Abundance, solubilities and sources of PM2.5-associated hazardous elements in the megacity of Beijing, China.

While the solubility of atmospheric elements critically influences their toxicity and environmental mobility, global understanding of water-soluble hazardous elements (WSHEs) in PM2.5 remains limited. To address this, PM2.5 samples were collected from July 2021 to March 2022 across six major cities in the Beijing-Tianjin-Hebei region and its surroundings (BTHs). Total elemental concentrations (all cities) and water-soluble fractions (Beijing) were quantified via inductively coupled plasma mass spectrometry (ICP-MS), focusing on Cd, V, Cr, Ni, Se, As, Mn, and Pb. Regional analyses revealed synchronized pollution patterns across BTHs, driven by shared industrial/energy activities. In Beijing, WSHEs constituted 25.0-39 .3 % of total concentrations, with Cd (35.7 %) and Mn (39.3 %) showing peak solubility. WSHE levels rose during air quality deterioration but declined during dust storms due to crustal sources with low solubility. Key drivers of solubility included particle acidity, liquid water content, and organic carbon, emphasizing atmospheric processing (e.g., acid dissolution) as a bioaccessibility amplifier. Positive Matrix Factorization identified vehicular emissions (38 %) and coal combustion (20 %) as dominant WSHE sources, followed by waste incineration (20 %), metal smelting (14 %), electronics manufacturing (6 %), and dust (2 %). Coal and vehicular sources contributed disproportionately to bioaccessible elements, whereas dust-derived elements exhibited minimal solubility. These results highlight the elevated health risks from anthropogenic WSHEs and the urgency of prioritizing sector-specific controls (e.g., accelerating vehicle electrification, phasing out residential coal) over broad PM2.5 reduction strategies. The study establishes a mechanistic link between emission sources, atmospheric aging, and elemental bioaccessibility, offering actionable insights for mitigating toxic elements exposure in megacities.