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”).

Advanced Reading Box 2.1 Terminal Velocity of Aerosol Particles

      It is easy to show [2] that for large (micron‐sized or more) particles with a diameter d and a density ρ_p, their terminal velocity due to gravity in a still gas with a density ρ_g is:

      (2.1)

      where η is the viscosity of the gas (η = 1.81 × 10⁻⁵ Pa s for air at Standard Conditions) and g is the acceleration due to gravity. For 1 μm diameter particles with a typical density (1000–5000 kg/m³), this gives ~0.1 mm/s. The equation, however is only valid for relatively large particles. In its derivation, it is assumed that the gas velocity at the particle surface is zero, which is invalid for very small particles whose size is less than the mean‐free path of the gas molecules. To put it crudely, very small particles “slip” through the gaps between the gas molecules and fall faster than predicted by the equation. As the particles get smaller, an increasing slip correction factor needs to be applied and this can get to be a factor of 10 or more. Even so, the fact that the terminal velocity decreases as d² ensures that small particles do drop more slowly. Applying the slip correction factor to 10 nm diameter particles falling through the air gives a terminal velocity of ~0.1 μm/s (or about a meter every four months). For all practical purposes, nanoparticles can be assumed to be suspended in the atmosphere. This discussion however is only relevant to the settling of particles by gravity. Accumulation, rainout, and washout will remove them more rapidly.

Till recently, naturally occurring nanoparticles in the atmosphere have been relatively overlooked because most natural processes that generate particles produce a wide size range that encompasses pieces up to macroscopic dimensions. Within such a wide distribution, the mass fraction (or volume fraction) of particles belonging to the nanoworld (<100 nm) is a tiny proportion of the whole distribution. For many processes in which the particles are interacting with their environment, however, it is the number density that is the important parameter and here the nanoparticles dominate. Figure 2.2 shows the concentration of particles as a function of their diameter in a typical urban aerosol using three different measures. The lower curve shows the total volume of particles in cubic μm per cubic centimeter of air. In this way of measuring the aerosol concentration, it would appear that particles with sizes in the range 0.5–10 μm dominate the distribution. These tend to be mechanically generated, for example, tire dust, wind‐blown sand grains, etc. In contrast, the upper graph shows the distribution of the same population when we measure simply the number of particles per cubic centimeter. The distribution is almost entirely in the nanoworld and dominated by nanoparticles with a typical size of 10–20 nm with larger particles being virtually absent on the same scale. These are mostly produced by combustion sources or by chemical reactions resulting in nitrates and sulfates. Some are smaller sea‐salt crystals produced by a bubble‐bursting mechanism, described below, and carried overland by air currents. In situations where each particle does something, for example, in respiratory problems, this upper graph is the relevant distribution. Note that according to the figure each lungful of air in the urban environment contains millions of nanoparticles. This is a figure to bear in mind when we talk of the hazards of nanoparticles resulting from nanotechnology. It is hard to imagine nanotech industries producing loose aerosol over the world on this scale.