Mount Etna volcanic emissions signature on the chemical composition of bulk atmospheric deposition in Sicily, Italy.

Mt. Etna, on the eastern coast of Sicily (Italy), is one of the most active and most intensely monitored volcanoes on the Earth, widely recognized as a big source of volcanic gases, such as CO2, SO2, halogens, and many trace elements, including technological critical elements (TCEs), to the atmosphere on a regional and global scale. Mt. Etna emissions account for a significant percentage of the worldwide average volcanic budget and especially during eruptive periods, its products can be dispersed over great distances and they influence the chemical composition of the atmosphere of other continents too. The current knowledge about the geochemical cycle of TCEs is still scarce, nevertheless, recent studies [Brugnone et al., 2020], evidenced a contribution from the volcanic activity for some of them (Te, Tl, and REE). Here we report the arithmetic mean of the volume-weighted mean concentration values of each sampling site of both volcanic gas-derived anions SO42-, Cl-, and F-, and of some TCEs (i.e. Te and Tl). These were determined on bulk deposition samples collected, on monthly basis, during three different research projects: (1) SEW, from July 2017 to July 2018; (2) CISAS, from June 2018 to June 2019; (3) HEAVEN, which started in March 2021 and still ongoing. All the samples were analysed for major ion contents and many trace elements by IC, ICP-OES, and ICP-MS. During the first project, samples were acquired with 3 bulk collectors, located on the eastern slope of Etna, i.e., the slope toward which the volcanic emissions are usually dispersed by the prevalent regional winds. During this period, Etna showed ordinary outgassing activity and occasional ash emissions. Volume-weighted-mean (VWM) concentrations of 3.37, 6.87, and 0.48 mg L-1 were measured for SO42-, Cl-, and F- respectively (maxima up to 12.74, 44.80, and 2.55 mg L-1, respectively). High concentrations of Te and Tl were measured especially at the sampling sites closest to the central craters (VWM 0.012 µg L-1 and maximum 0.129 µg L-1 for Te; VWM 0.122 µg L-1 and maximum 0.978 µg L-1 for Tl). During the CISAS project, atmospheric bulk depositions were collected through a network of 11 bulk collectors, which were installed in the area of Siracusa, a town on the east coast of Sicily, about 80 km SSE of Mt. Etna, and in Milazzo, a town on the northern coast of Sicily, about 55 km NNE of Mt. Etna. Between 24-30 December 2018, a major eruption of Etna occurred, characterized by lava fountains and ash emissions. The samples collected in the study area of Siracusa during the period straddling the eruptive event were characterized by high concentrations of SO42- (up to 6.68 mg L-1), Cl- (up to 19.00 mg L-1), and F- (up to 0.88 mg L-1). In the same samples, the maximum concentrations were 0.025 µg L-1 and 0.164 µg L-1 for Te and Tl, respectively, showing values one order of magnitude higher than the median concentrations measured in the samples of the other monitoring campaigns carried out in the same study area. The study area of Milazzo, due to the prevailing winds from the North direction during the period of the eruption, has not been affected by the volcanic plume, and therefore the signature of the eruption was not visible in the samples collected in that area. From March 2021 atmospheric bulk deposition samples were collected through a network of 10 bulk collectors, which were installed on Mt. Etna, at various distances from the summit craters and on different slopes of the volcano, near the city of Catania, in the Siracusa area and near the village of Cesarò, in the Nebrodi Natural Regional Park. Mt Etna experienced two long sequences of 53 short-living lava fountain episodes between December 2020 and March 2021 and April to October 2021. Other episodes occurred more recently, between February and May 2022. Volcanic emissions associated with these paroxysmal events have been dispersed over great distances, even reaching other continents (e.g., Asia), and have been important contributors to the chemical composition of atmospheric deposition at all monitoring sites during the first year of the research. VWM concentrations of 3.26 mg L-1 (maximum 189.60 mg L-1), 5.78 mg L-1 (maximum 244.60 mg L-1), and 0.43 mg L-1 (maximum 40.66 mg L-1) were recorded for SO42-, Cl-, and F-, respectively. High concentrations of Te and Tl were also recorded, especially at sites closer to the central craters of Mt. Etna, with VWM concentrations of 0.018 µg L-1 and 0.121 µg L-1 and values up to 0.369 µg L-1 and 2.101 µg L-1, respectively. Based on our finding, we highlight that volcanic emissions must be considered among the major contributors to the chemistry of the atmospheric bulk deposition in sites close to active volcano emissions, but also at considerable distances from the vents during high-magnitude eruption events.