Chimie d'éléments minéraux en traces dans les pluies méditerranéennes

The aim of this study is to show how some selected elements are dissolved by the rain water. We chose a set of elements for their importance in the biogeochemical cycles and the rain water chemistry (Na, Mg, Al, Si, P, S, Cl, K, Ca, Mn, Fe, Cu, Zn, Pb). To carry this work off, we have developed working methods in ultra clean conditions, being insistant about the washing of our material. The sampling and analytical methods are then described. Next, we discuss about the origin of the elements incorporated in the rainwater droplets, and we conclude that Al, Si, Fe and Mn are coming from the Saharan eolian erosion, Cu, Zn, Pb from the human activities, and P from both the sources with equivalent contributions. All the scavenged material is coming, either from local marine aerosol, or for long range transport, without any significant local emissions, for crustal and anthropogenic elements. The studies of the associated air-masses trajectories confirm this conclusion and show the frontal type of most rain events. This facilitates mixings between the European pollutants and the African crustal dusts. The pH variations show that its value is determined by the neutralization of nitric and sulfuric acids, which are coming from polluted regions, with calcium carbonat e which is coming from the Saharan eolian erosion. Na, S and Cl are completly in the solu b le phase in ours samples. Fe and Al are in an equilibrium state with a coating of their hydr o xyde . Zn, Pb and Cu make an adsorption-desorption equilibrium on hydrated oxydes of iron, manganese or al u minium . At last, the other studied elements show a solubility strongly dependent on their origin: the marine and polluted fractions of each element is soluble and, by opposite, their crustal contribution is less soluble, or even i nsoluble . It is now a strong evidence that if an element is in an equilibrium state, its behaviour is dependent on the receiving environment, on the contrary of the elements which solubility is controled by their origin.L'objet de ce travail est de déterminer les causes de la partition de certains éléments-trace entre les phases aqueuse et insoluble . Ces éléments ont été choisis, d'une part pour leur importance dans les cycles biogéochimiques de la terre, et d'autre part pour leur intérêt dans la compréhension de la chimie de la pluie. Il s'agit des éléments Na, Mg, Al, Si, P, S, Cl, K, Ca, Mn, Fe, Cu, Zn et Pb. Pour mener à bien ces recherches, nous avons mis au point des méthodes de travail dans des conditions ultra propres, en insistant particulièrement sur le protocole de lavage du matériel utilisé. Nous décrivons ensuite les méthodes de prélèvement des pluies, de séparation des phases aqueuse et insoluble , et d'analyse des éléments étudiés. Ce développement a été rendu nécessaire pour pouvoir obtenir des données fiables sur les métaux traces. Afin de pouvoir exploiter au mieux les résultats obtenus, nous essayons de déterminer les mécanismes par lesquels la matière s'incorpore dans une goutte de pluie. Puis, en liaison avec les études conjointes sur l'aérosol, nous discutons de l'origine de cette matière et nous en concluons à la prédominance des apports sahariens pour Al, Si, Mn et Fe, et à l'importance des sources de pollution pour Cu, Zn, Pb, le phosphore montrant une origine mixte crustale et de pollution bien partagée. Les sources crustales et de pollution sont toutes suffisamment éloignées du point de prélèvement pour que la matière recueillie ait subi un transport à longue distance. Seule la source mari n e est présente localement. L'étude trajectographique des masses d'air associées à chacune des pluies, confirme à chaque fois la possibilité d'un tel transport, tout en indiquant le caractère presque systématiquement frontal des situations rencontrées. Ainsi, le mélange des polluants en provenance de l'Europe avec les produits de l'érosion éolienne des sols africains se trouve très favorisé dans la zone de front où prend naissance la précipitation. L'étude des balances ioniques et du pH montre alors de manière nette cette incorporation simultanée, dans les gouttes d'eau de pluie, des acides provenant des zones polluées et des composés crustaux (en particulier la calcite) neutralisant cette acidité. Enfin, la partie la plus importante de ce travail concerne la partition entre les phases soluble et insolubl e des éléments choisis. Nous avons pu montrer que certains éléments, comme le phosph o re et le mang a nèse , présentent une fraction d'origine de pollution entièrement soluble se séparant de la fraction crustale très peu soluble, voire insoluble. A l'opposée, l'al u minium e t le fer , d'origine uniquement crustale, sont en équilibre de solubilité avec l e ur hydroxyde situé à la surface des particules solides en suspension, pourvu qu'il ne soit pas entièrement dissous; pour le fer, cet hydroxyde représente en moyenne 15% de sa masse. D'une même f a çon, le c uivre , le zinc et le plomb sont en équilibre d'adsorption-désorption entre la phase aqueuse et des oxydes hydratés de fer, de manganèse ou d'aluminium présents à la surface des particules solides en suspension dans la goutte d'eau de pluie. Ainsi, le devenir des éléments en équilibre dans la goutte de pluie dépendra fortement du milieu récepteur qui, en fixant de nouvelles conditions physico-chimiques, déplacera les équilibres de ces éléments entre les phases solubles et insolubles

Determination of optima conditions for atmospheric aerosols analysis by X-Ray Fluorescence

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On the dynamic of the atmospheric input of copper and manganese into the Western Mediterranean Sea

International audienceDaily 24-h aerosol samples have been collected at a coastal location in northwestern Corsica, between February 1985 and March 1988. Total deposition has been sampled between February 1985 and October 1987 with a time span of about 15 days.Three-dimensional air-mass trajectory analysis indicates that atmospheric Cu and Mn over the western Mediterranean are long-range transported from continental source regions.We show that the scavenging of atmospheric Cu and Mn by rain generates a seasonal cycle with high aerosol concentrations during the dry season and lower aerosol concentrations during the wet season. Sporadic but intense Saharan dust transport events are responsible for the highest atmospheric concentrations of Mn.The ratios between the atmospheric deposition of Cu and Mn and the corresponding mean daily precipitation rate reach a maximum during the Mediterranean summer. We attribute this behaviour to the wet scavenging of a more loaded atmosphere during the dry season than during the wet season. The precipitation frequency (Fp) is a major factor influencing the seasonal variability of the atmospheric Cu deposition and, this is in agreement with what we observe for the atmospheric Pb deposition.Our results also suggest that the relationship between the seasonal variability of Mn deposition and the Fp is less direct. This is due to the occurrence of Saharan dust transport events which strongly influence the variability of the Mn total deposition

Determination of optima conditions for atmospheric aerosols analysis by X-Ray Fluorescence

International audienc

Atmospheric Deposition Over the Caribbean Region: Sea Salt and Saharan Dust Are Sources of Essential Elements on the Island of Guadeloupe

International audienceDust emitted from North Africa is transported over long distances and has a strong impact on large areas over the North Tropical Atlantic Ocean. Sea salt emitted by the sea surface is the second source of essential elements transported in the atmosphere and plays a major role in the cycles of alkaline-earth metals in the ecosystems of tropical North Atlantic Islands. The total atmospheric deposition fluxes were continuously sampled on a weekly basis in Guadeloupe, Lesser Antilles, from March 2015 to August 2018 (41 months). Elemental deposition fluxes, including Al, Ca, K, Mg, Fe, Na, P, S, and Zn, were measured for all samples to provide the first long time series of atmospheric elemental deposition fluxes over the Lesser Antilles region. It is shown that: (a) the three sources of atmospheric deposits in Guadeloupe for the presented elements are sea salt (for K, Ca, Mg, Na, and S), long-range transported Saharan dust (for Al, Ca, K, and Fe), and biogenic particles (for P and Zn); (b) the average deposition mass fluxes of sea salt and Saharan dust are 17.4 and 11.2 g.m−2.year−1, respectively, without noticeable inter-annual variations; (c) a pronounced seasonality is found for the Saharan dust deposition, for which maximum flux values are observed between June and July each year and 85% of the annual deposition flux occurs between April and September; (d) the deposition flux of sea salt is strongly correlated to local wind speed, without seasonality

Factors influencing aerosol solubility during cloud process

International audienceThe water-soluble fraction of an aerosol determines its chemical and physical properties and also its behaviour. The origin of the aerosol and its atmospheric transport influence its solubility. Cloud process simulations have been conducted on both Saharan and anthropogenic aerosols. The rate of solubilisation was followed for native and processed aerosol particles; it is controlled by the pH variations due to release of acids or bases. It appears that one condensation/evaporation cycle increases the solubility of aerosol particles. Increasing the number of cloud process simulations does not affect the solubility profile. The solubility depends only on the conditions of the last cloud cycle and, in particular, on the factor controlling pH during this process

Zinc partitioning in Mediterranean rainwater

International audienceEleven rainwater samples were collectedWin the Western Mediterranean region in clean conditions in order to study the partitioning of zinc between soluble and insoluble forms. The pH of the rainwater sampled ranges between 4 and 7; the high values of pH resulting from the neutralization of acidic species by the calcium carbonate present in desertic dusts. The soluble zinc ranged between 15 and 99% of the total zinc; the higher percentage are observed for the lower pH, and the lower percentage for the higher pH. This variable solubility can be explained by an adsorption-desorption equilibrium which is highly sensitive to the value of pH in rainwater

Factors influencing aerosol solubility during cloud process

International audienceThe water-soluble fraction of an aerosol determines its chemical and physical properties and also its behaviour. The origin of the aerosol and its atmospheric transport influence its solubility. Cloud process simulations have been conducted on both Saharan and anthropogenic aerosols. The rate of solubilisation was followed for native and processed aerosol particles; it is controlled by the pH variations due to release of acids or bases. It appears that one condensation/evaporation cycle increases the solubility of aerosol particles. Increasing the number of cloud process simulations does not affect the solubility profile. The solubility depends only on the conditions of the last cloud cycle and, in particular, on the factor controlling pH during this process

The pH-dependent dissolution of wind-transported Saharan dust

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Revisiting atmospheric dust export to the Southern Hemisphere ocean: Biogeochemical implications

International audienceAerosol concentrations in the Southern Hemisphere are largely undersampled. This study presents a chemical and physical description of dust particles collected on board research vessels in the southeast Pacific (SEPS) and the Southern Ocean (SOKS). Concentrations of dust were 6.1 ± 2.4 ng m À3 for SEPS and 13.0 ± 6.3 ng m À3 for SOKS. Dust fluxes, derived from those concentrations, were 9.9 ± 3.7 mg m À2 d À1 for SEPS and 38 ± 14 mg m À2 d À1 for SOKS and are shown to be representative of actual fluxes in those areas. Dust and iron deposition are up to 2 orders of magnitude lower than former predictions. A map of dust deposition on the Southern Hemisphere is proposed by incorporating those in situ measurements into a dust model. This study confirms that dust deposition is not the dominant source of iron to the large high-nutrient low-chlorophyll Southern Ocean