Pollutants Ozone (O3)

Description:
Ozone (O₃) is a gas that forms when three atoms of oxygen are combined. O₃ is a highly unstable gas that may smell sweet. In the upper-atmosphere, ozone is found naturally and protects the earth from harmful ultraviolet radiation. But ground-level ozone is a key ingredient in forming urban smog and is considered one of the greenhouse gases.

In terms of health and air quality, we are concerned with both ground-level, or bad, ozone and upper-atmospheric, or good, ozone.

Source:
Upper-atmospheric ozone (good ozone)
Roughly 6-30 miles above the earth's surface lies the stratosphere where ozone occurs naturally. In this layer of our atmosphere, ozone is formed when ultraviolet radiation dissociates some O₂ molecules into two free oxygen atoms. These free oxygen atoms then combine with other O₂ molecules to create O₃. Since O₃ is in turn dissociated by UV rays, a net balance is kept with continual formation and destruction of the oxygen molecules. In the upper atmosphere this "good" ozone protects life on Earth by absorbing some of the sun's ultraviolet rays. The discovery of a hole in the ozone layer in the last 20 years has been a source of great environmental concern.

Ground-level ozone (bad ozone)
Ozone also occurs in the part of the atmosphere directly above the earth's surface, called the troposphere. Unlike the stratospheric ozone layer, this ground level ozone is harmful to human health. Ground-level ozone is caused by the release of volatile organic compounds (VOC) and nitrogen dioxides (NO₂) into the air. When these chemicals are released into the air they react with sunlight and heat to create ozone. Consequently, as temperatures rise, ozone levels rise as well.

VOC + NO₂ + heat + sunlight = ground level ozone (O₃)

Whereas NO₂ helps form ozone, nitrogen oxide (NO)--a pollutant primarily emitted from motor vehicles--contributes to the destruction of ozone (see discussion of nitrogen dioxide). Therefore, areas with high NO emissions, such as urban centers, have lower concentrations of ozone than rural areas.

Ozone is also considered a greenhouse gas. Ozone absorbs infrared radiation emitted by the Earth's surface and thereby traps heat and warms the troposphere.