Nuclei mode

The distribution of aerosols (shown in Figure 2—per unit mass) can also be expressed in terms of the total number of particles, which places greater emphasis on the smaller particles and provides information about the nuclei mode and the process of accumulation. Aerosol concentrations are sometimes expressed in terms of aerosol surface area, which is closely related to visibility impacts. 

Atmospheric particles have spherical equivalent diameters (Dp) ranging from 1 nm to 100 pm. Plots of particle number concentration (as well as surface area and volume) as a function of particle size usually show that an atmospheric aerosol is composed of three or more modes, as illustrated in Figure 1. By convention, particles are classified into three approximate categories according to their size Aitken (or transient) nuclei mode (Dp <0.1 pm), accumulation mode (0.1 < Dp < 2.5 pm), and coarse mode (Dp > 2.5 pm) (Seinfeld and Pandis 1998). Particles smaller than 2.5 pm are generally classified as fine. The terms PM2.5 and PMio refer to particulate matter with aerodynamic equivalent diameters under 2.5 and 10 pm, respectively. These terms are often used to describe the total mass of particles with diameters smaller than the cutoff size. 

Nuclei mode particles in this mode are formed by nucleation of atmospheric gases in a supersaturated atmosphere and their size is of the order of nanometres. 

F12 Brownian coagulation of nuclei-mode particles with themselves to produce accumulation-size (chemically) externally mixed particles. 

F15 cloud formation yielding nuclei-mode interstitial... 

More recently, Whitby (1978) has analyzed the results of much more numerous size distribution observations carried out mainly by his group who used a combination of expansion chamber, electrical mobility and optical counter techniques. This analysis clearly shows that the complete size distribution is composed of three separate log-normal distributions as demonstrated in Fig. 26. Whitby speculates that the first distribution, the nuclei mode, is controlled by the condensation of vapour, (predominantly H2S04) formed by chemical reactions. Thus, the concentration of these small particles was found to be very significant in irradiated polluted atmosphere. On the other hand, the so-called accumulation... 

Size distribution of the volume of aerosol particles (solid line) according to Whitby (1978). Dotted line corresponds to the power law of Fig. 26. V particle volume n nuclei mode a accumulation mode < coarse particle mode tlrGV geometric volume mean size. (By courtesy of Atmospheric EnvironmentI... 

This means that the nuclei mode was not studied. 

The phenomena that influence particle sizes are shown in an idealized schematic in Figure 2.7, which depicts the typical distribution of surface area of an atmospheric aerosol. Particles can often be divided roughly into modes. The nucleation (or nuclei) mode comprises particles with diameters up to about 10 nm. The Aitken mode spans the size range from about lOnm to lOOnm (0.1 pm) diameter. These two modes account for the... 

The mass distribution of continental aerosol not influenced by local sources has a small accumulation mode and no nuclei mode. The PM10 concentration of rural aerosols is around 20 pg m-3. 

Nuclei Mode Accumulation Mode Coarse-Particle Mode... 

The contribution to the total scattering coefficient bSCM can be calculated for each of the modes of the different aerosol classes in Table 15.4 from the theory in Section 15.3. The accumulation mode is found to be the dominant scattering mode, with the coarse mode contributing a small amount and the nuclei mode a negligible amount. For the two background aerosol cases ... 

An important feature of atmospheric aerosol size distributions is their multimodal character. Mass distributions, measured in urban centers, are characterized by three modes with a minimum between 1.0 and 3 The size range of particles larger than the minimum (supermicron particles) is termed coarse, while the smaller particles are called fine. The three modes present in the mass distribution of Figure 7.14 correspond to the nuclei mode (particles below 0.1 /zm), accumulation mode (0.1 < Dp < /.tm), and coarse mode Dp > )Lim) (Whitby and Sverdrup, 1980). Thus the fine particles include both accumulation and nuclei modes. The boundaries between these sections are not precise (recall in Chapter 2 that we divided fine and coarse modes at 2.5 /zm diameter). Note that our definition of modes has been based on the mass (or volume distribution). The location of modes may be different if they are based on the number or surface distribution. 

The mass concentrations of the accumulation and coarse particle modes are comparable for most urban areas. The nuclei mode, with the exception of areas close to combustion sources, contains negligible volume (Figures 7.12 and 7.14). Most of the aerosol surface area is in particles of diameters 0.1 to 0.5 /xm in the accumulation mode (Figure 7.12). Because of this availability of area, transfer of material from the gas phase during gas-to-particle conversion occurs preferentially on them. 

Aerosols in rural areas are mainly of natural origin but with a moderate influence of anthropogenic sources (Hobbs et al., 1985). The number distribution is characterized by two modes at diameters about 0.02 and 0.08 /xm, respectively (Jaenicke, 1993), while the mass distribution is dominated by the coarse mode centered at around 7 /xm (Figure 7.17). The mass distribution of continental aerosol not influenced by local sources has a small accumulation mode and no nuclei mode. The PM lo concentration of rural aerosols is around 20 /xg m . 

While vehicles are the major source of UFP within cities, other indoor sources also contribute to UFP within homes. Hoek et al. (2008) studied the particle number concentration in homes in four major European cities. It was observed that UFP number concentrations in the study participants homes were poorly correlated with central site measurements during the day. This correlation improved slightly at night. The difference between the indoor and outdoor UFP concentrations was attributed to the presence of numerous indoor sources. Koponen et al. (2001) measured the indoor and outdoor size distribution of UFP and demonstrated that accumulation mode particles (>90 nm) are directly related to outdoor sources while nuclei mode particles (<50 nm) originate from indoor sources. 

Fushimi A, Hasegawa S, Takahashi K et al (2008) Atmospheric fate of nuclei-mode particles estimated from the number concentrations and chemical composition of particles measured at roadside and background sites. Atmos Environ 42 949-959 Gehin E, Ramalho O, Kirchner S (2008) Size distribution and emission rate measurements of fine and ultrafine particle from indoor human activities. Atmos Environ 42 8341-8352 Graham S, McCurdy T (2004) Developing meaningful cohorts for human exposure models. J Expo Anal Environ Epidemiol 14 23 3... 

Nuclei mode has a geometric mean radius between 0.0025 and 0.020 /a and probably results from the condensation of gaseous carbon moieties. 

Accumulation mode encompasses particles in the size range 0.075-0.25 /a and apparently results from the coagulation and condensation of the nuclei mode particles. 

Washout by rain greatly reduces the Aitken nuclei mode and the coarse particle mode but has little effect on the accumulation mode in the trimodal size distribution (Whitby, 1975). The origin of each mode of atmospheric aerosol size distribution can be associated with various aerosol formation mechanisms, such as Brownian motion of the particles smaller than 0.1 pm in diameter, which causes the particles to diffuse and by collisions to coagulate to larger sizes. Coagulation generates multimodal distributions and affects the shape and the chemical composition of the particles. 

The aerodynamic size distribution of radionuclide-associated aerosol particles is, as mentioned in Section 2, a surface distribution, and so it is trimodal, the first mode being the so-called Aitken nuclei mode, the second the accumulation mode and the third the coarse particle mode. Analytically, these modes can be summarised as follows ... 

The nuclei or Aitken nuclei mode accounts for most of the Aitken nuclei count and originates primarily from the condensation and coagulation of highly supersaturated vapours. There is evidence that a prominent nuclei mode in the size distribution indicates the presence of substantial amounts of fresh aerosol. Many particles in the nuclei mode raise the Aitken nuclei. Usually, they do not greatly increase the aerosol mass concentration because the nuclei mode rarely accounts for more than a few percent of the total mass. 

Whitby et al. (1975) found that the nuclei mode may contain over 25 Jgm of aerosol. Whitby et al. (1976) also observed distributions in which the nuclei mode contained more volume than the accumulation mode. Because particies may serve as nuclei for the condensation of water vapour, condensation is an important growth mechanism for submlcrometre aerosol particles. Examples are fogs and hazes formed when the humidity exceeds 60%. 

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