Size distribution of water-soluble metals in atmospheric particles in Xi'an, China: Seasonal variations, bioavailability, and health risk assessment

The water solubility of potentially toxic elements is associated with biological effects and thus should be a major toxicity evaluation criterion for elements. The size distribution and potential health risk of particle-bound water-soluble metals (Cr, Ni, Zn, Fe, Pb, Ba, Mn, Cu, and Ti) in nine particle sizes (<0.43, 0.43–0.65, 0.65–1.1, 1.1–2.1, 2.1–3.3, 3.3–4.7, 4.7–5.8, 5.8–9, and >9 μm) were investigated from December 2018 to November 2019 in Xi'an, China. Water-soluble metals exhibited a typical bimodal distribution, with a dominant peak in the coarse mode (4.7–5.8 μm) and a minor peak in the fine mode (0.43–0.65 μm). Most measured water-soluble metals (Ni, Fe, Pb, Ba, Mn, Cu, and Ti) exhibited a peak-shifting tendency from 0.43 to 0.65 to 0.65–1.1 μm (summer to winter) because of the high relative humidity and high concentrations of gaseous precursor. The bioavailability index (BI) indicated that Zn (44.3%), Mn (37.3%), Ba (34.3%), and Pb (33.6%) had higher bioavailable fractions. Cu (29.4%) and Fe (10.4%) had the highest BI at <0.43 μm in autumn. Water-soluble Cr and Pb exhibited obvious noncarcinogenic health risks to both children and adults. Water-soluble Cr also had a concerning incremental lifetime carcinogenic health risk of 8.68 × 10⁻⁶ caused primarily by the elements in fine particles (<0.65 μm). Furthermore, the relative contributions of elements in fine particles had a larger effect on bioavailability and health risks than those in coarse particles did, indicating that closer attention should be paid to fine-particle-size elements in Xi'an.