Understanding PM-bound trace element solubilities: Controls, anthropogenic and natural sources

Heavy metal pollution in fine particulate matter (PM_2.5) is a subject of great concern due to rapid urbanization and industrialization. To evaluate the chemical characteristics and sources of PM_2.5-bound trace elements (TEs) in PM_2.5, chemical fractions of two-year (2019-2020) key TEs (Al, Si, Ca, Fe, Ti, K, S, P, Mn, Ni, V, Cr, Cu, Zn and Pb) were investigated. A four-step sequential extraction method was adopted to extract and analyse TEs exchangeable, reducible, oxidisable and residual fractions using inductively coupled plasma optical emission spectrometry. Positive Matrix Factorization was applied to the fractional concentrations of individual elements to apportion TE sources in PM_2.5. The model resolved factors were identified as i) mineral dust ii) agricultural crop burning residues iii) vehicular emission iv) coal combustion residues, and v) industrial emission. Mobility of elements from geogenic sources increased during 2020 (COVID 19 lockdowns year), likely due to the formation of soluble secondary inorganic species, while mobility of elements from anthropogenic sources decreased. These findings suggest that anthropogenic activities influence not only TE abundance but also their solubility and bioavailability.