Chemical partitioning of fine particle-bound metals on haze–fog and non-haze–fog days in Nanjing, China and its contribution to human health risks

• PM2.5 was extremely elevated with a mean concentration of 281 μg/m3 on HF days.
• All elements had higher contents and many metals had higher enrichment on HF days.
• High temperature and humidity may increase the metal bio-accessible fraction.
• Human health risk was assessed combined with metal forms on HF and non-HF days.

Information on chemical partitioning and associated risk of airborne metals, particularly during a haze–fog episode, is limited. Fine particulate matter (PM2.5) was collected during a severe haze–fog event in winter and non-haze–fog periods in summer and fall from an urban region of a typical Chinese mega-city, Nanjing. The particulate-bound metals (Al, As, Ba, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Mo, Na, Ni, Pb, Sr, Ti, V, and Zn) were chemically fractionated in a four-step sequential extraction procedure and human health risk was assessed. During the haze–fog episode, PM2.5 was extremely elevated with a mean concentration of 281 μg/m3 (range: 77–431 μg/m3), whereas the mean PM2.5 concentrations in summer and fall periods were 86 μg/m3 (range: 66–111 μg/m3) and 77 μg/m3 (range: 42–131 μg/m3), respectively. All elements had significantly higher concentrations and many metals exceeded relevant limits on haze–fog days. K, Na, Sr, Zn, Mo, Ca, Cd, Mg, Mn, Cu, Ba, Cr and As all showed relatively high proportions of the soluble and exchangeable fraction and strong bio-accessible potential. High temperature and humidity may increase the bio-accessible fraction of many airborne metals. The hazard index for potential toxic metals was 0.115, which was lower than the safe limit (1). However, the combined carcinogenic risk was 1.32 × 10− 6 for children and 5.29 × 10− 6 for adults, with both values being higher than the precautionary criterion (10− 6). Results of this study provide information for the behavior and risk mitigation of airborne metals.