Mobile sources of air emissions

Combustion of fuels can cause the release of hydrocarbons (HC), carbon monoxide (CO), oxides of nitrogen ( NO_x), particulate matter (PM), toxics, sulfur dioxide (SO₂), and greenhouse gases such as carbon dioxide (CO₂) and nitrous oxide (N₂O).

Mobile sources produce air pollution from exhaust emissions, as well as evaporative emissions and refueling losses from gasoline engines. The type and amount of pollution depends on many factors:

---

• The type of engine or vehicle (e.g., passenger car, heavy-duty truck, lawnmower, locomotive, etc.);
• The type of fuel used (e.g., gasoline, diesel, or alternative fuels);
• The type and condition of emission control devices (e.g., catalytic converters); and
• How the engine is used/run.

For example, diesel engines used in trucks, buses, locomotives, and ships tend to emit more NO_x and PM than gasoline engines. Engines with catalytic converters (e.g., passenger cars) emit much less HC, CO, NO_x, and toxics than similar engines without catalysts (i.e., the catalytic converter is an important pollution control device to reduce exhaust emissions).

EPA regulates air pollution from motor vehicles, engines, and the fuels used to operate them. The agency also encourages travel choices that minimize emissions. Under EPA rules, “mobile sources” include cars and light trucks, large trucks and buses, non-road recreational vehicles (such as dirt bikes and snowmobiles), farm and construction equipment, lawn and garden equipment, marine engines, aircraft, and locomotives.