ngs. One of the largest groups of particulates is sulfate aerosols which derive from the sulfur dioxide that is produced by burning coal and oil. Another is nitrate aerosols which are produced by vehicle emissions and, in major population centers with heavy traffic, such as Los Angeles, can account for "about one-third of fine particulates" (Romm & Ervin, 1996, p. 391).
Ozone is the most important of a number of photochemical oxidants, pollutants that "result from chemical reactions driven by heat and sunlight" (Romm & Ervin, 1996, p. 392). It was first identified in the 1950s as an important element of smog and is now one of the major air pollution problems. In their gaseous state, sulfur oxides, another group of photochemical oxidants, can also injure the human respiratory system (Romm and Ervin, 1996). Sulfur oxides also dissolve in water to produce acid, thus "their presence in the air turns rain into acid" (Take a deep breath, 1994, p. 92).
The principal source of both particulates and sulfur oxides is the use of fo
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