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" From Mason and Moore (1982).

" From Mason and Moore (1982).

of 1500. (Similar considerations are expected to apply to manganese in some polluted urban areas where (methylcyclopentadienyl)manganese tricarbonyl, MMT, is added to gasoline to improve octane and act as an antiknock agent (e.g., see Loranger and Zayed, 1997; and Wallace and Slonecker, 1997). Intermediate between these two extremes is vanadium, with an MMD of f.44 /am and EFcrust 14. Vanadium is produced by the combustion of fuel oil, which is expected to generate small particles, but is also found in the earth's crust (Table 9.12), which should lead to its being found in large particles as well. Clearly, the relative contribution of these two processes will depend on the particular location and conditions.

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 pm and a geometric standard deviation of 3, about 5% of the particles will have diameters below the 2.5-yu,m 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).

In addition to crustal elements being found in airborne particles due to weathering processes, in marine areas one also finds particles characteristic of sea salt. Wave action entrains air and forms bubbles that rise to the surface. As they rise, dissolved organics may become adsorbed on them. The bubbles burst on reaching the surface, producing small droplets that are ejected into the air. Two types of drops have been distinguished—jet drops and film drops. Jet drops are produced from the jet of water that rises from the bottom of the collapsing bubble; film drops are produced from the bursting of the bubble water film. Particles with a wide range of sizes, from less than 0.1 pm to greater than 100 ¡xm, are formed (Blanchard and Woodcock, f980; Blanchard, 1985).

It might be assumed that initially the composition of the liquid drops would approximate that of seawater, given in Table 9.13. As evaporation of water in the droplet into the surrounding air mass occurs, the salt

FIGURE 9.34 Amounts of elements (wt%) found in the earth's crust for atomic numbers up to 92 (adapted from Mason and Moore, 1982).
TABLE 9.13 Typical Sea Water Composition"

Species

Concentration (mg L 1 )

Al

1 x 10 2

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