Page 226 - Fundamentals of Air Pollution

IV. Static Sampling Systems 189

The collection technique involves the removal of particles from the air
stream. The two principal methods are filtration and impaction. Filtration
consists of collecting particles on a filter surface by three processes—direct
interception, inertial impaction, and diffusion (5). Filtration attempts to
remove a very high percentage of the mass and number of particles by
these three processes. Any size classification is done by a preclassifier,
such as an impactor, before the particle stream reaches the surface of the
filter.

IV. STATIC SAMPLING SYSTEMS

Static sampling systems are defined as those that do not have an active
air-moving component, such as the pump, to pull a sample to the collection
medium. This type of sampling system has been used for over 100 years,
Examples include the lead peroxide candle used to detect the presence of
SO 2 in the atmosphere and the dust-fall bucket and trays or slides coated
with a viscous material used to detect particulate matter. This type of system
suffers from inability to quantify the amount of pollutant present over
a short period of time, i.e., less than 1 week. The potentially desirable
characteristics of a static sampling system have led to further developments
in this type of technology to provide quantitative information on pollutant
concentrations over a fixed period of time. Static sampling systems have
been developed for use in the occupational environment and are also used
to measure the exposure levels in the general community, e.g., radon gas
in residences.
The advantages of static sampling systems are their portability, conve-
nience, reliability, and low cost. The systems are lightweight and can be
attached directly to individuals. Nonstatic sampling systems can, of course,
also be attached to individuals, but are less convenient because the person
must carry a battery-powered pump and its batteries. Static sampling sys-
tems are very reliable, and the materials used limit the costs to acceptable
levels.
Two principles are utilized in the design of static samplers—diffusion
and permeation (6, 7). Samplers based on the diffusion principle depend
on the molecular interactions of N 2, O 2, and trace pollutant gases. If a
concentration gradient can be established for the trace pollutant gas, under
certain conditions the movement of the gas will be proportional to the
concentration gradient (Pick's law of diffusion), and a sampler can be de-
signed to take advantage of this technique. Figure 13-4 illustrates this princi-
ple. The sampler has a well-defined inlet, generally with a cylindrical shape,
through which the pollutant gas must diffuse. At the end of the tube, a
collection medium removes the pollutant gas for subsequent analysis and