Page 223 - Fundamentals of Air Pollution

186 13. Ambient Air Sampling

downstream for collection on a filter or other medium. These diffusion
denuders are used to sample for SO 2 or acid gases in the presence of
particulate matter. This type of sampling has been developed to minimize
the interference of gases in particulate sampling and vice versa.
The third technique, shown in Fig. 13-l(c), involves collection of an
aliquot of air in its gaseous state for transport back to the analytical labora-
tory. Use of a preevacuated flask permits the collection of a gas sample in
a specially polished stainless steel container. By use of pressure-volume
relationships, it is possible to remove a known volume from the tank for
subsequent chemical analysis. Another means of collecting gaseous samples
is the collapsible bag. Bags made of polymer films can be used for collection
and transport of samples. The air may be pumped into the bag by an inert
pump such as one using flexible metal bellows, or the air may be sucked
into the bag by placing the bag in an airtight container which is then
evacuated. This forces the bag to expand, drawing in the ambient air
sample.

B. In Situ Sampling and Analysis

The fourth sampling technique involves a combination of sampling and
analysis. The analytical technique is incorporated in a continuous monitor-
ing instrument placed at the sampling location. Most often, the monitoring
equipment is located inside a shelter such as a trailer or a small building,
with the ambient air drawn to the monitor through a sampling manifold.
The monitor then extracts a small fraction of air from the manifold for
analysis by an automated technique, which may be continuous or discrete.
Instrument manufacturers have developed automated in situ monitors for
several air pollutants, including SO 2, NO, NO 2/ O 3, and CO.

III. SAMPLING SYSTEMS FOR PARTICULATE
POLLUTANTS AND PM 10

Sampling for particles in the atmosphere involves a different set of param-
eters from those used for gases. Particles are inherently larger than the
molecules of N 2 and O 2 in the surrounding air and therefore behave differ-
ently with increasing diameter. When one is sampling for particulate matter
in the atmosphere, three types of information are of interest—the mass
concentration, size, and chemical composition of the particles. Particle size
is important in determining adverse effects and atmospheric removal pro-
cesses. The U.S. Environmental Protection Agency has specified a PM 10
sampling method for compliance monitoring for the National Ambient Air
Quality Standards (NAAQS) for particulate matter. This technique must
be able to sample particulate matter with an aerodynamic diameter less
than 10 ju,m with a prescribed efficiency.