Source and origin of atmospheric trace elements entrapped in winter snow of the Italian Eastern Alps

Abstract. Trace elements concentrations were determined in shallow snow samples from 21 sites in the Italian Eastern Alps in order to identify the sources of the contaminants present in the tropospheric winter boundary layer. The collection of superficial snow layers was carried out weekly at altitudes between 1000 and 3000 m next to meteorological stations, far away from villages, roads and ski slopes. Ultra clean procedures were adopted in order to avoid contamination of the snow during the different experimental phases. Trace elements (Ag, Ba, Bi, Cd, Co, Cr, Cu, Fe, Mo, Mn, Pb, Sb, Ti, U, V and Zn) were determined by Inductively Coupled Plasma Sector Field Mass Spectrometer (ICP-SFMS). Ancillary parameters such as major ions (SO₄²⁻, NO₃⁻, Ca^2+;, Mg²⁺, K, Na⁺, Cl⁻) were measured by Ion Chromatography (IC) and were useful in identifying the trace elements sources. The structure of the data was studied by using Principal Component Analysis (PCA) applied on the ranked data set matrix in order to minimize the weight of the outliers. Although concentrations were low at high altitudes in the heart of the Alps, and higher at lower altitudes (Pre-Alps), the structure of the chemical content deposited by wet/dry mechanisms, resulted as rather uniform over the territory studied during the time considered. PCA shows that the chemical content of the snow is characterised by an anthropogenic component (V, Sb, Zn, Cd, Mo, Pb, Ag, Bi, SO₄²⁻, NO₃⁻), mainly originating from the traffic in the adjacent Alpine valleys and the nearby heavily industrialised area of the Po Valley, a crustal component (Ca²⁺, Mg²⁺, Mn, U, Ti, Fe, Cr, Co, Cu and Ba) mainly from the geological carbonate background (Dolomites) of the Eastern Alps, and a marine component (Na⁺, Cl⁻) from the Mediterranean Sea. It is likely that transport and mixing of trace elements in the winter boundary layer occurred at a local (~10 km) and regional (~100 km) scale, and was due not only to the weak convection within the winter boundary layer but also to orographically induced winds and turbulences arising in the Alpine valleys due to the action of the synoptic wind.