Sources of Pollutants in the Ambient Air - Mobile Sources

Figure:On-road sources

"Mobile sources" is a term used to describe a wide variety of vehicles, engines, and equipment that generate air pollution and that move, or can be moved, from place to place. Mobile sources are classified as on-road and nonroad sources. "On-road" or highway sources include vehicles used on roads for transportation of passengers or freight. On-road sources include light-duty vehicles (LDVs, also referred to as passenger cars), heavy-duty vehicles (HDVs), and motorcycles that are used for transportation on the road. On-road vehicles may be fueled with gasoline, diesel fuel, or alternative fuels, such as alcohol or natural gas. "Nonroad" sources include gasoline and diesel powered vehicles, engines, and equipment used for construction, agriculture, transportation, recreation, and many other purposes. These sources emit both criteria pollutants and other hazardous air pollutants.

Mobile sources account for more than half of all the air pollution in the United States. The primary mobile source of air pollution is the automobile. EPA studies show that today's cars emit 75 to 90 percent less pollution (for each mile driven) than their 1970 counterparts, thanks largely to advancements in vehicle and fuel technology. But today's motor vehicles are still responsible for up to half of all the emissions released into the air. The specific pollutant categories include 45 percent of the volatile organic compound (VOC) emissions, 50 percent of the nitrogen oxides (NOX) emissions, approximately 60 percent of the carbon monoxide (CO) emissions and 50 percent of the hazardous air pollutants in urban areas.

Figure: Types and sources of air pollutants

Mobile sources pollute the air through combustion and fuel evaporation. These emissions contribute greatly to air pollution nationwide and are the primary causes of air pollution in many urban areas. Combustion is the process of burning. Motor vehicles and equipment typically burn fuel in an engine to create power. Gasoline and diesel fuels are mixtures of hydrocarbons, which are compounds that contain hydrogen and carbon atoms. In "perfect" combustion, oxygen in the air would combine with all the hydrogen in the fuel to form water and with all the carbon in the fuel to form carbon dioxide. Nitrogen in the air would remain

unaffected. In reality, the combustion process is not "perfect," and engines emit several types of pollutants as combustion byproducts. Evaporation is the process by which a substance is converted from a liquid into a vapor. "Evaporative emissions" occur when a liquid fuel evaporates and fuel molecules escape into the atmosphere. A considerable amount of hydrocarbon pollution results from evaporative emissions that occur when gasoline leaks or spills, or when gasoline gets hot and evaporates from the fuel tank or engine.

Perfect Combustion
Fuel (hydrocarbons) + Air (oxygen and nitrogen) Carbon dioxide (CO2) + water (H2O) + unaffected nitrogen

Typical Engine Combustion
Fuel + Air Unburned Hydrocarbons + Nitrogen Oxides (NOx) + Carbon monoxide (CO) + Carbon dioxide + water

More information on mobile sources can be found at http://www.epa.gov/air/topics/comotaq.html