THE POLLUTANTS

Pollution control is directed at five major air pollutants: carbon monoxide (CO), hydrocarbons (HC), oxides of nitrogen (NO_x), sulfur dioxide (SO₂), and particulates. A sixth pollutant, photochemical oxidants, is formed by the action of sunlight on NO_x and HC and does not come from sources on the ground. Control of NO_x and HC would automatically lead to control of photochemical oxidants. As Table 6-1 shows, different sources cause widely differing proportions of the pollutants. Transportation, mostly automobiles, causes more than three-quarters of the CO emissions and more than half the HC and NO_x totals. Stationary fuel combustion, mostly in power plants generating electricity, produces SO₂. Eighty percent of the SO₂—an especially dangerous pollutant when combined with particulates—comes from such sources. Industry gives off about half of all particulate emissions.

The two pollutants caused chiefly by transportation—carbon monoxide and hydrocarbons—are reasonably well known: CO is a poisonous gas and HC is one of the primary ingredients of photochemical smog. These are mainly produced by the combustion of gasoline in auto engines. Sulfur dioxide comes from sulfur in a fuel being burned in the presence of oxygen. A typical power plant using coal containing 1 percent sulfur would produce 38 pounds of SO₂ per ton. Particulates are particles of various sizes produced along with SO₂ in the combustion of fossil fuels. Larger particles appear as soot or dirt. The same