Environmental Engineering FOURTH EDITION

Measurement of Air Quality 38 1

important when there is a question of compliance with air quality standards and when
analyzing for hazardous air pollutants.

GRAB SAMPLES

It is often necessary to obtain a sample of a gas for analysis in the laboratory, and obtain-

ing this sort of grab sample of air presents some difficulties. Grab samples of exhaust

gases, as from automobiles, are relatively straight forward to collect; care must be taken

only that the container can withstand the temperature of the exhaust gas being sampled.

Plastic or aluminum-coated bags are often used. The gas is pumped or exhausted with

some positive pressure into the bag, and allowed to escape through a small hole. By

displacing several volumes of gas in this way, contamination problems can be avoided.

Evacuated containers can be used for grab samples. The air to be sampled is

allowed to be drawn into a previously evacuated container, usually attached to a vacuum

system. Some contamination is always possible, since no container can be completely

evacuated and the concentration of air pollutants being sampled is usually small. If

gases being sampled are not soluble in water, allowing gas to displace water in a

container is a useful sampling method. Unfortunately, most air pollutants measured in

grab samples are water soluble.

STACK SAMPLES

Stack sampling is an art worthy of individual attention. Sampling of gas directly from a
stack is necessary to evaluate compliance with emission standards and to determine the
efficiency of air pollution control equipment. In moderate- to large-diameter industrial
stacks, when the gas is exhausted at relatively high temperatures, the concentration of
exhaust gas constituents is not uniform across the gas stream, and the sampler must
take care in getting a representative sample. A thorough survey is made of the flow,
temperature, and pollutant concentration both across the effluent stream and at various
locations within the stack. A train of instruments, as shown in Fig. 19-7, is often used
for stack sampling, so that a number of measurements may be determined at each
positioning of the intake nozzle.

Velocity

Collection Flow

of pollutants measurement

Figure 19-7. A stack sampling train.