Chemistry of atmospheric depositions over two polluted industrial areas of Sicily (Italy)

Abstract

Dry and wet depositions provide the pathways through which particles and gases emitted into the atmosphere return to the Earth’s surface. Bulk deposition is defined as the sum of dry and wet deposition, which is characterized by water-soluble and insoluble chemical species. Recent studies (e.g. Castillo et al., 2017) showed that most of the atmospheric deposition in Europe occurs in the wet form, but in southern European regions the role of dry contribution can be as significant as that of wet deposition. In the framework of the multidisciplinary project “CISAS” (Centro Internazionale di Studi Avanzati su Ambiente, ecosistema e Salute umana), bulk depositions were monthly sampled over two polluted industrial areas of Sicily (Milazzo and Priolo), through a network of 12 bulk collectors, from June 2018 to July 2019. In that period, 137 samples were collected and subdivided into 4 aliquots for different analytical determinations: (i) unfiltered aliquots for the total alkalinity; (ii) filtered aliquots (0.45 µm filters) for major anions by ionic chromatography (IC); (iii) filtered and acidified (with Ultrapure HNO3) for the analysis of major cations and a large suite of trace elements, respectively by ICP-OES and ICP-MS; (iv) unfiltered and acidified aliquots for the same suite of trace elements, to evaluate the contribution of less-soluble species. The filtration of the rain samples allows the separation of the water-soluble fraction from the particulate fraction (> 0.45 µm), which is retained by the filter. The comparison between filtered and unfiltered aliquots allowed to highlight the different geochemical mobility of the trace elements, and their distribution among the two fractions. The unfiltered aliquots, therefore, represent the less-soluble species found in the particulate under typical rainwater pH values (4 – 8), that becomes soluble due to the acidification of the samples (pH < 1). Results showed that the less-soluble fraction represents a significant part of the bulk deposition for several trace metals, especially during dry-dominated periods which are characterized by a long-range transport of geogenic particles (e.g. desert dust). In particular, Al, B, Ba, Fe, Li, Sr, and Ti showed significant enrichment if the less-soluble fraction is included. Similar enrichments are noteworthy also for As, Cr, Co, Cu, Ni, Pb, Se Tl, Te and V which can be attributed to the local industrial atmospheric pollution, as well as to Mt. Etna that is a permanent source of several volcanogenic elements (Brugnone et al., 2020).