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What are Mobile 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.
"On-road" or highway sources include vehicles used on roads for transportation of passengers or freight. "Nonroad" (also called "nonroad") sources include vehicles, engines, and equipment used for construction, agriculture, transportation, recreation, and many other purposes. Within these two broad categories, on-road and nonroad sources are further distinguished by size, weight, use, and/or horsepower.
On-Road Vehicles
This category of mobile sources includes light-duty vehicles, light-duty trucks, heavy-duty vehicles, and motorcycles, 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.
Light-duty vehicles: 
Passenger cars
Light-duty trucks: 
Includes pickup trucks, minivans, passenger vans, and sport-utility vehicles. Those up to 6,000 pounds Gross Vehicle Weight (which includes passenger and cargo weight in addition to the weight of the vehicle) are known as "light light-duty trucks"; those that are 6,001 to 8,500 pounds Gross Vehicle Weight are known as "heavy light-duty trucks."
Heavy-duty vehicles: 
Vehicles of 8,501 pounds Gross Vehicle Weight and higher that are equipped with heavy-duty engines. Examples of heavy-duty vehicles include large pick-ups, buses, delivery trucks, recreational vehicles (RVs), and semi trucks.
Medium duty passenger vehicles:
Vehicles between 8,500 and 10,000 pounds Gross Vehicle Weight that are designed primarily to transport people. Medium-duty passenger vehicles are a subset of heavy-duty vehicles, and consist primarily of large sport-utility vehicles and passenger vans.
Motorcycles: 
2 or 3 wheeled vehicles designed for on-road use.
Nonroad Vehicles, Engines, and Equipment
This category of mobile sources includes nonroad gasoline equipment and vehicles, nonroad diesel equipment and vehicles, aircraft, marine vessels, locomotives, and assorted other engines and vehicles.
Nonroad gasoline, diesel, and "other" equipment and vehicles:These categories of mobile sources include equipment and vehicles fueled with diesel fuel, gasoline, propane, or natural gas in the following sectors: recreational, construction and mining, industrial, lawn and garden, farm, commercial, logging, airport service, railway maintenance, and recreational marine vessels.
Recreational engines and vehicles: 
Includes nonroad motorcycles (dirt bikes), all-terrain vehicles (ATVs), golf carts, snowmobiles, and engines used in specialty vehicles, such as go-karts.
Construction equipment and vehicles: 
Includes asphalt and concrete pavers and paving/surfacing equipment, tampers/rammers, plate compactors, concrete pavers, rollers, scrapers, paving equipment, surfacing equipment, signal boards, trenchers, bore/drill rigs, excavators, concrete/industrial saws, cement and mortar mixers, cranes, graders, nonroad trucks, crushing/processing equipment, rough terrain forklifts, rubber-tired loaders and dozers, tractors/loaders/backhoes, crawler tractors, skid steer loaders, nonroad tractors, dumpers/tenders, and other construction equipment.
Industrial equipment: 
Includes aerial lifts, forklifts, sweepers/scrubbers, and other general industrial material-handling equipment
Lawn and garden equipment: 
Includes lawnmowers, weed trimmers, brush cutters, leaf blowers/vacuums, rear-engine riding mowers, front mowers, chainsaws (under 6 horsepower), tillers (under 6 horsepower), shredders (under 6 horsepower), lawn and garden tractors, wood splitters, snowblowers, chippers/stump grinders, and commercial turf equipment.
Farm equipment:
Includes two-wheel tractors, agricultural tractors and mowers, combines, sprayers, balers, tillers (over 6 horsepower), swathers, hydropower units, and other agricultural equipment.
Commercial equipment:
Includes generator sets, pumps, air compressors, gas compressors, welders, and pressure washers.
Logging equipment:
Includes chainsaws and shredders (over 6 horsepower), skidders, and fellers/bunchers.
Airport service equipment and vehicles: 
Includes ground support equipment used in airport operations, such as maintaining and fueling aircraft, transporting and loading cargo, transporting passengers, handling baggage, servicing lavatories, and serving food.
Railway maintenance equipment:
Includes specialized equipment used for installing and maintaining railroad track.
Recreational marine vessels: 
Includes pleasure boats and larger non-commercial vessels with inboard and outboard engines, stern drive engines, and sailboat auxiliary inboard and outboard engines.
Aircraft: 
All types of aircraft (ground equipment not included)
Marine vessels: 
Includes auxiliary and propulsion engines used by all types of commercial marine vessels, including harbor vessels, tugs, ocean-going ships, and commercial fishing vessels.
Locomotives:
Includes diesel-powered engines only (coal-and wood-fired not included) used in freight and passenger rail, line-haul, local, and switch yard service.