Hosted and free-floating metal-bearing atmospheric nanoparticles in Mexico City

Nanoparticles (NPs) are ubiquitous in the atmosphere. Because of their small sizes, they can travel deeply into the lungs and other parts of the body. Many are highly reactive which, combined with their large surface areas, means they can seriously affect human health. Their occurrences in the atmosphere and their biological effects are not well-understood. We focus on NPs that were either free-floating or hosted within large aerosol particles (aerodynamic diameter 50-300 nm) and consist of or contain transition or post-transition metals (m-NPs). The samples were collected from ambient air above Mexico City (MC). We used transmission electron microscopy to measure their sizes and compositions. More than half of the 572 m-NPs that we analyzed contain two or more metals, and Fe, Pb, or Zn occurs in more than 60%. Hg occurs in 21% and is especially abundant in free-floating m-NPs. We find that m-NPs are common in polluted air such as in the MC area and, by inference, presumably other megacities. The range and variety of compositions of m-NPs that we encountered, whether free-floating or hosted within larger aerosol particles, indicate the complicated occurrences that should be considered when evaluating the health effects of m-NPs in complex urban areas.