Chemical and isotopic characterisation of the atmospheric deposition in volcanic, urban, industrial, and rural environments in Sicily, Italy

The chemical composition of atmospheric deposition is influenced by several factors, including the chemical composition of gases and particulate matter from natural and anthropogenic sources, chemical and physical reactions during pollutant transport, and removal processes. This study aimed at investigating the atmospheric deposition by analysing rainwater pH and the concentration of major (F-, Cl-, HCO3-, NO3-, SO42-, Na+, K+, NH4+, Ca2+, and Mg2+), trace (Li, B, Al, Si, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Br, Rb, Sr, Mo, Cd, Sn, Sb, Cs, Ba, Tl, Pb, U), ultra-trace elements (Sc, Ge, Te, Y, Nb, Zr, Hf, Th, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu), and of the isotopic composition of B (δ11B/10B) and Sr (87Sr/86Sr). A network of fifteen bulk collectors was used to collect 301 monthly atmospheric deposition samples for almost two years from four different contexts: urban, industrial, rural, and volcanic. Different techniques, including titration, ionic chromatography, inductively coupled plasma-optical emission and mass spectrometry, and multi-collector mass spectrometry, were employed to analyse these samples.The results showed that the natural acidity of rainwater in the Etna area was derived from the dissolution of volcanic acid gases, while in urban and industrial areas, the acidity was increased by the dissolution of anthropogenic SOx and NOx and generally neutralised by Ca2+ and Mg2+ of crustal origin. Principal Component Analysis and Positive Matrix Factorisation were used to process the major ion concentrations, revealing that natural sources were responsible for the emissions of Na+, Cl-, and partly Mg2+ (dissolution of sea-salt aerosols), Ca2+, HCO3- and, to some extent, Mg2+ and K+ (dissolution of crustal materials), and of F-, and partly Cl-, and SO42- (volcanic emissions). Conversely, K+ (biomass burning), NH4+ (agricultural activities), NO3- and SO42- (domestic heating, vehicular traffic, industrial emissions) were derived, mainly, from anthropogenic sources. Deposition rates of major ions were also calculated, with the highest values observed at coastal sites for Na+ and Cl-, and close to Mt. Etna for F- and SO42-.In terms of trace elements, volcanic activity significantly contributed to the enrichment of rainwater in many trace elements, especially during the paroxysms that occurred between 2021 and 2022, resulting in intense volcanic ash deposition. The industrial area of Milazzo had the highest deposition fluxes of Br and B (marine source), as well as Ni, Mo, and Cr (industrial emissions). The urban area of Palermo had the highest flux of Sb, resulting from vehicle brake wear and tear. Comparing our data to those obtained for some trace elements in European rainwater showed that lower concentrations of Pb, Fe, and Al were found for rainwater in Sicily than in Europe, not considering the rainwater samples from Etna in which strong Fe and Al enrichments were measured. On the contrary, significant enrichments were observed for Zn, V, Cu, Ni, Cr, and As due to local inputs from urban and industrial emissions.The concentration of trace elements in rainwater obtained from filtered (0.45 µm) aliquots only allows quantification of the contribution of the most soluble atmospheric particulate fraction. To have a complete picture of atmospheric deposition, it is necessary to measure the less soluble atmospheric particulate fraction in rainwater. This can be done by measuring the concentration of trace elements in two additional matrices: (i) solution obtained by the acidic mineralisation of the insoluble fraction deposited on the filters, and (ii) solution obtained by the dissolution of the material adhering to the surface of the bulk collector at the end of each sampling period. The insoluble fraction constituted for many elements the main contribution to the bulk atmospheric deposition, reaching up to 96.3%, 95.5%, and 86.8% for Ti, Fe, and Al, respectively. The relative contributions of the recovery solution reached values up to 9.37% for Pb. The samples obtained during dry and rainy periods revealed significant differences.Technology-critical elements (TCEs) include ultra-trace elements, Sc, Zr, Nb, Ge, Y, Te, Hf, and Th, and the lanthanoids, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Rainwater in urban areas had the highest concentrations of Sc, Zr, Nb, and Hf, while that of the Etna area had the highest concentrations of Ge, Y, and Te. Concentrations of Th were similar between the different contexts. Rainwater from Etna had the highest concentrations of all lanthanoids, indicating that volcanic sources and leaching of volcanic ash enriched rainwater in these usually very low-concentration elements. Boron and strontium isotopic compositions in rainwater samples were different in the Mt. Etna area compared to other study areas. Two sources of atmospheric emissions of B and Sr were identified. The marine source was predominant in urban and industrial areas close to the coastline, while the volcanic isotopic signature was prevalent at all sites on Mt. Etna and detectable up to 35 km from the summit craters.