Ozone Pollution: Clean Air Facts Air Facts

WHAT'S WRONG WITH OUR AIR?

Ozone - Good Up High, Bad Nearby

The word "ozone" conjures up both good and bad images. In fact, both perceptions are correct since this invisible gas exists in both the upper and lower atmospheres in two very different ways. “Good” ozone – otherwise known as the ozone layer – exists naturally in the Earth's upper atmosphere, or stratosphere, where it acts as a filter to protect people and the environment from harmful ultraviolet radiation. Ground-level ozone, however, is created in the Earth's lower atmosphere when common air pollutants react in the presence of heat and sunlight. This is “bad” ozone and high concentrations of it near the ground's surface can be harmful to people, animals and the environment.

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The Basics of Ground-Level Ozone

Ground-level ozone is a colorless, odorless gas and the main ingredient of urban and regional smog. It is considered a secondary pollutant because it is not emitted directly from a source, but instead forms in the lower atmosphere as a result of a chemical reaction between common air pollutants in the presence of heat and sunlight. The primary pollutants factoring into the formation of ground-level ozone are volatile organic compounds (VOC's) and nitrogen oxides (NOx), which may be found in a variety of sources. Since heat and sunlight are also important factors in ground-level ozone formation, ozone pollution is primarily a concern during daylight hours from April through October. Fortunately, it tends to dissipate after sunset due to its unstable nature.