PM_2.5-bound metallic elements in village and urban airs in North China during wintertime: Concentration, morphology, mixing state, and size distribution

Atmospheric metal pollution is a significant environmental issue in China. Understanding the microscopic characteristics of metallic elements are crucial for investigating their sources and health effects, but this information is still limited. In this study, transmission electron microscopy (TEM) combined with bulk analysis method were employed to investigate the microscopic characteristics and mass concentrations of atmospheric metallic elements at a village site and an urban site located in North China Plain during wintertime. Our results reveal that the total mass concentrations of 16 metallic elements (including nine toxic heavy metallic elements) in PM_2.5 were 3439.18 ± 1101.24 ng/m³ (1129.6 ± 376.85 ng/m³) at the village site and 3555.1 ± 916.71 ng/m³ (1295.26 ± 446.39 ng/m³) at urban site, accounting for 2.23 % and 2.76 % of PM_2.5, respectively. We found that K, Ca, and Fe were the dominant elements, constituting over 75 % of total metal mass. TEM analysis indicates that these metallic elements were mainly enriched in six types of individual particles, including K-rich, mineral, fly ash, Fe-rich, Zn-rich, and Pb-rich particles. Mineral particles were predominant in all individual metal particles at urban site, while K-rich particles became the dominant metal particles influenced by residential biomass burning at village site. Moreover, we found toxic metal particles (Fe-rich, Zn-rich, Pb-rich, and fly ash) presented smaller mean sizes (243 nm) in village air compared to urban air (337 nm), suggesting that metal particles in village air might pose a higher health risk to the residents. These results emphasize that atmospheric metal pollution in the village areas need more attentions in the future.