Ozone is a Dr. Jekyll and Mr. Hyde Molecule

As much as we need ozone in the stratosphere to protect us from the harmful rays of the sun, we do not need ozone close to the ground. It is a strong oxidizing agent and damages living things÷both plants and animals. The process that leads to ozone in the lower atmosphere begins with the formation of NO₂. Internal-combustion automobile engines are a major source of NO₂, and so is the lightning that occurs during thunderstorms. In the presence of high-energy ultraviolet radiation, nitrogen dioxide decomposes to form NO and atomic oxygen.

Two things can happen next. The atomic oxygen (O) can react with an oxygen molecule (O₂) to form ozone:

Also, the atomic oxygen can react with an unburned hydrocarbon molecule to form a radical, C_xH_yO. This, in turn, can react with a molecule of O₂ to form an acyl peroxy radical, the precursor of photochemical smog. In either case, the outcome is a toxic combination of gases that make summer days so unpleasant, and dangerous, in many of America's largest cities.

By the way, NO₂ has a strange role in the story of ozone depletion. To read about just how NO₂ figures into the saga, read Why Antarctica?

Nobel Prize in Chemistry 1995 — featuring the science and autobiographies of Paul Crutzen, Mario Molina and F. Sherwood Rowland, from the Nobel Foundation.