Causes of enhanced fluoride levels in Alpine ice cores over the last 75 years: Implications for the atmospheric fluoride budget

A continuous high-resolution record from a Col du Dame (Mont Blanc massif, 4250 m above sea level (asl), French Alps) ice core in addition to disconti nuous samples from a Colle Gnifetti (Monte Rosa massif, 4450 m asl, Swiss A lps) ice core were used to reconstruct the history of the atmospheric fluor ide pollution at the scale of Europe. Such studies are mandatory by large u ncertainties in our understanding of the natural fluoride cycle which have confounded assessment of the environmental impact of anthropogenic emission s. For fluoride, advantages of Alpine ice core records with respect to the Greenland ones include less efficient post-depositional effects in relation with higher snow accumulation rates, and less contamination by quasi-perma nent passive volcanic HF emissions at midlatitudes compared to the situatio n at high northern latitudes. Hence Alpine ice records permit detailed exam ination of natural sources of fluoride for the free troposphere over Europe and the impact of anthropogenic sources such as aluminium smelters, coal b urning, and contribution of the stratospheric reservoir built up from the c hlorofluorocarbon (CFC) degradation since the beginning of the twentieth ce ntury. At Col du Dame (CDD), fluoride concentrations in summer snow layers were close to 0.30 ng g(-1) in 1930, started to increase in the late 1930s, reaching 1.4 ng g(-1) in 1940 and 2.4 ng g(-1) in the late 1960s. From 197 0 to 1980 they were strongly decreased, exhibiting a plateau value close to 1.3 ng g(-1) between 1980 and 1995. It is shown that at the scale of Europ e in summer, soil dust emissions dominated the atmospheric fluoride budget prior to 1880. In the late 1960s the soil contribution decreased to 6 +/- 1 % due to enhanced release of fluoride by aluminium smelters and coal burnin g which accounted for 86 +/- 3% and 8 +/- 2% of the total fluoride content, respectively. From 1970 to 1980, effective precautions have been taken to minimize the release of fluoride from aluminium smelters to the atmosphere. Thus, over the 2 last decades, 26 +/- 8% of the fluoride summer level of A lpine snow was due to coal combustion. The remaining part was related to th e release from the less pollutant aluminium smelters as well as other anthr opogenic processes (cement and phosphate industrial processings) (56 +/- 11 %) and to soil dust emissions (18 +/- 2%). Winter levels close to 0.10 ng g (-1) or less in 1930 were gradually increased after the late 1930s, reachin g a maximum of 0.4 ng g(-1) in 1970. Similarly to the summer level, the win ter one has then strongly decreased (similar to0.12 ng g(-1) in 1980). A ma jor difference between summer and winter trends is the reincrease of winter level up to 0.4 ng g(-1) (i.e., similar to the 1970 maximum) in 1990. Such a very recent change of fluoride background levels may be partly related t o the impact via stratosphere/troposphere exchanges of the growing HF strat ospheric load related to the CFC degradation.