On hot, sunny days, the gases form ozone, which drifts eastward across the continent as smog.

Atomic oxygen can then combine with more molecular oxygen to form ozone.

When the hydrocarbon molecules in gasoline and other volatile chemicals are exposed to sunlight, they form ozone.

With sunlight and heat they form ground-level ozone.

Hydrocarbons from motor vehicles react with the sun to form ozone.

It is when these two forms of oxygen recombine to form ozone that they release the heat found in the stratosphere.

It takes so many hours to form ozone.

The higher temperatures also speed up the photochemical reactions that form ozone, the study says.

The energy given off is sufficient to form ozone from diatomic oxygen:

In the presence of sunlight, methane interacts with nitrogen oxides to form ozone.