Page 245 - Fundamentals of Air Pollution 3E

208 14. Ambient Air Pollutant Analysis and Measurement

Fig. 14-7. Distribution of sensitivity to ethylene sulfide odor in 33 individuals. The abscissa
is the percentage of the individuals who detected the presence of ethylene sulfide at various
levels. Source: Dravnicks, A., and Jarke, F., /. Air Pollut. Control Assoc. 30, 1284-1289 (1980).

dilutions using some intensity rating method (Fig. 14-7). The threshold
value may be selected at some value (e.g., zero intensity) and the concentra-
tion determined with the dilution ratio.

IV. ANALYSIS AND MEASUREMENT OF VISIBILITY

Impairment of visibility is a degradation of our ability to perceive objects
through the atmosphere. As discussed in Chapter 10, several components
influence our concept of visibility: the characteristics of the source, the
human observer, the object, and the degree of pollution in the atmosphere.
Our attempts to measure visibility at a given location can take two ap-
proaches: human observations and optical measurements. In pristine loca-
tions such as national parks, use of human observers has permitted us to
gain an understanding of the public's concept of visibility impairment.
Although it is difficult to quantify the elements of human observations,
this type of research, when coupled with optical measurements, provides
a better measure of visibility at a given location (18).
Optical measurements permit the quantification of visibility degradation
under different conditions. Several instruments are capable of measuring
visual air quality, e.g., cameras, photometers, telephotometers, transmis-
someters, and scattering instruments.
Photography can provide a permanent record of visibility conditions at
a particular place and time. This type of record can preserve a scene in a
photograph in a form similar to the way it is seen. Photometers measure
light intensity by converting brightness to representative electric signals
with a photodetector. Different lenses and filters may be used to determine
color and other optical properties. When used in combination with long-
range lenses, photometers become telephotometers. This type of instru-