the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 23 Jan 2025
Contrasting solubility and speciation of metal ions in total suspended particulate matter and fog from the coast of Namibia
Abstract. The west coast of southern Africa is a region of particular climate interest and a crossroad for aerosols of different origins as well as fog occurrences. In this study, we present a comparison between the solubility of trace metals in pairs of total suspended particulate (TSP) and fog water samples collected in Henties Bay, Namibia, during the AErosols, Radiation and CLOuds in southern Africa (AEROCLO-sA) field campaign in September 2017. From inductively coupled plasma mass spectrometry measurements, we found that Al, Fe, Ni, Cu, and Cr have enhanced solubility in fog samples compared to the TSP samples. We found that thermodynamic modelling predicts the formation of soluble complexes with inorganic and organic ligands in fog for Cu, Cr, and Ni, but it would predict Al and Fe to precipitate as hydroxides given the neutral pH of fog. Contrastingly, X-ray absorption near edge structure measurements showed the presence of oxalate of Fe complexes that could explain its enhanced solubility in fog samples, despite a neutral pH. In addition, transmission electron microscopy and dynamic light scattering measurements revealed the presence of nano-sized colloidal particles containing Fe and Al in filtered fog samples that may appear as soluble in ICP-MS measurements. We hypothesise that those complexes are formed in the early stages of particle activation into droplets when water content and, therefore, pH are expected to be lower and then remain in fog in a kinetically stable form or lead to the formation of colloidal particles.
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RC1: 'Comment on egusphere-2024-4140', Anonymous Referee #1, 05 Mar 2025
This study investigates the differences in metal concentrations between TSP (total suspended particulate) and fog samples collected concurrently at a coastal site in 2017. Using ICP-MS analysis and thermodynamic modeling, the authors attribute these variations to distinct complexation behaviors with inorganic/organic ligands. Key findings include enhanced Al/Fe solubility in fog water under neutral pH conditions, hypothesized to result from aqueous-phase processing during droplet activation. XANES analysis confirmed Fe-organic complexes despite thermodynamic predictions favoring hydroxide species. TEM/DLS measurements further supported colloidal Fe nanoparticles as an additional phase. I have the following comments.
Major comments:
- The manuscript does not explicitly state whether total or water-soluble metals were measured via ICP-MS. Metal solubility should be quantified as the soluble fraction relative to total metal content. Equating bulk concentration to "solubility" is problematic. A revised discussion addressing this distinction is needed.
- The method section lacks critical information for ICP-MS analysis. The manuscript only has one sentence for this. E.g. how are the samples prepared? What are the instrumental parameters. This affects how the metal data should be interpreted.
- Current spectral plots in Figure 5 are challenging to interpret due to overlapping lines/symbols and low contrast colors/lineshapes. Consider stacked panels or splitting into subfigures for key comparisons. For example, it is hard to tell from the figure that Fe was predominantly present in the (III) oxidation state. Additionally, descriptions on how the simulations were done (i.e. parameters) should be added to the method section.
- The discussion on pH and metal solubility is largely lacking. It is known that acidification of metal species in the presence of sulfate is an important mechanism to make insoluble metal become soluble. The author should consider adding relevant discussions. It would be useful to plot the pH against/with metal solubility.
Minor comments:
- Provide the year for this reference: “Formenti et al, this issue”. This appears many times in the manuscript.
- “…is discussed in section 0.” Where is section 0? This also appears many times in the manuscript.
- Revise single-sentence paragraphs at e.g. Lines 91/125/141 by integrating them logically into adjacent sections or expanding context where appropriate.
- In section 4.1, “temperature with a narrow range (<5°C) and humidity (RH 95%)”. Provide a range for humidity.
- Section 4.3, line 282, “higher concentrations of Al, Fen and Ni..” Missing Cu here?
- Define LWC upon first use.
Citation: https://doi.org/10.5194/egusphere-2024-4140-RC1 -
RC2: 'Comment on egusphere-2024-4140', Akinori Ito, 11 Mar 2025
General comments
Model predictions of metal speciation and solubility in aerosols and fog are highly uncertain. The authors collected total suspended particulate (TSP) and fog water samples at Henties Bay and seawater on the seashore during the period of 3-11 September. They combined thermodynamical modelling with field measurements using X-ray absorption near edge spectroscopy (XANES), Transmission electron microscopy (TEM), Dynamic Light Scattering (DLS), and Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Previous study at the Henties Bay Aerosol Observatory (HBAO) showed strong link between MSA (methane sulfonic acid) and iron solubility up to 20% but not between oxalate and iron solubility. Conversely, their XANES results showed that photochemical reduction to Fe(II) was not a significant process involved in the enhanced iron solubility in the fog samples but the presence of iron-oxalate complexes that could explain its enhanced solubility in fog samples. Their TEM and DLS measurements revealed the presence of nano-sized colloidal particles containing Fe and Al in filtered fog samples that may appear as soluble in ICP-MS measurements. The modeling exercises with direct measurements performed in this paper may help us to advance our understanding of metal speciation and solubility in aerosols and fog. I have some comments and questions to improve this paper.
Specific comments
l.61: Please specify the photo-reduction processes of marine biogenic emissions and explain the role of MSA (methane sulfonic acid) and oxalate in enhanced solubility to elucidate the complement to those findings.
l.68: What is the role of butenes in the formation of metal-ligand complexes?
l.83, l.310: Please correct section 0.
l.89: What are the differences in the leaching method from the previous study at HBAO? Please explain the leaching protocol whether nano-sized colloidal particles are measured as soluble in ICP-MS measurements.
l.108: Please explain the differences in the operational definition of “soluble” and “dissolved” to elucidate dissolved metals in fog and soluble metals in TSP. Please specify whether dissolved or soluble trace metals in your method include nano-sized colloidal particles.
l.161: Why don’t you include organic complexation of Fe with humic-like substances, as you mentioned in l.290?
l.193: This is probably the major reason of low pH calculated in sea salt and Ca-containing aerosols under high RH and thus resulting in free form for a large fraction of the metal ions as, as you mentioned in l.322. It is highly recommended to use the model which considers the alkaline minerals.
l.214: How did you determine the oxidation state?
l.248: What do you mean by scarce vegetation sources in this Namib region? Please rephrase this.
l.256: What are the marine phytoplanktonic emissions of NO? Do you mean photochemical production of nitrogen oxides? Please rephrase this.
l.257: Please explain why weaker correlation of nitrate with MSA than oxalate supports this hypothesis.
l.281: Please explain the conversion of the unit in the method.
l.286 and Table 3: It is not clear whether the metal solubility is enhanced or the metal concentration is larger. How do you explain the lower values of the ratio between the mass concentration of dissolved metals in fog samples and soluble metals in the corresponding TSP samples than the unity? Please show the metal solubility in aerosol and fog samples, separately. Please also compare the solubility with the previous study at the HBAO.
l.297: How can you explain the lowest Fe solubility differences observed in the H1001 sample collected during daytime? Is this photochemical reduction consistent with the results on l.387, “photochemical reduction to Fe(II) was not a significant process involved in the enhanced iron solubility in the fog samples”? Please compare Fe solubility in fog samples and in the corresponding TSP samples quantitatively.
l.335 and Figure S6: Figure S6 did not show Fe(III) being predominantly complexed with oxalate. Please indicate the organic ligands and sampling date to compare with Figure 2.
l.375: How can you explain the lowest Fe solubility differences observed in the H1001 sample, which showed larger amounts of iron-oxalates in fog than aerosol, as opposed to previous studies cited on l.298? Please show the results for the fog sample H1002, which indicated the largest Fe solubility differences.
l.411: Can you tell the differences in colloidal materials between insoluble hydroxides and humic-like substances?
Citation: https://doi.org/10.5194/egusphere-2024-4140-RC2
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