Key Air Emissions

Key Air Emissions

Transportation in 2001 emitted 66 percent of the nation’s carbon monoxide (CO), 47 percent of nitrogen oxides (NOx), 35 percent of volatile organic compounds (VOC), 5 percent of particulates, 6 percent of ammonia, and 4 percent of sulfur dioxide.1 Highway vehicles emitted almost all of transportation’s share of CO in 2001, 79 percent of the NOx, and 78 percent of all VOC (figure 10-1). Marine vessels and railroad locomotives each contributed 10 percent of transportation’s NOx emissions, and other nonroad vehicles2 had a 20 percent share of VOC emissions. With the exception of ammonia, transportation air emissions have declined since 1991 (figure 10-2). NOx shows only a slight decrease between 1991 and 2001.

Gasoline powered highway vehicles experienced the greatest decline in NOx emissions, while diesel-powered highway vehicles and aircraft show increases between 1991 and 2001 (figure 10-3). New, tightened NOx emissions standards for diesel and gasoline trucks are due to go into effect in 2007 and 2008 [1]. In addition, new, tightened NOx standards will apply to certain marine engines built in 2004 or later. NOx emissions standards for locomotives went into effect in 2000, and tightened standards will apply to locomotives built in 2005 and later [2].

These key air emissions—generated during the use of various vehicles, locomotives, aircrafts, and vessels—affect the nation's air quality and are the most widely used indicator of transportation's impact on the environment and human health (box 10-A).


1. U.S. Department of Transportation, Bureau of Transportation Statistics, National Transportation Statistics 2002 (Washington, DC: 2003), tables 4-30–4-32, also available at, as of June 2004.

2. U.S. Department of Transportation, Federal Railroad Administration, personal communication, July 2003.

1 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.

2 Other nonroad vehicles include recreational marine vessels, airport service vehicles, and road maintenance equipment.