Magnetic and Isotopic Analyses Unravel the Abundance and Sources of Magnetic Nanoparticles in Urban PM _2.5

Abstract Magnetic nanoparticles (NPs) are increasingly recognized as an emerging threat to ecosystems and human health, yet their mass concentrations and sources in urban atmosphere remain poorly characterized. Here, we developed a robust methodology for the extraction, quantification, and source apportionment of magnetic NPs in urban PM 2.5 . Our results show that these particles (0.57–5.75 μg/m 3 ) constitute 0.65%–7.9% of PM 2.5 , which is an order of magnitude higher than previous estimates based on iron concentrations, highlighting their substantial contribution to particulate matter. Integrated morphological, magnetic, and geochemical analyses reveal that these particles are primarily spherical nanoscale (<100 nm) magnetite formed through high‐temperature processes. By combining magnetic coercivity analysis with lead isotopic fingerprinting, we further trace the primary anthropogenic sources of these NPs to gasoline (59.2%) and diesel (40.8%) vehicle exhaust. These findings provide critical insights for devising targeted strategies to mitigate urban airborne magnetic particle pollution.