mineral dust?
l.621: Please discuss the reasons of zero fluoride concentrations from marine emissions even though the MSA concentrations were higher during P2 and P3 than P1. How could you explain the lower solubility during P2 and P3 than P1 by supplementing the processing by DMS described for iron?
Technical comments
l.65, 72, and 74: Please add the references.
l.592: Please correct a typo.
Citation: https://doi.org/10.5194/egusphere-2025-446-RC2 -
AC1: 'Reply on RC2', Paola Formenti, 11 Jul 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-446/egusphere-2025-446-AC1-supplement.pdf
-
AC1: 'Reply on RC2', Paola Formenti, 11 Jul 2025
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