9: Pollution
AIR POLLUTION
Measurement Units
The most common form of expressing air pollutants is parts per million (ppm); 1
ppm represents one particle of a pollutant for every 999,999 particles of air. The
symbol μ is often used to represent one millionth. This concentration is
equivalent to one drop of ink in 40 gallons (150 L) of water or one second in 280
hours.
Primary Pollutants
Primary pollutants are emitted directly into the air.
Figure 9.1 Primary pollutantsSecondary Pollutants
Secondary pollutants result from the reaction of primary pollutants in the
atmosphere forming a new pollutant. Examples include sulfur trioxide (SO 3 ),
sulfuric acid (H 2 SO 4 ), ozone (O 3 ), and chemicals found in photochemical smog,
such as PANS (peroxyacyl nitrates).