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nitrate are again seen to be much larger in the coarse particles. These data also illustrate again that soil components such as Si, Al, Mg, Fe, and Ca tend to be found mainly in the coarse particle mode, whereas sulfur (i.e., sulfate) is primarily in fine particles. Artaxo and co-workers have deconvoluted these elemental distributions using principal factor analysis into contributions from different sources, including soil, dust, and two other sources that appeared to be plant related.

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 /Am, respectively. Table 9.15 shows the concentrations of nitrate, sulfate, and ammonium in each size range, from which the predominance of nitrate, as well as sulfate and ammonium, in the accumulation mode is evident.

Continental

Marine

Continental

Marine

FIGURE 9.36 Average size distributions for sulfate in continental and marine aerosol. A M M-r is the fraction of the total mass of sulfate (Mt) found in each particle size range (adapted from Milford and Davidson, 1987).

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