How does ground-level ozone form from vehicle and industrial emissions?

Ground-level ozone is a secondary pollutant formed in the lower atmosphere through complex photochemical reactions involving primary pollutants emitted by vehicles and industrial sources.123

The process typically follows these steps:

• Emission: Automobiles, power plants, and industrial processes release nitrogen oxides ($N{O}_x$) and volatile organic compounds (VOCs), such as gasoline vapors and chemical solvents, into the atmosphere.34
• Reaction: In the presence of strong sunlight, these nitrogen oxides and VOCs undergo chemical reactions. Specifically, nitrogen dioxide ($N{O}_2$) reacts with sunlight and hydrocarbons to produce ozone ($O_3$).1542
• Conditions: This formation is most prevalent in urban areas with heavy traffic, stagnant air, and warm, sunny weather, which drives the photochemical reactions.14

Because ground-level ozone is a key component of photochemical smog, air pollution control efforts focus on reducing the emissions of its precursors—nitrogen oxides and volatile organic compounds—through vehicle emission controls, cleaner fuels, and industrial regulations. It is essential to distinguish this harmful surface-level ozone from the naturally occurring ozone layer found in the stratosphere, which protects life on Earth by blocking harmful ultraviolet radiation.1356