# Chemical Speciation of Antarctic Atmospheric Depositions
## 3 Results
### Speciation of the Main Components of Atmospheric Deposition Samples  
The application to the atmospheric deposition samples of a chemical model, which considers the species listed in Tables 1 and 2, allowed us to draw species distribution diagrams as a function of pH. Iron was considered as Fe(III) supposing a complete oxidation of Fe(II) during the collection and storage of the samples. The speciation distribution diagrams were drawn for each sample in the pH range 3-6, which was chosen to be representative of rainwater pH. The diagrams show that most cations would occur as aquoions in the whole pH range, while Fe(III) would occur as hydrolytic forms. The results obtained by plotting the percentage of Fe(III) occurring as different species for three different samples (D1-1, D5-1 and D3-2) are shown in Figure 4a-c. The three samples under consideration are characterized by different concentration levels of total iron, namely 4.03 \(\times\) 10\({}^{-8}\) mol L\({}^{-1}\) (D1-1), 1.58 \(\times\) 10\({}^{-8}\) mol L\({}^{-1}\) (D5-1) and 4.64 \(\times\) 10\({}^{-9}\) mol L\({}^{-1}\) (D3-2). The formation of the solid species Fe(OH)\({}_{3\text{ (s)}}\) seems to be related to the concentration of Fe(III) in solution (the higher the [Fe(III)], the more Fe(OH)\({}_{3\text{ (s)}}\) is formed). The presence of complexes with organic acids is negligible at the concentration levels found in the samples, whereas the hydrolysis equilibria are predominant. However, one should consider that this finding applies to samples obtained by dissolution of solid depositions in a quite high water volume (see experimental section), which is poorly representative of chemical equilibria that may take place on deliquescent atmospheric particles. Therefore, further application of the chemical model was devoted to the understanding of what would happen by increasing the component concentration (i.e., a scenario of components dissolution in much lesser water volume), as it may be the case of a thin water film covering deliquescent atmospheric particles, which happens when the particles act as cloud condensation nuclei (CCN) in the atmosphere. A multiplicative factor was thus applied to all the original concentration values in the extraction water, thereby ensuring that the concentration increase would maintain constant ratios between the components. By so doing, the role of inorganic and organic ligands became evident. An example of this behavior is shown for Cu(II) (see Figure 5a,b). Speciation diagrams were thus built by multiplying the components concentrations of the samples D1-1 and D3-2 by 10, 100, and 1000 times (multiplicative factors as per the above discussion). Samples D1-1 and D3-2 were initially chosen for this test, because they are characterized by quite different composition. In particular, sample D3-2 shows high concentration values of ammonium, acetate, formate, and zinc, as highlighted above. Figure 6 shows the percentage of the free cations of each element, which would occur in solution as a function of the concentration levels of the solution components (i.e., detected concentrations times the multiplicative factors), obtained from speciation diagrams. For cations with charge 2, one observes a quite evident decrease of the percentage of the dissolved free cation as a function of the component concentration, while alkaline ions are less sensitive to concentration changes. Fe is not displayed in the figure, because it is predominantly present as hydroxy species or in complexed form. Figures 7 and 8 show the percentages of cations involved in the different chemical forms, as a function of the concentration level of the components (multiplicative factors) at pH = 6.0. In the case of sample D1-1 (Figure 7) one observes important roles of the inorganic anions (chloride, sulfate, and hydrogen phosphate) in complex formation with all the cations, while organic acids such as acetate and formate play quite negligible roles. At pH = 6.0, Fe(III) is in the solid form.