Page 460 - Fundamentals of Air Pollution 3E

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Emission Standards

L SUBJECTIVE STANDARDS

Limits on emissions are both subjective and objective. Subjective limits
are based on the visual appearance or smell of an emission. Objective limits
are based on physical or chemical measurement of the emission. The most
common form of subjective limit is that which regulates the optical density
of a stack plume, measured by comparison with a Ringelmann chart
(Fig. 25-1). This form of chart has been in use for over 90 years and is
widely accepted for grading the blackness of black or gray smoke emissions.
Within the past four decades, it has been used as the basis for "equivalent
opacity" regulations for grading the optical density of emissions of colors
other than black or gray.
The original Ringelmann chart was a reflectance chart; the observer
viewed light reflected from the chart. More recently, light transmittance
charts have been developed for both black (1) and white (2) gradations of
optical density which correlate with the Ringelmann chart scale. It is now
common practice in the United States to send air pollution inspectors to a
"smoke school" where they are trained and certified as being able to read
the density of black and white plumes with an accuracy that is acceptable
for court testimony.
Before the widespread acceptance of the Ringelmann scale, smoke was
regulated by prohibiting the emission of black smoke. Now regulatory

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