Chemical Composition Of Tropospheric Aerosols

As we have seen in our earlier discussion of the size distribution of tropospheric particles, the chemical components are not generally distributed equally among all sizes but, rather, tend to be found in specific size ranges characteristic of their source. Generally, the smallest ultrafine particles are produced by homogeneous nucleation and hence tend to contain secondary species such as sulfate and likely organics (see Section A.2). Particles in the Aitken nuclei range are produced

FIGURE 9.33 Size distribution of particles in clouds (solid line) and below the clouds (dashed line), showing two modes (adapted from Hoppel et al., 1994).

by combustion processes, by coagulation of smaller particles, and by condensation of low-vapor-pressure products of gas-phase reactions. Hence these particles and accumulation mode particles tend to contain elements such as carbon and some trace metals such as V, which are characteristic of combustion, as well sulfates, nitrates, and polar organics. Finally, because mechanical processes are primarily responsible for coarse particles larger than about 2.5 jim, these larger particles typically contain elements in soil, sea salt, etc.

In the following, we illustrate these principles using data from studies in various locations and characteristic of various types of environments from polluted urban to the free troposphere.

1. Inorganic Species a. Size Distribution

Table 9.11 shows the aerodynamic mass median diameter (MMD) for some typical inorganics that are common components of tropospheric particles. Also shown are the calculated crystal enrichment factors, EFcrusl. These are a measure of the enrichment of the element in the airborne particles compared to that expected for the earth's crust, using aluminum as the reference element. Thus EFtrust for a particular element X is defined as

EFcrust {Xair Alair} {Xcrusl A1crust}' (HH)

where "air" indicates the concentration in airborne particles and "crust" that in the earth's crust. A value of EFtrusl of approximately unity indicates that the composition of the particles is consistent with that expected if they were formed by erosion of the earth's surface. In this case, one would also expect that they will fall in the coarse particle range, having typical diameters above about 2.5 yum.

The data in Table 9.11 are based on an extensive review of the literature through 1985 by Milford and Davidson (1985). Because they represent composites of many different studies carried out in many different locations by many different investigators, they will not match any particular sample of airborne particulate matter; indeed, for many of these elements, the size distribution is multimodal, which is not reflected in the median values shown in Table 9.11. On the other hand, such data demonstrate very clearly some characteristics of tropospheric particles that are common to many conditions.

For example, the most common elements in the earth's crust (Table 9.12 and Fig. 9.34) are O, Si, Al, Fe, Mg, Ca, Na, K, and Ti. These elements have MMDs > 3 /am and enrichment factors that are generally less than three (Table 9.If). That the enrichment

TABLE 9.11 Aerodynamic Mass Median Diameters of Tropospheric Particles Containing Various Elements Observed in a Number of Studies and Enrichment Factors"

Element

mmd ((am)

efcrust6

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