the Creative Commons Attribution 4.0 License.
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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Status: open (until 20 May 2025)
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RC1: 'Comment on egusphere-2025-446', Anonymous Referee #1, 18 Apr 2025
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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
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