FIGURE 46.5 Emission sources of criteria air pollutants for the United States. Recent EPA data show that on-road vehicles, categorized as “transportation,” are the largest source of (a) carbon monoxide and (b) nitrogen oxides. The major source of (c) anthropogenic sulfur dioxide is the generation of electricity, primarily from coal. Among the sources of (d) particulate matter are road dust, industrial processes, electricity generation, and natural and human-made fires.

(Data from http://www.epa.gov/air/emissions/index.htm)