Particles modes

As previously mentioned, past studies used non-filtered air with unknown concentrations of trace gases at unknown relative humidities. Also, many of the studies used plastic aging chambers that may have introduced volatile monomers into the air. These unknown factors are important to determine in order to fully understand the nature of the ultrafine particle mode. According to the classical thermodynamic theory of ion cluster formation (Coghlan and Scott, 1983), the relative humidity and trace gases will affect the existence of condensation nuclei. Megaw and Wiffen (1961) observed an increase in nuclei formation with the presence of sulfur dioxide. 

It is seen that the distribution is bimodal, with the coarse mode dominating the aerosol volume concentrations. The 1979 average volume concentration of aerosol less than 10 ym diameter was 32.4 ymVcm. From its large standard deviation, it is clear that the coarse particle mode exhibited considerable variation throughout the year. Records show that high coarse mode volume concentrations accompanied moderate-to-high wind speeds. The coarse material was very likely wind-blown dust of crustal composition. 

Particles in the coarse particle mode originate primarily from mechanical processes while those in the fine particle mode are produced by condensation and coagulation processes (24, 25). The work of many investigators (19, 20, 26, 27) indicates that manganese, particularly coarse particle manganese, is largely... 

Indeed, based on the number, surface, and volume distributions shown in Fig. 9.6, Whitby and co-workers suggested that there were three distinct groups of particles contributing to this atmospheric aerosol. Particles with diameters >2.5 yu,m are identified as coarse particles and those with diameters 2.5 pm are called fine particles. The fine particle mode typically includes most of the total number of particles and a large fraction of the mass, for example, about one-third of the mass in nonurban areas and about one-half in urban areas. The fine particle mode can be further broken down into particles with diameters between 0.08 and 1-2 yxm, known as the accumulation range, and those with diam-... 

FIGURE 9.7 Schematic of an atmospheric aerosol size distribution showing four modes. The original hypothesis of Whitby and co-workers is shown by the solid, trimodal curves, and the fourth, ultrafine particle mode, as well as the two peaks sometimes observed in the accumulation mode are shown by the dashed lines (adapted from Whitby and Sverdrup, 1980). 

FIGURE 9.10 Modified particle modes and growth processes for sulfate particles involving aqueous-phase reactions in low altitude fogs and in higher altitude clouds upon advection of boundary-layer air upwards. (Adapted with permission from Ondov and Wexler, 1998. Copyright 1998 American Chemical Society.)... 

A word of caution is also in order with respect to assigning a particular particle to the fine or coarse particle modes. Since the size distributions can generally be described as log-normal, they do not have sharp cutoffs. A few particles at the top end of the fine mode distribution will have diameters larger than 2.5 [jlm and a few at the bottom end of the coarse mode will have diameters smaller than this. For example, as Lodge (1985) points out, for a coarse particle distribution with a geometric mean diameter of 15 fim and a geometric standard deviation of 3, about 5% of the particles will have diameters below the 2.5-gm fine particle cutoff. This may be responsible for observations that while Si and Ca dominate the coarse particle mode, they are also often found at significant levels in fine particles (e.g., see Katrinak et al., 1995). 

Figure 9.35 shows a typical set of mass size distributions for total suspended particles (TSP), Na, Cl, Al, V, NO-, S04, and NH4 at Chichi in the Ogasawara (Bonin) Islands, about 1000 km southeast of the main island of Japan (Yoshizumi and Asakuno, 1986). As expected for a marine site such as this, Na and Cl from sea salt predominate, and both the TSP and Na and Cl components peak in the coarse particle range. Al is also found primarily in the larger particles and is attributed to a contribution from soil dust. On the other hand, vanadium, non-sea salt sulfate (nss-S04 ), and ammonium are primarily in the fine particles. The vanadium levels are extremely low and likely reflect long-range transport of an air mass containing the products of combustion of fuel oil, which contains V because it is likely associated with a combustion source, it would be expected in the fine particle mode, consistent with Fig. 9.35. 

Sulfate shows two peaks, one in the coarse particle mode associated with sea salt, and one in the fine particle mode. The smaller peak is expected since the nss-S04 is formed from the oxidation of S02 and other sulfur-containing compounds such as dimethyl sulfide. The small size of particles containing ammonium reflects the uptake of the gaseous base NH3 into the smaller acidic particles. 

Similar data for sulfate have been reported in many studies. Figure 9.36, for example, shows overall average sulfate distributions measured in marine areas as well as at continental sites (Milford and Davidson, 1987). The marine data show two modes, a coarse mode associated with sea salt and a fine mode associated with gas-to-particle conversion. Sulfate in seawater, formed, for example, by the oxidation of sulfur-containing organics such as dimethyl sulfide, can be carried into the atmosphere during the formation of sea salt particles by processes described earlier and hence are found in larger particles. The continental data show only the fine particle mode, as expected for formation from the atmospheric oxidation of the S02 precursors. 

On the other hand, nitrate is often a major component of the fine particle mode as well, especially in more polluted urban areas. Figure 9.37, for example, shows the frequency of observations of sulfate and nitrate in various particle sizes during a study in southern California in the summer of 1987 (John et al., 1990). Three distinct peaks are seen for both sulfate and nitrate, at 0.2 0.1, 0.7 0.2, and 4.4 1.2 m,... 

Afterfilter data. As indicated in Table I, the minimum D50 in this study was about 0.5 pm, and particles smaller than this were collected on an afterfilter. Aerosols from combustion of pulverized coal typically are distributed bimodally, with a fine-particle mode at about 0.1 pm and a large-particle mode at supermicrometer sizes the modal diameter of the latter depends strongly on the efficiency characteristics of the control device. The elemental concentrations in the fine-particle mode are of interest in health-impact and source-apportionment studies because of the typically high enrichment of the concentrations of many potentially toxic elements and useful tracer elements in particles in this size range. Large-particle con-taimination of the afterfilter due to particle bounce can, however, limit the value of these data. 

In addition to the three modes described above, recent measurements have shown that there is often a distinct particle mode under 10-nm diameter (Fig. 2). There is no current agreement for the name of particles in this mode, which are interchangeably called ultrafme particles, nanoparticles, or nucleation mode particles. There are also alternative definitions for these terms, which can be a source of confusion. For example, the term ultrafme particles is sometimes employed to refer solely to particles with Dp = 3 -10 nm (e.g., in nucleation studies) or to all particles with Dp < 100 nm (e.g., in health and emission studies). Similarly, the term nanoparticles is sometimes employed as a description for all particles of Dp < 50 nm (regardless of mode), sometimes for particles of 10-nm diameter or less, and occasionally for any particle with Dp < 1 pm. In this review we use the common current definitions of ultrafme particles as those with Dp < 100 nm and nanoparticles as those with Dp < 50 nm. 

The different size modes reflect differences in particle sources, transformations, and sinks (Finlayson-Pitts and Pitts 2000). For example, coarse particles are generated by mechanical processes such as wind erosion of soil, wave action in the oceans, and abrasion of plant material. In contrast, many of the fine particles in the atmosphere are produced from either primary emissions from combustion sources or via atmospheric gas-to-particle conversions (i.e., new particle formation). The relative and absolute sizes of particle modes, as well as the number of modes, can vary greatly in different locations and at different times. In addition, the chemical composition of particles within one size... 

We will show that for the retrieval of a stored field state only excitations in a certain quasi-particle mode, the dark-state polariton, are of importance. Thus all storage states with the same projection to this mode are equivalent. Due to the existence of these equivalence classes there is no enhanced sensitivity to (individual) decoherence processes in an N-atom system as compared to a single atom system. 

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... 

Atmospheric aerosols consist of particles ranging in size from a few tens of angstroms (A) to several hundred micrometers. Particles less than 2.5 pm in diameter are generally referred to as fine and those greater than 2.5 pm diameter as coarse. The fine and coarse particle modes, in general, originate separately, are transformed separately, are removed from the atmosphere by different mechanisms, require different techniques... 

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