Ground-level ozone in most major united states cities results primarily from

it results primarily from nitrogen dioxide which comes from Combustion and industrial emissions

Explanation: The most important constituent in the production of ozone is nitrogen dioxide. Nitrogen dioxide comes primarily from Combustion of cars and heavy industrial emissions from factories and industries. It can also come from natural sources which includes wildfires.

Ground-level ozone in most major United States cities results primarily from motor vehicle exhausts.

Explanation:

Ozone is composed of three oxygen atoms. It is a pale blue gas.There are two types:

1)Stratospheric ozone:(naturally occurring) also famously known as ozone layer,that forms high in the atmosphere.This is considered as a 'good ozone' as it protects earth from harmful sun's ultraviolet radiation.

2)Ground-level ozone: forms just above the earth's surface.This is considered dangerous and harmful for both humans and ecosystem health. It is formed when nitrogen oxides photochemically react with volatile organic compouds. Nitrogen dioxide, the most significant component in the creation of ozone, comes fundamentally from motor vehicles and industrial emissions.