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.

The shungites of Karelia (Russia) form a large, diverse group of black Precambrian rocks, all of which contain an intriguing type of poorly crystalline carbon. Wide differences of opinion exist about its structural state and its relation to carbon from other geological environments and origins. We used a variety of measurement techniques to determine the structural features of the carbon in shungite samples and to relate them to other natural sources of carbon. Although there is a wide range of types of shungite rocks, it appears as if the structure of their carbon is similar throughout in respect to high-resolution transmission electron microscopy (HRTEM) images, and electron and X-ray diffraction patterns. Other samples whose carbon is indistinguishable using these techniques include those from the Erickson gold mine (Canada), the Sovetskaya gold mine (Russia), and the Sudbury impact structure (Ontario). Carbon samples from different localities of the Shunga district are characterized by containing curved layers, similar to samples from natural and synthetic cokes. The HRTEM images and nanodiffraction patterns of shungites suggest that some 3-dimensional closed shells occur but, more commonly, there are fractions of such shells or regions of structure that are highly disordered into bent stacks of graphene layers.

Nanocrystals of magnetite (Fe3O4) in a meteorite from Mars provide the strongest, albeit controversial, evidence for the former presence of extraterrestrial life. The morphological and size resemblance of the crystals from meteorite ALH84001 to crystals formed by certain terrestrial bacteria has been used in support of the biological origin of the extraterrestrial minerals. By using tomographic and holographic methods in a transmission electron microscope, we show that the three-dimensional shapes of such nanocrystals can be defined, that the detailed morphologies of individual crystals from three bacterial strains differ, and that none uniquely match those reported from the Martian meteorite. In contrast to previous accounts, we argue that the existing crystallographic and morphological evidence is inadequate to support the inference of former life on Mars.

Certain chemical and mineral features of the Martian meteorite ALH84001 were reported in 1996 to be probable evidence of ancient life on Mars. In spite of new observations and interpretations, the question of ancient life on Mars remains unresolved. Putative biogenic, nanometer magnetite has now become a leading focus in the debate.