Bioavailability of heavy metals bounded to PM2.5 in Xi'an, China: seasonal variation and health risk assessment.

Studying the characteristics and health risks of heavy metals in atmospheric fine particulate matter (PM2.5) is a crucial component of understanding atmospheric pollution in China. Integrated 24 h PM2.5 samples were collected in winter and summer 2016 in Xi'an, China. The pollution levels, speciation, and health risks of seven PM2.5-bound metal elements (Al, As, Cd, Cr, Ni, Pb, and Zn) were investigated in this study. The average concentration of PM2.5 was 50.1 ± 30.4 μg m-3 and was higher in winter than in summer. Significant seasonal variations in the elements were also observed. The average concentration ratios of Al, As, Cd, Cr, and Pb decreased in summer by 17.5%, 6.4%, 42.5%, 34.1%, and 61.4% compared with their concentrations in winter, respectively, whereas Ni and Zn increased by 37.7% and 7.6% in summer. The soluble and exchangeable fraction (F1) accounted for large proportions of Cd and Pb concentrations, indicating their greater hazard to the environment and human health. Al, As, and Cr mainly existed in the residual state (F4), which had relatively high stability in particulate matter. Ni was consistently distributed in different forms (F1-F4). The bioavailability evaluation indicated that Pb, Cd, Ni, and Zn were potential bioavailable element which exhibited strong biological toxicity. Although the concentration of Pb was very low, its BI value was the highest. The carcinogenic and non-carcinogenic risks of Cr and As were relatively high, and thus require attention from the government and environmental management departments.