Les éléments présents à l'état de traces et les hydrocarbures aromatiques polycycliques dans la neige et la glace prélevées au col Gnifetti, massif du Mont Rose (4450 m) : implications environnementales et climatiques
Year: 2008
Type: dissertation
Abstract: A new melting device for on-line decontamination and continuous analysis of alpine firn/ice cores has been designed, built and tested. Melt water from the inner part of each ice core section was pumped to an ICP-QMS and a conductivity micro-cell for continuous measurements of trace elements concentrations and conductivity, respectively. Discrete samples were also collected for trace elements, Pb isotopes and 239Pu determinations by ICP-SFMS and ICP-OES. Melt water from the outer section was extracted on-line by solid-phase cartridges for semi-continuous Polycyclic Aromatic Hydrocarbons (PAHs) analysis. Pronounced seasonal variations were observed for all elements, both crustal (Mg, Al) and anthropogenically enriched (Pb). To understand short-time variations, air mass back trajectories are an important parameter which must be considered. Also the knowledge of the temperature inversion dynamics and the boundary layer features is extremely important because they play a key role in the transport and dispersion of aerosol and gases from low-altitude emission sources. The largest emissions of Pb through history occurred during the 19th and 20th centuries and especially between the 1950s and 1970s. To determine if changes observed in Colle Gnifetti core faithfully reflect changes in emissions from nearby European countries, we have compared snow/ice data with emissions data present in the literature. For example, from 1800 AD to the first decade of 20th century Pb concentrations progressively increased, reaching a maximum in the 1920s. During the 1920s Pb concentrations suddenly halved and remained low for the next two decades. After the Second World War, Pb depositions increased dramatically with the introduction of Pb additives in gasoline, peaking in the mid-1970s. From 1975, Pb concentrations in Colle Gnifetti ice began to decrease in accordance with the first environmental policies in Europe which started to limit pollutant emissions. Before 1875 PAHs levels were very low: the pre-1750's PAHs concentrations were assumed to be the background level. ΣPAHs in the 1945-1955 ten-year period were 10 times greater than background values with ΣPAHs* greater by about 40-50 times. From 1900, PAHs concentrations increased exponentially, reaching a maximum in 1920. In the 1920s, after the First World War economic stagnation in Europe depressed industrial activities which were converting to civilan production. From the mid-1930s PAHs rapidly doubled reaching their maximum concentrations from 1940 to 1950. The heaviest ΣPAHs* concentrations decreased by a factor 5 from 1950 to 1975 while for total ΣPAHs the concentrations halved. From 1975 to 2003 ΣPAHs rose again and nearly returned to 1910s values. Although the general PAHs trends are strongly correlated with anthropogenic emission variation, the fine shape of the profile is less clear and could be influenced by several parameters. Plutonium is present in the environment as a consequence of the atmospheric nuclear testing carried out in the 1960s, the production of nuclear weapons and nuclear industry releases over the past 50 years. The shape of 239Pu profile reflects three main periods of atmospheric nuclear weapons testing. The 206Pb/207Pb ratio for the pre-1700 background period ranged between 1.18 and 1.20, in agreement with the composition of local rocks. Although Pb deposition on Colle Gnifetti after 1900 was almost totally due to anthropogenic emissions, the 206Pb/207Pb ratio does not decline intensely until 1975. This is due to the average Pb isotopic composition of industrial emissions and gasoline additives used which was very similar to the crustal composition in local rocks and soil. After 1975, a sudden and intense 206Pb/207Pb ratio depletion is recorded. This value reached a minimum of 1.11 in 1979-1980. This behaviour is characteristic of the ILE experiment (Isotopic Lead Experiment) - a large scale isotopic tracer experiment using Pb isotopes that was carried out between 1975 and 1980 in the Piedmont region of northwest Italy centred on Turin.
Author: Jacopo Gabrieli
Language: French
Subfield: Atmospheric Science
Field: Earth and Planetary Sciences
Domain: Physical Sciences
SDG: Climate action
Open Access status: green
OpenAlex ID: W372639839