Characteristics, sources, and health risks of PM2.5-bound trace metals in northern Zhejiang Province: The effects of meteorological variables based on machine learning

Atmospheric metals have recently attracted the attention of governments recently due to their significant health risks. In retrospect, a five-year (2017–2021) continuous measurement of airborne metals (13 elements) was conducted in northern Zhejiang Province. A declining trend of total metals (22%) was found from 2017 to 2021, with K, Fe, Zn, Ca, and Mn being its predominant compositions (95.3%). The Random Forest model simulation demonstrated the important roles of meteorological conditions in the trace metal reduction, such as Ca, As, K, Cr, and Zn. In addition, the emission controls contributed more than 50% to most of the remaining metals, especially for Cu (92.6%) and V (85.9%), which demonstrated the effectiveness of the government policies. The relative humidity, wind speed and wind direction were found to be the most important variables affecting the ambient concentration of trace metals. Meteorology was demonstrated to be the worst for trace metal diffusion in 2018. The primary noncarcinogenic metal was Mn (85.9%), while the major carcinogenic metal was Cd (63.4%). Six sources were resolved by positive matrix factorization (PMF). Anthropogenic source emission was largest in winter, followed by spring, summer, and autumn. The reduction of ferrous metal smelting and coal combustion emissions might be the major goal for reducing the public health risk of trace metals in the future.