Dust in snow cores: an integrated study on an Italian alpine glacier

On a global scale, mineral dust accounts for the major contribution to the mass load of airborne particles in the atmosphere. It consists of micron-sized minerals mainly deflated from arid and semiarid regions by eolian processes. It can affect glaciers reducing the albedo and it can transform the snowpack leading the formation of melting-refreezing crusts and hoar crystals weak layers. To evaluate these processes, snow and ice cores provide a privileged archive to reconstruct the Earth’s past atmospheric composition. Moreover, they can be powerful tools to assess the impact of human activities on the environment, also enabling albedo evaluation and avalanches forecasting. For these reasons we performed, for the first time, an integrated study on three shallow firn cores (3-4 m long), representative of the previous winter season, collected in June 2015, 2016, and 2017 on the top of the Rutor glacier (Aosta Valley, Italy, 46.637° N, 7.016° E - 3200 m a.s.l.). On these cores we carried out the optical characterization at the EuroCold laboratory (University of Milano-Bicocca) and chemical analyses at the Institute of Polar Science and Ca’ Foscari University of Venice. The physical quantities were analyzed with the SPES (Single Particle Extinction and Scattering) method and with a hyperspectral imaging device. The results were related to the stratigraphic data and compared to the chemical analysis. The SPES method provides extinction cross-section and optical thickness of single particles and their relative concentration in each sample. The particle-by-particle approach and the simultaneous measurement of two optical parameters for each particle pave the way for characterizing the samples avoiding the problem of inverting the particle size distribution. Moreover, from the two dimensional distribution, absorbing particles can be isolated in each sample. Furthermore, the snow cores were analyzed by a non-destructive hyperspectral imaging device to quantify the Aerosol Optical Depth (AOD) and other optical parameters of the snow useful for albedo evaluation. To better assess the particle sources, on the same samples a high temporal-resolution chemical analysis was performed. Specifically, we found that Levoglucosan, a key biomass burning tracer, is mildly correlated with the AOD anomalies, whereas Polycyclic Aromatic Hydrocarbon (PAH) diagnostic ratios clearly suggest their pyrogenic origin from combustion sources. The integration of the chemical results, together with the physical properties of the particles, can provide a detailed characterization of the glacier. This is important for the determination of the anthropogenic contamination and the snow ablation linked to climate changes.