Metal Ion Release from Fine Particulate Matter Sampled in the Po Valley to an Aqueous Solution Mimicking Fog Water: Kinetics and Solubility

Metals are among the key aerosol components exerting adverse health effects. Their toxic properties may vary depending on their chemical form and solubility, which can be affected by aqueous processing during aerosol atmospheric lifetime. In this work, fine particulate matter (PM2.5) was collected in the city centre of Padua in the Po Valley (Italy), during a winter campaign. Part of the sampling filters were used to measure the kinetics by which metal ions and other elements can leach from PM2.5 to an aqueous solution mimicking fog water in the winter in temperate climate regions (pH 4.7, 5 degrees C). The leaching process was interpreted by a first order kinetics, and the fitting of the experimental data allowed to obtain the leaching kinetic constants and the equilibrium concentrations (i.e., at infinite time) for all elements. The remaining filter parts were mineralised, through two subsequent extraction steps, and the extracts were analysed by ICP-MS to gain the total elemental content of PM for a large number of elements. We found that elements can leach from PM with half times generally between 10-40 minutes, which is a timescale compatible with atmospheric aqueous processing during fog events. For instance, aluminium(III) in PM2.5 dissolved with an average k = 0.0185 min(-1), and t(1/2) = 37.5 min. Nevertheless, a fraction of each element was immediately solubilised after contact with the extraction solution suggesting that metal ion solubilisation may already had started during particle lifetime in the atmosphere.