Page 233 - Fundamentals of Air Pollution

196 14. Ambient Air Pollutant Analysis and Measurement

final evolutionary step has been the concurrent collection and quantification
of SO 2. An example of this is the flame photometric SO 2 analyzer, in which
SO 2-laden air is fed into an H 2 flame, and light emissions from electronically
excited combustion products are detected by a photomultiplier tube. Prior
calibration of the analyzer permits the rapid determination of SO 2. This is
but one of the many methods available for the measurement of SO 2.
Hundreds of chemical species are present in urban atmospheres. The
gaseous air pollutants most commonly monitored are CO, O 3, NO 2/ SO 2,
and nonmethane volatile organic compounds (NMVOCs). Measurement
of specific hydrocarbon compounds is becoming routine in the United States
for two reasons: (1) their potential role as air toxics and (2) the need for
detailed hydrocarbon data for control of urban ozone concentrations. Hy-
drochloric acid (HC1), ammonia (NH 3), and hydrogen fluoride (HF) are
occasionally measured. Calibration standards and procedures are available
for all of these analytic techniques, ensuring the quality of the analytical
results.

A. Carbon Monoxide
The primary reference method used for measuring carbon monoxide in
the United States is based on nondispersive infrared (NDIR) photometry
(1, 2). The principle involved is the preferential absorption of infrared
radiation by carbon monoxide. Figure 14-1 is a schematic representation of
an NDIR analyzer. The analyzer has a hot filament source of infrared
radiation, a chopper, a sample cell, reference cell, and a detector. The
reference cell is filled with a non-infrared-absorbing gas, and the sample cell
is continuously flushed with ambient air containing an unknown amount
of CO. The detector cell is divided into two compartments by a flexible
membrane, with each compartment filled with CO. Movement of the mem-
brane causes a change in electrical capacitance in a control circuit whose
signal is processed and fed to a recorder.
The chopper intermittently exposes the two cells to infrared radiation.
The reference cell is exposed to a constant amount of infrared energy which
is transmitted to one compartment of the detector cell. The sample cell,
which contains varying amounts of infrared-absorbing CO, transmits to
the detector cell a reduced amount of infrared energy that is inversely
proportional to the CO concentration in the air sample. The unequal
amounts of energy received by the two compartments in the detector cell
cause the membrane to move, producing an alternating current (AC) electri-
cal signal whose frequency is established by the chopper spacing and the
speed of chopper rotation.
Water vapor is a serious interfering substance in this technique. A mois-
ture trap such as a drying agent or a water vapor condenser is required to
remove water vapor from the air to be analyzed.