Impingement And Impaction

The impingers previously described for sampling gases and vapors (see Figure 5.10.3) can also be used for the collection of particles and, in fact, were originally developed for that purpose. However, in ambient air sampling, they are not used because their collection efficiency is low and unpredictable for the fine particles present in ambient air. The low sampling rates also make them less attractive than filters for general air sampling, but instances do arise when impingers can be satisfactorily used. When impingers are used, the correct sampling rates must be maintained since the collection efficiency of impingers for particles varies when flow rates are not optimal.

Impactors are more widely used in ambient air sampling. In these devices, air is passed through small holes or orifices and made to impinge or impact against a solid surface. When these devices are constructed so that the air passing through one stage is subsequently directed onto another stage containing smaller holes, the resulting device is known as a cascade impactor and has the capability of separating particles according to sizes.

Various commercial devices are available. Figure 5.10.5 shows one impactor that is widely used and consists of several layers of perforated plates through which the air must pass. Each plate contains a constant number of holes, but the hole size progressively decreases so that the same air volume passing through each stage impinges at an increased velocity. The result is that coarse particles are deposited on the first stage and successively finer particles are removed at each subsequent stage. Although these instruments do not achieve exact particle size fractions, they

FIG. 5.10.5 Cascade impactor (Andersen sampler).

do perform predictably when the characteristics of the aerosol being sampled are known.

In use, an environmental engineer assembles a cascade impactor after scrupulously cleaning each stage and applying, if necessary, a sticky substance or a removable surface on which the particles are deposited. After a period of sampling during which time the volume of air is me-tered, the stages can be removed, and the total weight of each fraction is determined, as well as its chemical composition. Such information is sometimes more useful than a single weight or chemical analysis of the total suspended particulate matter without regard to its particle size.

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