As this chart using the EPA’s National Emissions Inventory demonstrates, vehicles and other gasoline and diesel engines are a significant source of ozone precursors.
Mobile On-road sources of ozone precursors include light and heavy duty gasoline and diesel automobiles and trucks travelling on public highways. Mobile Off-road sources include railroad locomotives, aircraft, commercial marine vessels, farm equipment, construction equipment, recreational boating, lawn equipment and vehicles on roads other than highways.
Point sources of ozone precursors include stationary commercial or industrial operations that emit 100 tons or more per year of VOCs or NOx. Electrical generation is also a point source, but its NOx emissions are significant enough to present it as a category on its own. Personal and Commercial Use (often referred to as non-point or area sources) are typically too numerous or small to address individually, but may include (but are not limited to) activities such as dry cleaning, bakeries, graphic arts, auto refinishing and consumer product usage such as spray cans.