NOx is produced almost entirely as a by-product of high-temperature combustion.

Common sources of NOx include:

• automobiles, trucks, and marine vessels
• gasoline powered lawn equipment
• construction equipment
• power generation
• industrial processes
• natural gas furnaces

VOCs include many organic chemicals that vaporize easily, such as those found in gasoline and solvents. They are emitted from many sources, including:

• gasoline stations
• motor vehicles, airplanes, trains, boats
• petroleum storage tanks
• oil refineries
• biogenic, or natural emissions from trees and plants