Accumulation mode particles

As indicated above, there is a relationship between particle concentration, equilibrium factor and the amount of highly mobile radioactive particles. Removal of the accumulation mode particles may decrease the decay product exposure, but increase the dose because of the high effectiveness of the "unattached activity in dose deposition. Thus, air cleaning may not succeed in lower risk unless both factors are taken into account. Jonassen explores electrostatic filtration in this context. Finally, design considerations are presented for a possible alternative control system using activated carbon in an alternating bed system. 

Gillani, Leaitch, and co-workers (1995) carried out a detailed study of the fraction of accumulation mode particles (diameters from 0.17 to 2.07 /Am) that led to cloud droplet formation in continental stratiform clouds near Syracuse, New York. When the air mass was relatively clean, essentially all of the particles were activated to form cloud droplets in the cloud interior and the number concentration of cloud droplets increased linearly with the particle concentration. However, when the air mass was more polluted, the fraction of particles that were activated in the cloud interior was significantly smaller than one. This is illustrated by Fig. 14.40, which shows the variation of this fraction (F) as a function of the total particle concentration, Nun. In the most polluted air masses (as measured by large values of Nun), the fraction of particles activated was 0.28 + 0.08, whereas in the least polluted, it was as high as 0.96 + 0.05. The reason for this is likely that in the more polluted air masses, the higher number of particles provided a larger sink for water vapor, decreasing the extent of supersaturation. 

As a result of the particle size-dependent properties the accumulation mode particles having highest penetration efficiencies and lowest deposition rates tend to enter indoors most efficiently and remain suspended there, thus substantially contributing to indoor exposures. Another implication is that the particle size distribution indoors differs significantly from that outdoors, even in the absence of indoor sources. Finally particle infiltration varies from home to home, resulting in higher variability across homes in indoor particle concentrations compared to outdoor concentrations. 

Infiltration factors for UFP assessed by total particle number counts were somewhat lower than for PM25 in the four European cities included in the RUPIOH study (Table 1), but higher than for coarse particles. A large Canadian study in Windsor, Ontario in which total particle number counts were measured with PTraks reported infiltration factors of 0.16, 0.26, and 0.21 in the first summer, winter, and second summer, respectively, with a large variability for individual homes [19]. The lower infiltration of ultrafine particles is consistent with lower penetration and higher decay rates due to diffusion losses compared to accumulation mode particles. 

Kumar P, Fennell P, Britter R (2008) Effect of wind direction and speed on the dispersion of nucleation and accumulation mode particles in an urban street canyon. Sci Total Environ 402 82-94... 

As shown in Figure 1, within an atmospheric aerosol the smallest particles usually dominate the total number of particles, while the accumulation and coarse modes often determine the total surface area and volume (i.e., mass), respectively. For example. Figure 3 shows results from a study in Atlanta where nanoparticles (Dp = 3-10 nm) and nano- and ultrafme particles (Dp = 10-100 nm) contributed approximately 30 and 60%, respectively, to the total particle number concentration (Dp < 2 pm). However, in terms of particle mass, the accumulation mode particles were dominant, and nanoparticles with Dp < 10 nm contributed insignificantly. 

This section provides a conceptual framework and several examples of modeling and fieldwork on the growth of atmospheric nanoparticles. The growth of nanoparticles is an important source of Aitken mode and accumulation mode particles, including cloud condensation nuclei, especially in remote regions with few primary particle sources. For more quantitative descriptions of growth processes, as well as their parameterizations in models, see Kulmala (1993), Kulmala et al. (1993), Kerminen et al. (1997), Mattila et al. (1997), Vesala et al. (1997), Seinfeld and Pandis (1998), and Friedlander (2000). 

Accumulation mode particles in this mode originate from primary emissions as well as through gas-to-particle conversion, chemical reactions, condensation and coagulation. 

Figure 13.3 The volume distribution of the Piisadena. California aerosol measured on September 3, 1969. Photochemical processes acting on vehicular emissions lead to the increase in accumulation mode particles froin 4 A.M. to noon. Advection of cleaner air clears out the aerosol products in the afternoon. (After Whitby ct al., 1972.)...

F02 growth of existing accumulation-mode particles by the deposition of products of chemical reactions. 

The sources and chemical compositions of the fine and coarse urban particles are different. Coarse particles are generated by mechanical processes and consist of soil dust, seasalt, fly ash, tire wear particles, and so on. Aitken and accumulation mode particles contain primary particles from combustion sources and secondary aerosol material (sulfate, nitrate, ammonium, secondary organics) formed by chemical reactions resulting in gas-to-particle conversion (see Chapters 10 and 14). 

The main mechanisms of transfer of particles from the Aitken to accumulation mode is coagulation (Chapter 13) and growth by condensation of vapors formed by chemical reactions (Chapter 12) onto existing particles. Coagulation among accumulation mode particles is a slow process and does not transfer particles to the coarse mode. 

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. 

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