# Chemical Speciation of Antarctic Atmospheric Depositions
###### Abstract  
Both inorganic and organic complexation of metal cations in clouds or rainwater is essential to describe the global biogeochemical cycles of metals, because complexation can increase metal solubility and stabilize some of their oxidation states. Within a Project of the National Research Program in the Antarctica, atmospheric depositions were collected during the Antarctic summer 2017-2018 in eight sampling sites. The main ionic components occurring in water extracts of these atmospheric depositions were quantified, and a chemical model was applied, in order to identify the main species occurring in the samples. The speciation study showed that most cations were present as aquoions, except for Fe, which occurred predominantly in hydrolytic forms. The model allowed us to foresee the effect of an increase in the concentration levels of all the solution components, by simulating what could happen when the original particles act as cloud condensation nuclei. The role of inorganic anions as complexing agents becomes important when increasing total concentrations of all the solutes by a factor >100 compared to the water extracts, while the presence of organic acids acquires significance for samples having organic acid concentration higher than \(10^{-5}\) mol L\({}^{-1}\). Moreover, it was possible to pinpoint the formation constants that mostly affect the chemical system, and to gain insight into the behavior of metals in wet depositions, which is fundamental knowledge in atmospheric photochemistry studies and in the modeling of the biogeochemical cycles of metal cations. Antarctica; atmospheric depositions; metal complexes; metal speciation; chemical modeling; metal cations +
Footnote †: journal: _Sciences_  
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(HNLC), such as the Southern Ocean (SO) [4]. Mineral dust has been recognized to play a secondary role as a source of Fe and other metallic elements for the surface ecosystems of SO, when compared to inputs from sediments and the upwelling of nutrient-rich waters [5; 6]. However, the strong correlation between atmospheric mineral dust fluxes and climate conditions makes mineral dust a notable Fe source, especially for those regions that, like the SO, are quite susceptible to variations in the input magnitude [7]. The release of soluble species from mineral dust to ocean water can happen directly following atmospheric deposition, or occur after melting of snow and ice, on which the atmospheric particulate has previously accumulated. Part of the dust can be dissolved in atmospheric droplets or melting water, and thus the chemical processes that occur in the soluble fraction of the aerosol may affect the speciation, the photo-reactivity, and the bioavailability of metal cations [8; 9; 10]. The percentage of water-soluble Fe and Al in the Antarctic aerosol is quite low, and has been shown to vary from 10% to 20% with respect to the total concentrations [11]. However, inorganic and organic complexation can enhance the solubility of metal cations and stabilize some oxidation states, as shown in the case of oxalate that plays a key role in iron speciation [12; 13; 14]. [PERSON] et al. [10] concluded that the atmospheric complexation of iron in clouds or rainwater could be essential to describe its global biogeochemical cycles. Furthermore, they suggested that multidisciplinary measurements could provide more realistic formation constants for Fe complexes with organic and inorganic ligands, under conditions relevant to dust aerosol particles. This approach should be followed for all the main chemical species that characterize atmospheric depositions.