How is ozone produced?
Volatile organic carbons react with NOx when it is sunny
If we have volatile organic carbons in air and we have nitrogen oxides, photochemical smog -in the presence of sun- will form.
Hydrocarbons and other organic compounds that can vaporize easily are emitted from cars, power plants, etc.
VOC + NOx + Sunlight --> O3 and others.
If this reaction occurs near ground, it is harmful to public health.