Читать книгу Introduction to Nanoscience and Nanotechnology - Chris Binns - Страница 27

The particles in the Earth's atmosphere have an important influence on the climate, but also have a poorly understood effect on life and our health. Improving our understanding of the effect of airborne nanoparticles is becoming increasingly important in a world where nanotechnology is poised to become a major activity. Clearly, the amount of manufactured nanoparticles will increase, so it is wise to be aware of how they interact with life and with the environment. It is important to emphasize, however, that manufactured nanoparticles are normally bound up in some material and the number of “loose” particles produced by nanotechnology will not necessarily become significant compared to those produced by the natural processes described below. In this chapter, the discussion is extended to encompass nanoparticles that are generated by existing human activities not directly involving nanotechnology, such as power generation, transport, etc. Obviously, these are not naturally occurring in the normal sense of the phrase, but they are a component of a pre‐nanotechnology background of nanoparticles in which we live. The effect of naturally occurring nanoparticles on the environment is an enormous multidisciplinary subject and a rigorous discussion is well beyond the scope of this book. It is an important hot topic, however, as it encompasses climate change and nanoparticles are implicated in many of the feedback mechanisms involved in the Gaia hypothesis that treats the Earth as a living organism. The aim of this chapter is to describe, in general terms, where the nanoparticles come from and, as in the previous chapter, emphasize the special nature of particles belonging to the nanoworld (<100 nm – Figure I.1).

Figure 2.1 Sources of background nanoparticles. (a) Volcanoes and (b) forest fires produce nanoparticles in the atmosphere (aerosols). (c) Hydrothermal vents produce nanoparticles in the ocean (hydrosols). (d) Some bacteria produce nanoparticles such as this river‐dwelling bacterium that manufactures magnetic nanoparticles (black dots). (e) Supernova explosions such as the crab nebula shown here spread nanoparticles through space.

Source: (a) US Geological Survey. (b) Reproduced with permission from the government of British Columbia. (d) Reproduced with the permission of the Spanish Society for Microbiology from D. Schüler [1]. (e) NASA.

Naturally occurring nanoparticles are ubiquitous in land, sea and air and come from a number of processes (Figure 2.1), including volcanic activity, forest fires, ocean bed hydrothermal vents,¹ geological processes, living creatures, and human industrial activity. They are also to be found in space (though normally called dust by astronomers) and produced by, among other things, supernova explosions.²

To begin with, we will focus on atmospheric nanoparticles as these probably have the most immediate effect on living things. The general term for a tiny (solid or liquid) particles suspended in a gas is an aerosol. This term was first used in the 1920s to distinguish air‐suspended particles from liquid suspensions, or hydrosols. The term suspension implies that the particles are defying gravity, but this, of course, is not the case. The particles are falling through the gas (a viscous medium), but their terminal velocity due to gravity is so low that it may take years for them to settle (see Advanced Reading Box 2.1). In this regime, for all practical purposes, we can consider them to be suspended. Other processes can, however, remove nanoparticles from an aerosol. To begin with, if they have a sufficiently high concentration they will agglomerate and the larger particles will settle much more rapidly. In addition, in a humid atmosphere, nanoparticles will act as nuclei for the formation of water droplets (see below) and if these grow large enough to fall as rain, this will act as a removal mechanism (“rainout”). Alternatively, the particles can be incorporated into existing raindrops and removed (“washout”).