Page 224 - Fundamentals of Air Pollution

HI. Sampling Systems for Particulate Pollutants and PM 10 187

Particles in the atmosphere come from different sources, e.g., combus-
tion, windblown dust, and gas-to-particle conversion processes (see Chap-
ter 6). Figure 2-2 illustrates the wide range of particle diameters potentially
present in the ambient atmosphere. A typical size distribution of ambient
particles is shown in Fig. 2-3. The distribution of number, surface, and
mass can occur over different diameters for the same aerosol. Variation
in chemical composition as a function of particle diameter has also been
observed, as shown in Table 4-3.
The major purpose of ambient particulate sampling is to obtain mass
concentration and chemical composition data, preferably as a function of
particle diameter. This information is valuable for a variety of problems:
effects on human health, identification of particulate matter sources, under-
standing of atmospheric haze, and particle removal processes.
The primary approach is to separate the particles from a known volume
of air and subject them to weight determination and chemical analysis. The
principal methods for extracting particles from an airstream are filtration
and impaction. All sampling techniques must be concerned with the behav-
ior of particles in a moving airstream. The difference between sampling for
gases and sampling for particles begins at the inlet of the sampling manifold
and is due to the discrete mass associated with individual particles.

Behavior of Particles at Sampling Inlets
Sampling errors may occur at the inlet, and particles may be lost in the
sampling manifold while being transported to the collection surface. Figure

Fig. 13-3. The streamline flow patterns around a sampling inlet in a uniform-flow field.