Isotopic Compositions and Source Identification of Atmospheric Particulate Bound Cadmium in Chinese Megacities

Particulate matter (PM) bound cadmium (Cd) deposition on the Earth's surface poses significant ecological risks to humans and the environment. Cd stable isotopes are useful tracers for understanding sources and biogeochemical cycling in the Earth's ecosystems, yet knowledge of atmospheric Cd isotopic compositions remains limited. Here we present measurements of Cd isotopic compositions in fine particulate matter (PM 2.5 ) in five megacities selected to represent different anthropogenic Cd emission structures across China and explore potential atmospheric Cd sources. The mean (±1 SD) δ 114 Cd values of PM 2.5 were −0.231 ± 0.520‰, −0.192 ± 0.265‰, −0.003 ± 0.167‰, 0.005 ± 0.089‰, and 0.090 ± 0.121‰ in Chengdu, Lanzhou, Guangzhou, Jinan, and Beijing, respectively, more negative than δ 114 Cd signatures in the global remote atmosphere. Anticorrelations between δ 114 Cd and anthropogenic emission tracers (i.e., PM 2.5 ‐bound Cd concentrations and enrichment factors), and high Cd/As ratios in Chengdu and Lanzhou, indicate anthropogenic atmospheric Cd emissions mainly from nonferrous metal smelting. No such relationship was observed in other cities, where the δ 114 Cd and Cd/As ratios indicate coal‐fired emissions. Overall, the results of our study suggest that compared with the remote atmosphere, both coal combustion and nonferrous metal smelting cause a shift in PM 2.5 ‐bound Cd toward lighter isotopic compositions, with nonferrous metal smelting characterized by significantly more negative δ 114 Cd values and higher Cd/As ratios. Combined with Cd/As ratios, Cd isotopes provide a promising tool for identifying major anthropogenic atmospheric Cd sources.