The primary component of photochemical smog is ozone. Ozone (O3) is not emitted directly into the air. The ozone layer is formed through a complex series of reactions from nitrogen oxide, volatile organic compounds, and sunlight during periods when temperatures are elevated. The next result of these natural processes is the so-called ozone layer in the stratosphere at altitudes between 9 and 30 miles in which the concentration of the ozone is raised. Ground-level ozone levels increase in the spring and summer when there is more sunlight and temperatures are higher. Formation of ozone is lowest in the morning, and highest in the late afternoon when sunlight intensity and temperature peak. Ozone breaks down quickly when the sun goes down, and does not accumulate from day to day (NAS). Air in the stratosphere absorbs solar energy, or heat from the sun, which in turn creates a photochemical reaction that produces ozone - a benefit to the environment since ozone protects people, plants, and animals from harmful radiation.
The ozone layer is important because it absorbs most of the damaging ultra-violet radiation from the sun before it reaches ground level; where it can cause sunburn, skin cancer and cataracts. Research suggests that any additional UV-B (ultra-violet radiation) at ground level could depress our bodies" immune systems, damage the natural food chain and reduce crop yields (Ehrlich 115). Although ozone makes up less then 1ppm of all the gases in our planet's atmosphere, but it is essential to life on earth. Scientists assume that in the early days of the earth's evolution there was no atmosphere, but gases from planet surfaces and volcanoes slowly collected. At first, the gas was little protection from the sun's UV radiation. But according to some evolutionary theories, life forms on earth may have been able to develop in water that filtered out most of the UV rays but allowed enough visible lights for chemical reactions to take place.