Abstract
Metals in atmospheric aerosols play potentially an important role in human health and ocean primary productivity. However, the lack of knowledge about solubility and speciation of metal ions in the particles or after solubilisation in aqueous media (sea or surface waters, cloud or rain droplets, biological fluids) limits our understanding of the underlying physico-chemical processes. In this work, a wide range of metals, their soluble fractions, and inorganic/organic compounds contained in urban particulate matter (PM) from Padua (Italy) were determined. Metal solubility tests have been performed by dissolving the PM in water and in solutions simulating rain droplet composition. The water-soluble fractions of the metal ions and of the organic compounds having ligand properties have been subjected to a multivariate statistical procedure, in order to elucidate associations among the aqueous concentrations of these PM components in simulated rain droplets. In parallel, a multi-dimensional speciation calculation has been performed to identify the stoichiometry and the amount of metal-ligand complexes theoretically expected in aqueous solutions. Both approaches showed that the solubility and the aqueous speciation of metal ions were differently affected by the presence of inorganic and organic ligands in the PM. The solubility of Al, Cr, and Fe was strongly correlated to the concentrations of oxalic acid, as their oxalate complexes represented the expected dominant species in aqueous solutions. Oxalates of Al represented ∼98% of soluble Al, while oxalates of Cu represented 34-75% of the soluble Cu, and oxalates of Fe represented 76% of soluble Fe. The oxidation state of Fe can strongly impact the speciation picture. If Fe is present as Fe(II) rather than Fe(III), the amount of Cr and Cu complexed with diacids can increase from 75% to 94%, and from 32% to 53%, respectively. For other metals, the solubility depended on the formation of soluble aquo-complexes, hence with a scarce effect of the organic ligands. An iron-oxalate complex was also directly detected in aerosol sample extracts.
Highlights
Atmospheric conditions during the sampling campaign were characterised by low temperatures, close to 0 C during the first part of the campaign and reaching a maximum temperature of 14 C toward the end of the campaign, high relative humidity (RH), often above 90%, and high aerosol loading with PM2.5 concentrations ranging between 12 and 113 mg mÀ3 (Fig. 1, Tables S1 and S2)
PM2.5 concentrations were in the range of those observed in other northern Italian cities, such as Bologna in which average PM2.5 values were in the range 31e59 mg mÀ3 in the years 2011e2013 (Pietrogrande et al, 2014), and Milan in which average PM2.5 concentrations in the winter were 60 mg mÀ3 in the years 2006e2009 (Perrone et al, 2012)
We investigated the formation of metal-ligand complexes, and how this process may affect the solubility of metals, in urban atmospheric aerosol samples collected in the city centre of Padua (Italy) from the 5th December 2013 to the 1st April 2014
Summary
As many WSOC and inorganic compounds have coordinating properties towards metal ions, bioavailability depends on the stoichiometry and stability constants of the complexes formed in solution between the metal ions and the ligands contained in PM, after PM enters into contact with water (Giorio et al, 2017; Scheinhardt et al, 2013; Wei et al, 2019). The northern Italian Po Valley, a semi-closed basin surrounded by complex orography, represents a natural laboratory for studying aqueous phase processing of aerosol It is one of the major European air pollution hotspots and environmental conditions favour fog events during the winter. A multidimensional speciation calculation has been performed to evaluate the stoichiometry and the concentrations of metal-ligand complexes expected in aqueous solutions For those complexes expected at higher concentrations their detection was attempted by nano electrospray ionisation high-resolution mass spectrometry (nanoESI-HRMS) investigations
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