Particle diameter (jim)
Transient nuclei Ultra fine or Aitken nuclei particles i range i
Mechanically generated aerosol range
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).
can also be removed by washout. In the atmosphere, transport of coarse particles over long distances can occur, however, by convective processes. Chemically, their composition reflects their source, and hence one finds predominantly inorganics such as sand, sea salt, and so on in this range, although significant amounts of organics have also been reported associated with dust particles (e.g., Boon et al., 1998). Because the sources and sinks are different from those of the smaller modes, the occurrence of particles in this mode tends to be only weakly associated with the fine particle mode. The majority of biological particles, spores, pollens, and so on, tend to be in the coarse particle range. It should be noted, however, that this by no means indicates that compounds and elements associated with such mechanical processes are always found exclusively in the coarse particle range. For example, O'Dowd and Smith (O'Dowd and Smith, 1993b; Smith and O'Dowd, 1996)
report that sea salt was the major component of all particles with radii above 0.05 /jlm over the northeast Atlantic Ocean.
While particles in the coarse particle mode are generally sufficiently large that they are removed relatively rapidly by gravitational settling, there are large-scale mechanisms of transport that can carry them long distances during some episodes. For example, there are many studies showing the transport of dust in larger particles from the Sahara Desert to the northwestern Mediterranean, Atlantic Ocean, Bermuda, the Barbados, Ireland, the Amazon Basin, and the United States (see, for example, papers by Prospero and Nees, 1977, 1986; Prospero et al., 1981; Talbot et al., 1986; Wolff et al., 1986; Savoie et al., 1989; Mateu et al., 1994; Artaxo and Hansson, 1995; Dentener et al., 1996; Jennings et al., 1996; Gatz and Prospero, 1996; Chiapello et al., 1997; Perry etal., 1997; Moulin et al., 1997; and Li-Jones and Prospero, 1998). Similarly, dust transported from Asia has been recorded on a regular basis over the Pacific (e.g., see Duce et al., 1980; Uematsu et al., 1985; and Zhang et al., 1997) and in the Canadian Arctic (e.g., Kawamura et al., 1996b). Asian dust has been observed, usually during the spring, at the Mauna Loa Observatory in Hawaii (e.g., see Shaw, 1980; Braaten and Cahill, 1986; Zieman et al., 1995; and Holmes et al., 1997); the elemental signature in particles in the size range 0.5- to 3.0-/j,m, particularly in terms of the Si Fe and Ti Fe ratios, is very similar to those measured during dust storms in Beijing, consistent with long-range transport of these particles (Braaten and Cahill, 1986). The mineralogy of the dust has proven useful in source determination of particles after longrange transport (e.g., see Leinen et al., 1994; Merrill et al., 1994; Avila et al., 1997; and Caquineau et al., 1998). While these "dust storms" are episodic in nature, such long-range transport of dust has been proposed to play a significant role in S02 and NOx heterogeneous chemistry, in the photochemical cycles leading to 03 formation (Dentener et al., 1996), and, as discussed in Chapter f4, possibly in the radiative balance of the atmosphere as well.
Particles in the accumulation range with diameters from ~0.08 to ~l-2 ^m typically arise from condensation of low-volatility vapors (e.g., following combustion) and from coagulation of smaller particles in the nuclei range either with themselves or, more likely, with the larger particles in the accumulation range. The coagulation rates for particles in the nuclei range with the larger particles in the accumulation range are usually much larger than for self-coagulation of the small particles; this occurs because of the high mobility of the smaller particles combined with the larger target area of the bigger particles.
Because of the nature of their sources, particles in the accumulation range generally contain far more organics than the coarse particles (other than biologically derived particles) as well as soluble inorganics such as NH4, NO,, and S042 .
While many particle distributions show one peak in the accumulation range, many instances have been observed in which there are two peaks. For example, as seen in Fig. 9.8, John and co-workers (1990) observed two peaks within the traditional accumulation mode, one at 0.2 0.1 and one at 0.7 0.2 ftm, in studies of particles carried out in a relatively polluted urban area. This bimodal character of particles appears to occur quite frequently; for example, Hering et al. (1997) showed that during the summer in the Los Angeles area, two modes often occur, having diameters of about 0.26 /jum (range from 0.10 to 0.39 /¿m) and 0.65 /¿m (range from 0.46 to 0.90 /¿m), respectively.
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