Chemical characterization and speciation of the soluble fraction of Arctic PM10

Abstract

The chemical composition of the soluble fraction of atmospheric particulate matter (PM) and how these components can combine with each other to form different species affect the chemistry of the aqueous phase dispersed in the atmosphere: raindrops, clouds, fog, and ice particles. The study was focused on the analysis of the soluble fraction of Arctic PM₁₀ samples collected at Ny-Ålesund (Svalbard Islands, Norwegian Arctic) during the year 2012. The concentration values of Na⁺, K⁺, NH₄⁺, Ca²⁺, Mg²⁺, Mn²⁺, Cu²⁺, Zn²⁺, Fe³⁺, Al³⁺, Cl^-, NO₂^-, NO₃^-, SO₄^2-, PO₄^3-, formate, acetate, malonate, and oxalate in the water-soluble fraction of PM₁₀ were determined by atomic spectroscopy and ion chromatography. Speciation models were applied to define the major species that would occur in aqueous solution as a function of pH (2-10). The model highlights that (i) the main cations such as Na⁺, K⁺, Mg²⁺, and Ca²⁺ occur in the form of aquoions in the whole investigated pH range; (ii) Cu²⁺, Zn²⁺, and, in particular, Fe³⁺ and Al³⁺ are mostly present in their hydrolytic forms; and (iii) Al³⁺, Fe³⁺, and Cu²⁺ form solid hydrolytic species that precipitate at pH values slightly higher than neutrality. These latter metals show interesting interactions with oxalate and sulfate ions, too. The speciation models were also calculated considering the seasonal variability of the concentration of the components and at higher concentration levels than those found in water PM extracts, to better simulate concentrations actually found in the atmospheric aqueous phase. The results highlight the role of oxalate as the main organic ligand in solution.

Keywords: Chemometric data treatment; Iron speciation; Oxalate speciation; Polar aerosol; Water-soluble fraction.