Spatial and temporal distribution, source apportionment, and health risks of PM_2.5-bound heavy metals in a Chinese megacity, 2014-2022

Airborne particulate matter enriched with heavy metals constitutes a significant health threat across the developing world. To investigate the distribution, sources, and health risks of PM_2.5-bound heavy metals, ambient samples were collected in Baoding city from 2014 to 2022 and analyzed using inductively coupled plasma mass spectrometry (ICP-MS). Results showed that the annual mean PM_2.5 concentration in Baoding exceeded the World Health Organization (WHO) Interim Target-1. PM_2.5 levels generally declined from 2014 to 2022, with seasonal variation following the order: winter > autumn > spring > summer. Heavy metal concentrations peaked in winter, significantly exceeding those in spring, autumn, and summer, the latter exhibiting the lowest metal loading. Despite long-term air pollution controls, the annual average concentration of carcinogenic Cr(VI) may still surpass the WHO limit. Five major PM_2.5 sources were identified: coal combustion, secondary aerosols, vehicle emissions, dust, and industrial emissions. Coal combustion was the dominant source before 2017, after which secondary aerosols became predominant. Secondary aerosols also constituted the primary source in summer. Vehicle emissions and secondary aerosols contributed more to PM_2.5 in summer than in other seasons, while dust contributions were more pronounced in spring. Monte Carlo simulations indicated that PM_2.5-bound heavy metals pose non-carcinogenic risks to different populations at varying probabilities, with children exhibiting higher non-carcinogenic risks than adults. Manganese contributed most to non-carcinogenic risk. For carcinogenic risk, heavy metals showed a low probability of significant carcinogenic risk (SCR) or a high probability of acceptable risk (ACR). Carcinogenic risk probability ranked as: adult males > adult females > children, suggesting adult males may face the highest carcinogenic risk from PM_2.5-bound heavy metals. Among the five assessed metals, carcinogenic risk probability decreased in the order: As > Cr(VI) > Cd > Co > Ni, with As and Cr(VI) as dominant contributors. Both carcinogenic and non-carcinogenic risks were higher in winter than in other seasons. This study demonstrates that while coordinated air pollution controls in the Beijing-Tianjin-Hebei region have achieved some outcomes, Mn, Cr, and As still present notable potential health risks, requiring urgent attention.