Page 69 - Fundamentals of Air Pollution 3E

HI. Primary and Secondary Pollutants 45

Fig. 4-4. Urban trends in annual sulfur dioxide concentrations. Source: U.S. Environmen-
tal Protection Agency, 1992.

molecules such as polynuclear aromatic hydrocarbons and dioxins are re-
leased from industrial processes and combustion sources and are referred
to as toxic pollutants. Substances such as these, emitted directly from
sources, are called primary pollutants. They are certainly not innocuous, as
will be seen when their adverse effects are discussed in later chapters.
However, the primary pollutants do not, of themselves, produce all of the
adverse effects of air pollution. Chemical reactions may occur among the

Fig. 4-5. Annual second highest daily maximum 1-hr ozone concentration at 495 sites in
the United States for 1982-1991. Source: U.S. Environmental Protection Agency, 1992.