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Nanoparticles have no exact definition, but they are aggregates of atoms bridging the continuum between small molecular clusters of a few atoms and dimensions of 0.2–1 nm and chunks of solid containing millions of atoms and having the properties of macroscopic bulk material. In water, nanoparticles include colloids; in air, they include aerosols. Nanoparticles are ubiquitous. We pay to have them. We pay more to not have them. They occur as dust in the air, as suspended particles that make river water slightly murky, in soil, in volcanic ash, in our bodies, and in technological applications ranging from ultratough ceramics to microelectronics. They both pollute our environment and help keep it clean. Microbes feast on, manufacture, and excrete nanoparticles.
Understanding nanoparticle formation and properties requires sophisticated physics, chemistry, and materials science. Tailoring nanomaterials to specific applications requires both science and Edisonian inventiveness. Applying them to technology is state-of-the-art engineering. Tracing their transport and fate in the environment invokes geology, hydrology, and atmospheric science. Applying them to improving soil fertility and water retention links soil science and agriculture to surface chemistry. Understanding their biological interactions brings in fields ranging from microbiology to medicine. Probing the impact of nanoparticles on humans and of human behavior on the production and control of nanoparticles requires the behavioral and social sciences, e.g., in dealing with issues of automotive pollution. The purpose of this review is to describe some of the unique features of nanoparticles and to discuss their occurrence and importance in the natural environment.
Although we often think of the natural environment as that part of the planet which we can see, a somewhat broader definition includes the “critical zone”: the atmosphere, hydrosphere, and shallow portion of the solid earth that exchange matter on a geologically short time scale, on the order of tens to thousands of years. This critical zone affects us directly, and our activities influence it. Because of the active chemical reactions continuously taking place in the critical zone, and because its temperatures and pressures are relatively low and it is dominated by water, solids are constantly being formed and decomposed. Many of these solids start out as nanoparticles; many remain so. In a yet broader sense, our entire planet from crust to core, the solar system, and the galaxy are part of our environment.