Consistent histories of anthropogenic western European air pollution preserved in different Alpine ice cores

Abstract. Individual high-Alpine ice cores have been proven to contain a well-preserved history of past anthropogenic air pollution in western Europe. The question of how representative one ice core is with respect to the reconstruction of atmospheric composition in the source region has not been addressed so far. Here, we present the first study systematically comparing longer-term ice-core records (1750–2015 CE) of various anthropogenic compounds, such as major inorganic aerosol constituents (NH4+, NO3-, SO42-), black carbon (BC), and trace species (Cd, F−, Pb). Depending on the data availability for the different air pollutants, up to five ice cores from four high-Alpine sites located in the European Alps analysed by different laboratories were considered. Whereas absolute concentration levels can partly differ depending on the prevailing seasonal distribution of accumulated precipitation, all seven investigated anthropogenic compounds are in excellent agreement between the various sites for their respective, species-dependent longer-term concentration trends. This is related to common source regions of air pollution impacting the four sites less than 100 km away including western European countries surrounding the Alps. For individual compounds, the Alpine ice-core composites developed in this study allowed us to precisely time the onset of pollution caused by industrialization in western Europe. Extensive emissions from coal combustion and agriculture lead to an exceeding of pre-industrial (1750–1850) concentration levels already at the end of the 19th century for BC, Pb, exSO42- (non-dust, non-sea salt SO42-), and NH4+, respectively. However, Cd, F−, and NO3- concentrations started surpassing pre-industrial values only in the 20th century, predominantly due to pollution from zinc and aluminium smelters and traffic. The observed maxima of BC, Cd, F−, Pb, and exSO42- concentrations in the 20th century and a significant decline afterwards clearly reveal the efficiency of air pollution control measures such as the desulfurization of coal, the introduction of filters and scrubbers in power plants and metal smelters, and the ban of leaded gasoline improving the air quality in western Europe. In contrast, NO3- and NH4+ concentration records show levels in the beginning of the 21th century which are unprecedented in the context of the past 250 years, indicating that the introduced abatement measures to reduce these pollutants were insufficient to have a major effect at high altitudes in western Europe. Only four ice-core composite records (BC, F−, Pb, exSO42-) of the seven investigated pollutants correspond well with modelled trends, suggesting inaccuracies of the emission estimates or an incomplete representation of chemical reaction mechanisms in the models for the other pollutants. Our results demonstrate that individual ice-core records from different sites in the European Alps generally provide a spatially representative signal of anthropogenic air pollution trends in western European countries.