Chapter 19
Measurement of Air Quality
Air quality measurements are designed to determine the levels of all types of con-
taminants in the atmosphere we breathe with no attempt made to differentiate
between naturally occurring contaminants and those that result from human activity.
Measurements of air quality fall into three classes:
0 Measurement of emissions. This is called stack sampling when a stationary source
is analyzed. Samples are drawn out through a hole or vent in the stack for on-the-spot
analyses. Mobile sources like automobiles are tested by sampling exhaust emissions
while the engine is running and working against a load.
0 Meteorological measurements. The measurement of meteorological factors: wind
speed, wind direction, lapse rates, etc., is necessary to determine how pollutants travel
from source to recipient.
0 Measurement of ambient air quality. Ambient air quality is measured by a variety
of monitors discussed in this chapter. Almost all evidence of health effects of air
pollution are based on correlation of these effects with measured ambient air quality.
Modern monitoring and measurement equipment uses power-driven pumps and
other collection devices, and can sample a larger volume of air in a relatively shorter
time than older equipment could. Gas measurement is often by wet chemistry, in that
collected gas is either dissolved into or reacted with a collecting fluid. Modern monitors
provide continuous readout. The measurement of pollutant concentrations is almost
instantly translated by a readout device, so that the pollution may be measured while
it is happening.
MEASUREMENT OF PARTICULATE MATTER
Both total suspended particulate matter (TSP) and particles 10 pm in diameter or less
(PMlo) are measured when monitoring ambient air. Measurement devices used are
variants of high-volume, or hi-vol, samplers. Ahi-vol sampler (see Fig. 19-1) operates
much like a vacuum cleaner by pumping air at a high rate through a filter. About
2000m3 (70,000 ft3) can be pumped in 24h, so that sampling time can be cut to
between 6 and 24 h. Analysis is gravimetric; the filter is weighed before and after the
sampling period. The weight of particles collected is then the difference between these
two weights.
375