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
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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
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