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

Abstract

A continuous high‐resolution record from a Col du Dôme (Mont Blanc massif, 4250 m above sea level (asl), French Alps) ice core in addition to discontinuous samples from a Colle Gnifetti (Monte Rosa massif, 4450 m asl, Swiss Alps) ice core were used to reconstruct the history of the atmospheric fluoride pollution at the scale of Europe. Such studies are mandatory by large uncertainties in our understanding of the natural fluoride cycle which have confounded assessment of the environmental impact of anthropogenic emissions. 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‐permanent passive volcanic HF emissions at midlatitudes compared to the situation at high northern latitudes. Hence Alpine ice records permit detailed examination of natural sources of fluoride for the free troposphere over Europe and the impact of anthropogenic sources such as aluminium smelters, coal burning, and contribution of the stratospheric reservoir built up from the chlorofluorocarbon (CFC) degradation since the beginning of the twentieth century. At Col du Dôme (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 1970 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 Europe 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 burning 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 Alpine snow was due to coal combustion. The remaining part was related to the release from the less pollutant aluminium smelters as well as other anthropogenic 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, reaching a maximum of 0.4 ng g−1 in 1970. Similarly to the summer level, the winter one has then strongly decreased (∼0.12 ng g−1 in 1980). A major 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 to the impact via stratosphere/troposphere exchanges of the growing HF stratospheric load related to the CFC degradation.