Environmental Engineering FOURTH EDITION

Measurement of Air Quality 311

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Figure 19-2. Cascade impactor.

The cascade impactor, shown in Fig. 19-2, may also be used to measure fine
particles including respirable particles less than 1.0 pm in diameter - small enough
to penetrate the lung. The impactor consists of four tubes, each with a progressively
smaller opening, thus forcing progressively higher throughput velocities. A particle
entering the device may be small enough to follow the streamline of flow through
the first nozzle without bitting the microscope slide. At the next nozzle, however, the
velocity may be sufficiently high to prevent the particle from negotiating the turn, and
it will impinge on the slide.
Particle-measuring devices are usually fitted with an automatic computer input and
recording arrangement. The hi-vol sampler/computer recorder combination is often
referred to as a CAPS, or computer-assisted particle monitor.
The nephelometer (Fig. 19-3) can make continuous measurements of real-time
data. A nephelometer measures the intensity of light scattered by fine particles in the
air, and the scattered light intensity is proportional to the concentration of smoke or very
fine particulate matter in the air. Fine particles intedere with visibility by scattering
light; this scattering is what we know as haze. In a nephelometer, the scattered light
intensity is measured at a 90" angle from the incident light. The instrument can be
calibrated either in units of percent visibility decrease or in units of micrograms per
cubic meter (pg/m3).

MEASUREMENT OF GASES

Ambient concentrations of gaseous air pollutants are usually measured by reacting the
gas chemically with a colorimetric reagent and measuring the intensity of the reaction
product color.