Transportation vehicles, ships, aircraft, and locomotives emitted 58 percent of the nation's carbon monoxide (CO), 45 percent of nitrogen oxides (NOX), 36 percent of volatile organic compounds (VOC), 4 percent of particulates, 8 percent of ammonia, and 5 percent of sulfur dioxide in 20021 .
With the exception of ammonia emissions, which grew 54 percent, other transportation air emissions declined from 1992 to 2002 (figure 15-1). Generally, most declined by at least 30 percent, however, NOX emissions decreased only 8 percent between 1992 and 2000 but then fell to 18 percent by 2002.
In 2002, highway vehicles emitted almost all of transportation's share of CO, 78 percent of the NOX, and 77 percent of all VOC (figure 15-2). Marine vessels and railroad locomotives contributed 11 and 9 percent, respectively, of transportation's NOX emissions. Other vehicles, such as recreational boats, airport service vehicles, and road maintenance equipment, had a 22 percent share of VOC emissions.
These key air emissions affect the nation's air quality and are the most widely used indicator of transportation's impact on the environment and human health (box 15-A).
1. U.S. Environmental Protection Agency, Office of Air and Radiation, Air Trends, available at http://www.epa.gov/airtrends/, as of February 2005.
1 Starting with its 2001 updates, the U.S. Environmental Protection Agency is no longer estimating lead emissions. In 2000, transportation emitted 13 percent of the nation's lead emissions. Aircraft emitted almost 96 percent of all transportation lead emissions. While the substance is no longer used in most fuels, it is still present in aviation fuels.