Page 241 - Fundamentals of Air Pollution 3E

204 14. Ambient Air Pollutant Analysis and Measurement

the transport and removal of particles in the atmosphere and their deposi-
tion behavior in the human respiratory system. Their chemical composition
may determine the type of effects caused by particulate matter on humans,
vegetation, and materials.
Mass concentration units for ambient measurements are mass (/ug) per
3
unit volume (m ). Size classification involves the use of specially designed
inlet configurations, e.g., PM 10 sampling. To determine mass concentration,
all the particles are removed from a known volume of air and their total mass
is measured. This removal is accomplished by two techniques, filtration and
impaction, described in Chapter 13. Mass measurements are made by pre-
and postweighing of filters or impaction surfaces. To account for the absorp-
tion of water vapor, the filters are generally equilibrated at standard condi-
tions (T = 20°C and 50% relative humidity).
Size distributions are determined by classifying airborne particles by
aerodynamic diameter, electrical mobility, or light-scattering properties.
The most common technique is the use of multistage impactors, each stage
of which removes particles of progressively smaller diameter. Figure 14-6
shows a four-stage impactor. The particulate matter collected on each stage
is weighed to yield a mass size distribution or is subjected to chemical
analysis to obtain data on its chemical size distribution. Impactors are used
to determine size distributions for particle diameters of ~0.1 /xm and larger.
Electrical mobility is utilized to obtain size distribution information in the
0.01-1.0 ptm diameter range. This measurement method requires unipolar

Fig. 14-6. Schematic diagram of a four-stage cascade impactor. Source: Giever, P. M.,
Particulate matter sampling and sizing, in "Air Pollution," 3rd ed., Vol. Ill (A. C. Stern, ed.).
Academic Press, New York, 1976, p. 41.