Accumulation mode sources

On the other hand, in the literature reviewed for Table 9.11, particulate lead has an MMD of only 0.55 fim. This is consistent with the major source of lead to the atmosphere in the 1970s being combustion of leaded gasoline, the remainder being due mainly to smelting processes. As a result, Pb is primarily found in the accumulation mode and has a large enrichment factor... 

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. 

The usual TFT structure is shown in Fig. 10.7 and comprises the a-Si H channel, a gate dielectric, and source, drain, and gate contacts. N-channel accumulation mode operation using an undoped a-Si H channel is the only structure widely used. Depletion mode devices are prevented by the high defect density of doped material, which makes it difficult to deplete the channel. The much lower mobility of holes compared to electrons gives p channel devices a lower current by about a factor 100, which is undesirable. 

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). 

The ultrafine range is usually composed of emissions from local combustion sources or particles generated by atmospheric photochemical activity that leads to homogeneous nucleation. The principal mechanism of decay of the ultrafine range is attachment to particles in the accumulation mode by diffusion. Neglecting the Brownian motion of the coarse particles compared with the fine particles, the fractional rate of decay of particles in the ultrafine range is given by (Chapter 7)... 

The residence time distribution curve (Fig. 13.4) provides further insight into the origins of the bimodal distribution. The peak in the residence time curve falls in the size range 0.1 < dp < 1.0 (im corresponding to the accumulation mode. Although the coarse mode has a short residence time, it is continually reinforced by fresh injections of crustal material and. perhaps, anthropogenic sources. Thus the two modes are essentially uncoupled. 

Illustrated in Figure 24.4 is the output characteristic of a pentacene OFET with Au drain-source electrodes and a 200 nm Si02 dielectric [32]. The OFET exhibits unipolar p-type behaviour with a hole mobility = 0.165 cmWs, a threshold of = -4.5 V as well as an On/Off ratio of >10. These parameters have been derived from the respective transfer characteristics. The absence of an s-shaped feature in the linear range of the characteristic indicates ohmic contacts between the Au electrodes and the pentacene active layer. This is attributed to the good matching of the ionisation potential of the organic semiconductor and the Au work frmction. However, employing a Ca drain-soirrce metallisation, with an otherwise identical OFET device structure, the transistor did not exhibit any current in the electron accumulation mode. This is unexpected, since the metal work frmction is well matched to the electron affinity of pentacene. 

Urban aerosols are mixtures of primary particulate emissions from industries, transportation, power generation, and natural sources and secondary material formed by gas-to-particle conversion mechanisms. The number distribution is dominated by particles smaller than 0.1 pm, while most of the surface area is in the 0.1-0.5 pm size range. On the contrary, the aerosol mass distribution usually has two distinct modes, one in the submicrometer regime (referred to as the accumulation mode ) and the other in the coarse-particle regime (Figure 8.11). 

The mass concentrations of the accumulation and coarse particle modes are comparable for most urban areas. The Aitken and nucleation modes, with the exception of areas close to combustion sources, contain negligible volume (Figures 8.11 and 8.13). Most of the aerosol surface area is in particles of diameters 0.1-0.5 pm in the accumulation mode (Figure 8.11). Because of this availability of area, transfer of material from the gas phase during gas-to-particle conversion occurs preferentially on them. 

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 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. 

During the winter and early spring (February to April) the Arctic aerosol has been found to be influenced significantly by anthropogenic sources, and the phenomenon is commonly referred to as Arctic haze (Barrie 1986). During this period the aerosol number concentration increases to over 200 cm-3. The nucleation mode mean diameter is at 0.05 pm and the accumulation mode at 0.2 pm (Covert and Heintzenberg 1993)... 

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. 

The constituents of cloud water derive from two sources one is material incorporated with the condensation nuclei, the other is the dissolution of gases from the sm-rounding air. As the numbers of particles serving as cloud condensation nuclei are most numerous in the size range of the accumulation mode, cloud water generally represents a dilute solution of this fraction of the aerosol. But the components of the aerosol fraction are already fully oxidized and, therefore, not very reactive. On the other hand, many of the gases that dissolve in cloud water have a potential for further oxidation. The aqueous concentration of such substances depends on their abundance in the gas phase before cloud condensation sets in and on the individual gas-liquid (Henry s law) partition coefficients, which causes a redistribution of the substances between the two phases. The amoimt of liquid water in clouds is in the range 0.1-0.5 g/m , so that the volume of liquid... 

---