Are “Clean” Days Really Clean? Single Particle Elemental Signatures of PM_2.5 Unveil Their Pulmonary Health Risks

Ambient PM_2.5 mass concentrations inadequately reflect health risks due to compositional heterogeneity. This study utilized single-particle inductively coupled plasma time-of-flight mass spectrometry (spICP-TOF-MS) to characterize high-resolution elemental signatures of metal-containing fine particles (MCFPs) in PM_2.5 from an urban area with an intensive anthropogenic influence during different pollution levels in the winter and spring. Al-, Si-, Fe-, Mn-, and Pb-containing FPs accounted for approximately 80% of total MCFPs, with higher number concentrations in spring than in winter, increasing with pollution levels. Unlike Al- and Si-containing FPs, Fe-containing FPs were predominantly multimetal (mm)FPs (48-87%), with higher proportions in winter than spring and increasing with pollution levels. Notably, a larger fraction of mmFPs, particularly Fe-rich FPs, were associated with toxic metals (e.g., Mn and Pb) on clean days than on polluted days. Lung cytotoxic potencies, including oxidative stress and cytotoxicity, were up to 8.1 and 6.3 times greater on clean days than on polluted days. Fe-rich FPs and their associated toxic metals were identified as first-tier factors in regulating cytotoxic potency, playing a more critical role than organic/elemental carbon and dissolved metals. Machine learning-based source apportionment indicated that anthropogenic-sourced MCFPs, especially Fe-rich FPs, contributed more during winter than in spring, with peak contributions on clean days.