Characteristics and risk assessment of heavy metals and their acid-extractable fractions in atmospheric dustfall in urban and rural areas of Xi’an, China

This study presents a comprehensive investigation of 12 heavy metals in atmospheric dustfall from Xi’an, a major city in northwestern China, during 2021–2022. The results revealed severe contamination levels, with Cd being the most heavily contaminated element, exceeding the Shaanxi soil background value by 45.1 times, followed by Zn, Pb, and Cu. Notably, acid-extractable fractions of Zn and Ni showed increasing trends compared to levels from a decade ago, whereas Pb exhibited a decreasing trend. Spatiotemporal variations were more pronounced in acid-extractable fractions than in total metal concentrations. Bioavailability assessments identified Zn, Cd, and Mn as the most bioavailable metals, highlighting their enhanced mobility and potential toxicity. Source apportionment resolved three dominant sources in urban areas-natural sources (35.8%), industrial/traffic emissions (26.5%), and coal combustion (25.6%)-while industrial and coal-related sources predominated (31.1%) in surrounding counties. Health risk assessment underscored Cr as a major concern, presenting notable carcinogenic and non-carcinogenic risks. This work provides novel insights into the bioavailability and evolving trends of metal fractions in dustfall, offering a scientific basis for targeted pollution control in arid and semi-arid urban environments.