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" From Milford and Davidson (1985). h Calculated crustal enrichment factors.

" From Milford and Davidson (1985). h Calculated crustal enrichment factors.

factors are not all unity suggests that there is some contribution of other sources to the airborne particles. For example, road dust is thought to be a major source of Ca since crushed limestone is often used in road paving materials.

On the other hand, in the literature reviewed for Table 9.11, particulate lead has an MMD of only 0.55 /jum. 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

TABLE 9.12 Abundance in Earth's Crust of Some of the Most Common Crustal Elements and Those Commonly Found in Atmospheric Aerosols or Used as a Tracer"

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