Assessment of heavy metal characteristics and health risks associated with PM_2.5 in Xi’an, the largest city in northwestern China

Fine particulate matters (PM_2.5) samples were collected in Xi’an, northwestern China, from May 2015 to April 2016. The concentrations, seasonal variations, potential sources, and health risks for personal exposure for ten metallic elements (i.e., Ba, Zn, Cu, As, Ni, Pb, Mn, Cr, Cd, and Hg) bounded to PM_2.5 were determined and assessed in this study. The results showed that the average PM_2.5 mass concentration in Xi’an was 62.1 ± 35.0 μg m⁻³ during the sampling period. The annual concentration of the total quantified elements was 2459.5 ± 1789.8 ng m⁻³, with relatively higher values in winter (3334.9 ± 1690.9 ng m⁻³) and spring (2809.4 ± 2465.4 ng m⁻³), in comparison of those in summer (1857.6 ± 1162.7 ng m⁻³) and autumn (1252.5 ± 842.4 ng m⁻³). Two elements of Ba (678.0 ± 684.9 ng m⁻³) and Zn (1264.8 ± 725.3 ng m⁻³) had greater fluctuations in concentrations and were accounted for more than 80% of the concentration of total quantified elements for each season. The concentrations of As in Xi’an exceeded the national standard in China. The enrichment factors (EFs) of most target heavy metals were high, exceeding 100 for Zn, As, Pb, Cd, and Hg, attributed to strong influences from human activities. Moreover, the largest enrichment of heavy metals in PM_2.5 occurred in Xi’an in winter. Principle component analysis (PCA) was applied for source apportionment. Coal and other fuel combustion, vehicle exhaust, and industrial activities were the three major pollution sources which contributed 43.6, 29.9, and 15.3%, respectively, of the total variance of PM_2.5. The health risk assessment showed that the non-cancer risks of As, Pb, and Cr for children were greater than 1, as well as of As for adults. The cancer risks of As and Cr were higher than 1 × 10⁻⁶, indicating that the two elements had high potentials for both non-cancer and cancer risks. Our finding suggests that the PM_2.5 and related heavy metal pollutions in Xi’an were serious and posed high potential health risks. Effective controls and measures should be established in the studied area.