Ground-level Ozone
Ground-level ozone does not come from tailpipes and smokestacks.
Instead, it is a "secondary pollutant" - a byproduct of two other air pollutants, nitrogen oxides (NOx) and volatile organic compounds (VOCs or hydrocarbons). These pollutants, in turn, come from our vehicle emissions, industrial and utility plants and other sources.
Sunlight and warm temperatures drive NOx and VOCs to react to form ozone. That is why we see more ozone in the summer, especially in the afternoons and early evenings. However, ozone can form at other times when the weather is unseasonably warm. Ozone is a principal component of smog.