Читать книгу Introduction to Nanoscience and Nanotechnology - Chris Binns - Страница 28

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.

Figure 2.2 Size distribution of urban aerosol. The concentration of airborne particles (aerosol) as a function of their diameter in a typical urban environment using three different measures. The lower curve shows the total volume of particles in cubic μm per cubic centimeter of air. The middle curve shows the total surface area of the particles in square μm per cubic centimeter of air. The upper curve gives simply the number of particles per cubic centimeter of air. It is evident that while the total volume (and also the mass) contained in the particles per cubic centimeter is concentrated in large particles outside of the nanoworld. In terms of particle numbers per unit volume, the distribution is dominated by nanoparticles with a typical size of 10–20 nm diameter.

Source: Reproduced with the permission of Wiley from J. J. Seinfeld and S. N. Pandis [3].

A common process that produces nanoparticles in the atmosphere is gas‐to‐particle conversion (GPC). When a vapor is rapidly cooled, for example, in combustion where hot gases meet cool air, the atoms or molecules in the vapor condense to form particles. Alternatively, a vapor produced by some process may chemically react with the atmosphere to produce a less volatile compound, which then condenses into solid or liquid particles. Just about any combustion will generate a cloud of nanoparticles. For example, Figure 2.3 shows the particles generated by a “nightlite” type candle. Burning petrol and diesel also generate nanoparticles so in urban areas this is a major source. A particular issue is diesel engines, which generate most of their nanoparticles from sulfur contained in the fuel. A massive worldwide effort to reduce the level of sulfur in diesel and thus the number of nanoparticles produced has brought down the concentration from 5000 ppm before 1993 to 10 ppm today.

Figure 2.3 Nanoparticles produced by candles. The size distribution of particles produced by combustion from a standard “nightlite” type candle. The distribution was measured using an aerosol particle sizer and if plotted as the number density, consists mostly of nanoparticles with diameters around 10 nm. These are predominantly carbon particles. The method used to measure the size distribution (differential mobility analyzer) is described in Chapter 5, Section 5.1.7.