Page 227 - Fundamentals of Air Pollution

190 13. Ambient Air Sampling

Fig. 13-4. Static sampler based on the diffusion principle.

maintains a concentration gradient between the inlet of the tube and the
collection medium. The mathematical relationship (Pick's law) describing
this type of passive sampler is given by Eq. (13-1):

where R is the rate of transport by diffusion in moles per second, D the
diffusion coefficient in square centimeters per second, A the cross-sectional
area of the diffusion path in square centimeters, C the concentration of
species in moles per cubic centimeter, and x the path length in centimeters.
The ability of gases to permeate through various polymers at a fixed
rate depending on a concentration gradient has been used to create static
samplers. This principle was originally developed to provide a standard
calibration source of trace gas by putting that gas in a polymer tube under
pressure and letting the material diffuse or permeate through the wall to
the open atmosphere. Permeation samplers operate in the reverse direction.
Figure 13-5 illustrates this type of system. A thin film membrane is open
to the atmosphere on one side and to a collection medium on the other.
A pollutant gas in the atmosphere diffuses through the membrane and is
collected in the medium. The mathematical relationship for a permeation
sampler is given by Eq. (13-2):

Fig. 13-5. Static sampler based on the permeation principle.