Traffic emissions – such as from trucks, buses, and automobiles – also contribute. Airborne by-products from vehicle exhaust systems cause air pollution and are a major ingredient in the creation of smog in some large cities.
The major culprits are from transportation sources are carbon monoxide (CO),nitrogen oxides (NO and NOx), volatile organic compounds, sulfur dioxide,and hydrocarbons.These molecules react with sunlight, heat, ammonia, moisture, and other compounds to form the noxious vapors, ground level ozone, and particles that comprise smog.